AN INTERVIEW WITH

Diana H. WallAs a past president of the Ecological Societyof America and the American Institute of Bio­logical Sciences, and in many other nationaland international roles, Diana Wall has mademajor contributions to science and the publicinterest. She has also distinguished herself asa researcher on carbon cycling and otherewsystem processes, focusing on the tinyroundworms called nematodes, Dr, Wall hasB,A, and Ph,D, degrees from the University ofKentucky, She was a professor at the Univer­sity of california, Riverside before coming toColorado State University, where she is a pro­fessor of biology and senior research scientistat the Natural Resource Ecology laboratory,She does much of her fieldwork in Antarctica.

How did you get slarted in ecology?[n graduate school, in plant pathology, I stud­ied the interactions between two species ofnematodes and the plant roots they parasitize.Later, as a post-doc at UC Riverside, [partici­pated in the International Biological Program(161'). My job was to go to deserts in the west·ern U.S. and try to find out how soil nema­todes contributed to the energy balance inthose ecosystems.

What was the International BiologicalProgram?The IBP was the first truly global researchproject in ecology. The overall goal was to un­derstand Earth's productiVity by takingcomparable measurements across differentecosystems. (ProductiVity is the amount ofnew material-biomass-that's produced inan ecosystem.) Grassland researchers from allaround the world were measuring the samethings, desert researchers were measuring thesame things, and so forth, enabling us to com­bine all the data and make global compar­isons. It was a jump for me from studying theinteractions between two species to be askinghow such interactions fit into the globalscheme of things.

Why arl' nematodes of inll'f1'St to ecologists?Although many people know nematodes asharmful parasites of animals or plants, mostnematodes are good guys, espedally the free­liVing ones in soil. As part of my work in thedesert, [got interested in free-liVing soil nema­todes, whicll feed on fungi or bacteria, or some­times on smaller animals. The "good" nematodesaccelerate the turnover of organic matter byfeeding on soil microbes and tllen releasingcompounds of caroon and nitrogen to tile soil.Experiments have shown that soils withoutnematodes have slower rates ofdecomposition.

Nematodes are everywhere: in soils, instreams, in ocean sediments, and in many ani­mals and plants. It's been estimated that fourout of five animals on Earth are nematodes! Thegeographic distribution of different species ofnematodes is something we're studying. We'redoing a global-scale "latitudinal gradient" ex­periment, where we take soil samples at differ­ent latitudes-for example, from Sweden toSouth Africa. We're going into places that arehotspots of biodiversity above ground-andMcoldspDts" as well-and looking at what's be­low ground. We're classifying the nematodes tosee how many spedes there are in the soil andhow spedes distribution differs with latitude.

What impact have modern molecularmethods had on this kind of study?They've been great for research on the diversityof all kinds of microscopic organisms. We cannow go beyond asking general questions aboutwhat such organisms do in the soil and ask ex­actly what they are. With soil invertebrates suchas nematodes and mites, it can be very difficultto tell species apart morphologically. Now wecan look at DNA sequences and say, whoa, thisone is really different from that one, or, hey, thisone is endemic to Antarctica but it's got a reallyclose relative hanging out in Argentina.

Why is soil biodiversity important?From a practical perspective, soil biodiversity­the number and abundance of different speciesliving in soils-is probably critically importantfor soil fertility, and soil fertility is crucial for

feeding and clothing the world's people. There'sstill much to be learned about the role of soilbiodiversity in soil fertility. Simply how soilsdiffer over the range from a forest to a desert isstill a big question. Based on climate, geologichistory, and biology, there are about 13,000 kindsofsoil (called "series") in the United States alone.Each is a distinct habitat where microscopicsoil creatures have evolved, and so not everyspecies is widespread. Where the species aredifferent, can you scale up from a differencebetween two farms, say, or between a farm anda nearby forest, and generalize about the con­nections between soil biodiversity and soil fer­tility? How do species that are just abouteverywhere contribute to soil fertility? Thosequestions are wide open for more research.

Another important question is whetherthere's a correlation between biodiversity ahoveground and below ground. Can we predict thatan area ofgreat biodiversity above ground willhave a great diversit)' of mites, nematodes, andso forth below ground? And will their functionsin the ecosystem be predictable, to07 Because Idon't want to go around digging up all the soilsin the worldl Once we understand somethingabout that issue, we can start to think about therelationship between diversity and soil fertility.My big push now is to persuade people that )'oucan't think ofsoil biodiversity as being unrelatedtowhafs above ground. We used to think oftheocean as a dark garbage can, and we ~tilltend tothink of the soil that way. But we found outthere are a lot oforganisms in the ocean thatcarry out processes that benefit us, even if wedon't have names for all ofthem. I think thesame is true for soils.

How did you begin doing research inAntarctica? And what do you study there?After the IBP, [continued working in hotdeserts. They're a simpler system than a forest,for example, and therefore are more suitable forworking out the factors that determine differentfood webs in the soil. To tease out the contribu­tion of organisms other than plants, we wouldcompare areas with and without plants. Butthat's hard to do, because plant roots seem to go

everywhere, even in the desert. I wanted to findan even simpler s~'Stem, with fewer variables­soils where there are no plant roots. Iwrote to acolleague who was in Antarctica, and he sentme some soil for analysis. It certainly didn'thave plant roots, and it did have nematodes.

But when [searched the scientific literature,I read that the soils of the Antarctic Dry Val­leys, where my sample had come from, weresterile, with no organisms at all, not even pro­karyotes! Only in melt streams had some lifebeen found. Researchers had looked for bacteriain the soil, but only by trying to culture them,and nothing had shown up. This was beforemolecular methods had been developed foridentifying microorganisms from DNA alone.

We went down to Antarctica for the firsttime in 1989. Only 2% of Antarctica is actuallysoil; the rest is rock and ice. And the Dry Val­leys area has no visible vegetation; when you flyin, it looks like Mars. We had enough financialsupport for only one field season-so only tv.'omonths to find out what lived in the soil there.We adapted our hot-desert methods for thiscold desert, which has precipitation the equiva­lent of less than 3 cm of rain per year. And aswe'd hoped, we soon saw that there was anabundance of life in the soil. as much as in theChihuahuan Desert in New Mexico. We couldnot believe that other people had missed it.

Our method was simple: We would take ahandful of soil, about 100 g, stir it up with asugar solution, and centrifuge it. The nema­todes float in the solution, and the rock parti­cles sink. After rinsing off the nematodes, wecount them and determine their species. [t wasamazing to me to see these animals, knowingthat they spend nine months of the year in ahard, frozen, and dark environment. It turnedout that they live in water films around soilpores, and use survival mechanisms similar tothose of hot-desert nematodes. When they

receive certain environmental cues, such asdryness, they shrink their long bodies and curlup in a spiral, losing 99% of their water. Whenthey're that small, they can disperse in wind.So this is a mechanism for spreading, as well asfor surviving in a particular location.

These nematodes participate in very, verysimple food chains, with only one to three tiers.For example, we have one chain with only twospecies of nematode: The one at the bottomeats bacteria, and the one above it may eat thebacteria-eater. It's remarkably simple com­pared with what is found in the soils up here.

In the last two years, we've been lookingmore closely at the different habitats. We'vefound that one species is almost everywhere inthe valleys, Scottnema lindsa)'ae; it's reallytough, our "Rambo." But if there's too much saltin the soil or too much water, it's not present.We now have a model enabling us to predictwhere this species will be found around the val­leys. In Antarctica, unlike elsewhere, 1can lookat individual nematode species and see thatthey have different niches; there's little overlap.

How is climate change affecling Antarctica?Antarctica is the only continent that hasn't onaverage shown warming yet. The AntarcticPeninsula is warming rapidly, but the Dry Val­leys are cooling, at least partly an indirect effectof ocean currents. Some prople have made abig fuss about this cooling in attempts to dis­prove global warming. What we emphasize isthat, while the continent as a whole is notwarming, various regions are undergoing majorchanges, and there is warming and cooling oc­curring in different regions. [n the Dry Valleys,we're seeing changes in every component of theecosystem. For instance, we're seeing a decline inthe abundance of the widespread S. lindsayae.

To look at this decline over a period oftime,we've set up experiments in each valley basin.

We've made the soil wetter in places-because ifan unusual amount of melting occurs when cli­mate warms in this region, that's going to be thebiggest driver of change. We want to see whatthat does to the S. lindsayae population and tothe turnover of carbon in the soil. Because it'sthe only nematode species in many Dry Valleyareas, we can easily use a carbon-isotope tracerto see how much carbon that one species is as­similating. The nematode population has verylow biomass compared to what you see in thegrassland here in Colorado, but it assimilates agreater percentage of organic carbon from thesoil than do all the species here together. So it's avery important player in the ecosystem.

[n places where we've manipulated the en­vironment in the past, we're looking at howlong it takes the soils to return to their originalstates. We've also looking at the effects ofhu­man trampling because the numbers of scien­tists and tourists are increasing tremendously.\'({hat we see is that human movement along apath disturbs the soil enough to cause a signifi­cant decline in nematodes. Furthermore, new,potentially invasive organisms are coming inon people's shoes and and clothing.

Antarctica is not isolated from the rest ofthe planet. The continents are all connected byocean and atmosphere, and there is muchmore movement than we once thought. So fu­ture changes are something to worry about.

What is it like doing ecological researcMIt's fun. You have an idea, you come up with ahypothesis, and then rou get to test it in thelab or field. But as rou test the hypothesis, youlearn new things, and there are unexpectedchallenges. You have to try and fit everythingyou learn into a bigger picture. The process islike gathering all the blocks in a playpen andbuilding a structure-without having thewhole thing fall over.

Fieldwork has other kinds of pleasures.When you're in the field, you get to focus onone question that really interests you-duringthe Antarctic summer, there's a temptation towork 24 hours a day! At the same time, youusuallr have colleagues with different special­ties there, and it's a great opportunity for free­wheeling discussion and new insights. There'sa great feeling of camaraderie.

Of course, there are frustrations, too. Col­lecting your data may be uncomfortable or bor­ing. And you can spend a lot oftime in the fieldthat's not very productive or even a total fail­ure. But the result of analyzing the data from asuccessful field trip can be a satisfying contri­bution to what we know about the world.

Learn about an eKperiment by Diana Wall and acolleague in Inquiry Fgufl' 54, 19 on page 1210,

Leit to right: Diana Wall, Jane Reece, Rob Jackson

ahere.....lIIIIII

An I tro~·"""

to Eco 0

the.... Figure 52.1 Why do gray whales migrate?

KEY CONCEPTS

52.1 Ecology integrates all areas of biologicalresearch and informs environmental decisionmaking

52.2 Interactions between organisms and theenvironment limit the distribution of species

52.3 Aquatic biomes are diverse and dynamicsystems that cover most of Earth

52.4 The structure and distribution of terrestrialbiomes arc controlled by climate anddisturbance

r·'iji"'i~'.

The Scope of Ecology

High in the sky, a series of satellites circle Earth.These satellites aren't relaying the chatter of cell

phones. Instead, they are transmitting data onthe annual migration of gray whales (Figure 52.1). Leav­ing their calving grounds near Baja California, adult andnewborn gray whales (Eschrichtius robustus) swim sideby side on a remarkable 8,000-km journey. They areheaded to the Arctic Ocean to feed on the crustaceans,tube worms, and other creatures that thrive there in sum­mer. The satellites also help biologists track a secondjourney, the recovery of the gray whales from the brink ofextinction. A century ago, whaling had reduced the pop­ulation to only a few hundred individuals. Today, after 70years of protection from whaling, more than 20,000 travelto the Arctic each year.

What environmental factors determine the geographicdistribution of gray whales? How do variations in theirfood supply affect the size of the gray whale population?Questions such as these are the subject of ecology (fromthe Greek oikos, home, and logos, to study), the scientificstudy of the interactions between organisms and the en-

vironment. These interactions occur at a hierarchy ofscales that ecologists study, from organismal to global(Figure 52.2).

In addition to providing a conceptual framework for un­derstanding the field of ecology, Figure 52.2 provides the or­ganizational framework for our final unit. This chapter beginsthe unit by describing the breadth of ecology and some of thefactors, both living and nonliving, that influence the distribu­

tion and abundance oforganisms. The next three chapters ex­amine population, community, and ecosystem ecology indetail. In the final chapter, we'll explore both landscape ecol­

ogy and global ecology as we consider howecologists apply bi­ological knowledge to predict the global consequences ofhuman activities, to conserve Earth's biodiversity, and to re­

store our planet's ecosystems.

r:;:~::;i~~~~ates all areas ofbiological research and informsenvironmental decision making

Ecology's roots are in discovery science (see Chapter 1). Nat­uralists, including Aristotle and Darwin, have long observedorganisms in nature and systematically recorded their obser­vations. Because extraordinary insight can be gained throughthis descriptive approach, called natural history, it remains afundamental part of the science of ecology. Present-day ecol­ogists still observe the natural world, albeit with genes-to­globe tools that would astound Aristotle and Darwin.

Modern ecology has become a rigorous experimental sci­enceas well. Ecologists generate hypotheses, manipulate the en­vironment, and observe the outcome. Scientists interested in

the effects ofelimate change on tree survival, for instance, mightcreate drought and wet conditions in experimental plots instead

1148

~ Figure 52.2

•• • The Scope of Ecological Research

Ecologists work at different levels of the biological hierarchy, from individual organisms to the

planet. Here we present a sample research question for each level in the biological hierarchy.

1 Organismal EcologyOrganismal ecology, which includes the subdisciplines of physi­

ological, evolutionary, and behavioral &ology, is concerned with

how an organism's structure, physiology, and (for animals) behav­

ior meet the challenges posed by its environment.

... How do hammerhead sharks select a mare?

2 Population EcologyA population is a group of individuals of the same species livingin an area. Population ecology analyzes factors that affect popu­

lation size and how and why it changes through time.

... What environmental factors affect the reproductive rate of deermice?

3 Community EcologyA community is a group ofpopulations ofdifferent species in an area.

Community ecology examines how interactions between species,

such as predation and competition, affect community structure and

organization.

... What factors influence the diversity of SpeciB that make up a forBt?

4 Ecosystem EcologyAn ecosystem is the community of organisms in an area and the

physical factors with which those organisms interact. Ecosystem

ecology emphasizes energy flow and chemical cycling between

organisms and the environment.

... What factors control photosynthetic productivity in a temperategrassland ecosY5tem?

5 Landscape EcologyA landscape (or seascape) is a mosaic of connected ecosys­

tems. Research in landscape ecology focuses on the factors

controlling exchanges of energy, materials, and organisms

across multiple ecosystems.

... To what extent do the trees lining a river serve as corridors ofdispersalforanima~?

6 Global EcologyThe biosphere is the global ecosystem-the sum of all the planet's ecosys­

tems and landscapes. Global ecology examines how the regional exchange

of energy and materials influences the functioning and distribution of or­

ganisms across the biosphere.

... How does ocean circularion affect the global distribution of crustaceans?

C~"'PH~ fIHY·TWO An Introduction to Ecology and the Biosphere 1149

Troughs collect one-thirdof precipitation that fallson "dry" plot.

Pipes carry water from"dry" plot to "wet" plot.

"Ambient" plot receivesnatural amounts ofrainfall.

reducing the population size of thefungus-an ecological effect-and allow­ing the farmer to obtain higher yields fromthe crop. After a few years, however, thefarmer has to apply higher and higherdoses of the fungicide to obtain the sameprotection. The fungicide has altered thegene pool of the fungus-an evolutionaryeffect-byselectingfor individuals that areresistant to the chemical. Eventually, thefungicide works so poorly that the farmermust find a different, more potent chemi­cal to control the fungus.

.... Figure 52.3 Studying how a forest responds to altered precipitation. At theWalker Branch Watershed in Tennessee, researchers used a system of troughs and pipes to createartificial "dry" and "wet" conditions within parts of a forest.

Ecology and EnvironmentalIssues

of waiting decades for the dry or wet years that could be repre­sentative of future rainfaU. Paul Hanson and colleagues, at OakRidge National Laboratory in Tennessee, used just such an ex­perimental approach in a Herculean study that lasted more thanten years. In one large plot of native forest, they collected one­third ofthe incoming precipitation and moved it to a second plot,while leaving a third plot unchanged as a control (Figure 52.3).

By comparing the growth and survival of trees in each plot, theresearchers found that flowering dogwoods (Comus florida)were more likely to die in drought conditions than were memobers ofany other woody species examined.

Throughout this unit, you will encounter many more ex­amples of ecological field experiments, whose complex chal­lenges have made ecologists innovators in the areas ofexperimental design and statistical inference. As these exam­ples also demonstrate, the interpretation ofecological experi­ments often depends on a broad knowledge of biology.

linking Ecology and Evolutionary Biology

AJ:, wediscussed in Chapter 23, organisms adapt to their environ­ment over many generations through the process of natural se­lection; this adaptation occurs over many generations-the timeframe ofevolutionary time. The differential survival and repr<rduction of individuals that leads to evolution occurs in ecologicaltime, the minute-to-minute time frame of interactions bern'eenorganisms and the envirOlIDlent. One example of how events inecological time have led to evolution was the selection for beaksize in Galapagos finches (see Figure 23.1). Finches with biggerbeaks were better able to eat the large, hard seeds available duringthe drought. Smaller-beaked birds, which required smaller, softerseeds that were in short supply, were less likely to survive.

We can see the link between ecology and evolution allaround us. Suppose a farmer applies a new fungicide to protecta wheat crop from a fungus. The fungicide works well at first,

1150 U"IT EIG~T Ecology

Ecology and evolutionary biology help usunderstand the emergence ofpesticide-resistant organisms andmany other environmental problems. Ecology also provides thescientific understanding needed to help usconserve and sustainlife on Earth. Because of ecology's usefulness in conservationand environmental efforts, many people associate ecology withenvironmentalism (advocating the protection of nature).

Ecologists make an important distinction between scienceand advocacy. Many ecologists feel a responsibility to educatelegislators and the public about environmental issues. Howsociety uses ecological knowledge, however, depends onmuch more than science alone. If we know that phosphatepromotes the growth of algae in lakes, for instance, policy­makers may weigh the environmental benefits of limiting theuse of phosphate-rich fertilizers against the costs of doing so.This distinction between knowledge and advocacy is clear inthe guiding principles of the Ecological Society of America, ascientific organization that strives to "ensure the appropriateuse of ecological science in environmental decision making:'

An important milestone in applying ecological data to en­vironmental problems was the publication of Rachel Car­son'sSilcnt Spring in 1962 (Figure 52.4). In her book, which

.... Figure 52.4Rachel Carson.

I. Contrast the terms ecology and environmentalism.How does ecology relate to environmentalism?

2. How can an event that occurs on the ecological timescale affect events that occur on an evolutionary timescale?

3. -','!:f.jIIM A wheat farmer tests four fungicides on

small plots and finds that the wheat yield is slightlyhigher when all four fungicides are used together thanwhen anyone fungicide is used alone. From an evolu­tionary perspective, what would be the likely long-termconsequence of applying ail four fungicides together?

For suggested answers, see Appendix A.

was seminal to the modern environmental movement, Car­son (1907-1964) had a broad message: "The 'control of na­ture' is a phrase conceived in arrogance, born of theNeanderthal age ofbiology and philosophy, when it was sup­

posed that nature exists for the convenience of man:' Recog­

nizing the network of connections among species, Carsonwarned that the widespread use of pesticides such as DDTwas causing population declines in many more organismsthan the insects targeted for control. She applied ecologicalprinciples to recommend a less wasteful, safer use of pesti­cides. Through her writing and her testimony before the U.S.Congress, Carson helped promote a new environmentalethic to lawmakers and the public. Her efforts led to a ban onDDT use in the United States and more stringent controls onthe use of other chemicals.

CONCEPT CHECK 52.1

r;~";::::~o~~·~tweenorganismsand the environment limit thedistribution of species

Earlier we introduced the range of scales at which ecologistswork and explained how ecology can be used to understand,and make decisions about, our environment. In this section,we will examine how ecologists determine what controls thedistribution of species, such as the gray whale in Figure 52.1.

In Chapter 22, we explored biogeography, the study of thepast and present distribution of species, in the context ofevolutionary theory. Ecologists have long recognized globaland regional patterns in the distribution of organisms. Kan·garoos, for instance, are found in Australia but nowhere elseon Earth, Ecologists ask not only where species occur, but

also why species occur where they do: What factors deter­mine their distribution? In seeking to answer this question,ecologists focus on two kinds of factors: biotic, or living,factors-all the organisms that are part of the individual'senvironment-and abiotic, or nonliving, factors-all thechemical and physical factors, such as temperature, light,water, and nutrients, that influence the distribution andabundance of organisms.

Figure 52,5 presents an example of how both kinds of fac­tors might affect the distribution ofa species, in this case the redkangaroo (Macropus rufus), As the figure shows, red kangaroosare most abundant in a few areas in the interior of Australia,

where precipitation is relatively sparse and variable, They arenot found around most ofthe periphery ofthe continent, where

Kangarooslkm2

0-0.1

.0.1-1

• 1-5• 5-10

• 10-20

• >20limits ofdistribution

... Figure 52.5 Distribution andabundance of the red kangaroo inAustralia. based on aerial surveys.

Climate in northernAustralia is hot and wet.with seasonal drought.

Southern Australia hascool, moist winters andwarm, dry summers.

Red kangaroos occur inmost semi-arid and aridregions of the interior,where precipitation isrelatively tow and variablefrom year to year.

Southeastern Australiahas a wet, cool climate.

CHAPTER fifTY· TWO An Introduction to Ecology and the Biosphere 1151

Why is species Xabsentfrom an area?

I ~ Area inaccessibleor insufficient time

YesDoes disper5al r.. Habitat selection

limit itsDoes behavior

~ Predation, parasitism, Chemicaldistribution? competition, diseaseNo limit its Do biotic factors factors

distribution? No (other spe<ies)• limit its Do abiotic factors

distribution? No limit itsdistribution?

WaterOxygenSalinitypH5011 nutrients, etc.

... Figure 52.6 Flowchart of factors limiting geographic distribution. As ecologistsstudy the factors limiting a species' distribution, they often consider a series of questions like theones shown here,

D How might the importance of various abiotic factors differ for aquatic and terrestrialecosystems)

Physicalfactors

TemperaturelightSoil structureFireMoisture. elc.

the climate ranges from moist to wet. At first glance, this distri­bution might suggest that an abiotic factor-the amount andvariability of precipitation-directly determines where red kan­garoos live. However, it is also possible that climate influencesred kangaroo populations indirectly through biotic factors,such as pathogens, parasites, predators, competitors, and foodavailability. Ecologists generally need to consider multiple fac­tors and alternative hypotheses when attempting to explain the

distribution of species.To see how ecologists might arrive at such an explanation,

let's work our way through the series of questions in the flow­chart in Figure 52.6.

Dispersal and Distribution

The movement of individuals away from their area of origin orfrom centers of high population density, called dispersal, con­

tributes to the global distribution oforganisms. Abiogeographermight consider dispersal in hypothesizing why there are no kan­garoos in North America; Kangaroos could not get there be­cause a barrier to their dispersal existed. While land-boundkangaroos have not reached North America under their ownpower, other organisms that disperse more readily, such as somebirds, have. The dispersal oforganisms is critical to understand­ing both geographic isolation in evolution (see Chapter 24) andthe broad patterns ofcurrent geographic distributions ofspecies.

Natural Range Expansions

The importance of dispersal is most evident when organismsreach an area where they did not exist previously. For instance,200 years ago, the cattle egret was found only in Africa andsouthwestern Europe. But in the late 1800s, some of thesestrong-flying birds managed to cross the Atlantic Ocean andcolonize northeastern South America. From there, cattleegrets gradually spread southward and also northwardthrough Central America and into North America, reaching

1152 U"IT EIG~T Ecology

Florida by 1960(Figure 52.7). Today they have breeding pop­ulations as far west as the Pacific coast of the United Statesand as far north as southern Canada.

Natural range expansions clearly show the influence of dis­persal on distribution, but opportunities to observe such dis­persal directly are rare. As a consequence, ecologists oftenturn to experimental methods to better understand the role ofdispersal in limiting the distribution of species.

... Figure 52.7 Dispersal of the cattle egret in the Americas.Native to Africa, canle egrets were first reported in South America in 1877.

responsible (see Figure 52.6). In many cases, a species cannotcomplete its full life cycle if transplanted to a new area. This in­ability to survive and reproduce may be due to negative inter­actions with other organisms in the form of predation,

parasitism, or competition. Alternatively, survival and repro­

duction may be limited by the absence of other species onwhich the transplanted species depends, such as pollinators formany flowering plants. Predators (organisms that kill theirprey) and herbivores (organisms that eat plants or algae) arecommon examples of biotic factors that limit the distributionof species. Simply put, organisms that eat can limit the distri­bution of organisms that get eaten.

Let's examine one specific case of an herbivore limitingthe distribution of a food species (Figure 52.8). In certainmarine ecosystems, there is often an inverse relationship be­tween the abundance of sea urchins and seaweeds (large ma­rine algae, such as kelp). Where sea urchins that graze on

In ui~~52"

Does feeding by sea urchins limit seaweed distribution?Behavior and Habitat Selection

Species Transplants

To determine if dispersal is a key factor limiting the distribu­tion of a species, ecologists may observe the results of inten­

tional or accidental transplants of the species to areas where itwas previously absent. For a transplant to be considered suc­cessful, some of the organisms must not only survive in thenew area but also reproduce there. If a transplant is success­ful, then we can conclude that the potential range of thespecies is larger than its actual range; in other words, the

species could live in certain areas where it currently does not.Species introduced to new geographic locations often disrupt

the communities and ecosystems to which they have been intro­duced and spread far beyond the area of intended introduction(see Chapter 56). Consequently, ecologists rarely conduct trans­plant experiments across geographic regions. Instead, they docu­ment the outcome when a species has been transplanted for otherpurposes, such as to introduce game ani-mals or predators ofpest species, or when aspecies has been accidentally transplanted.

RESULTS Fletcher observed a large difference in seaweed growth between areaswith and without sea urchins_

Almost noseaweed grewin areas whereboth urchins andlimpets werepresent, or whereonly limpets wereremoved.

Removing bothlimpets andurchins orremoving onlyurchins increasedseaweed coverdramatically.

February1984

August1983

Only limpets removedControl (both urchinsand limpets present)

February1983

100

80

~• 60>0u~••, 40••~

20

0August1982

W. J. Fletcher, of the University of Sydney, Australia, reasoned that if seaurchinS are a limiting biotIC factor. then more seaweeds should Invade an area from whichsea urchins have been removed. To isolate the effect of sea urchins from that of anotherseaweed-eating animal. the limpet. he removed only urchins, only limpets. or both fromstudy areas adjacent to a control site.

CONCLUSION Removing both limpets and urchins resulted in the greatest increase in sea-weed cover, indicating that both species have some influence on seaweed distribution, Butsince removing only urchins greatly increased seaweed growth while removing only limpetshad little effect. Fletcher concluded that sea urchins have a much greater effect than limpetsin limiting seaweed distribution

EXPERIMENTAs transplant experiments show, some or­ganisms do not occupy all oftheir potentialrange, even though they may be physicallyable to disperse into the unoccupied areas.To follow our line of questioning fromFigure 52.6, does behavior playa role inlimiting distribution in such cases? Whenindividuals seem to avoid certain habitats,

even when the habitats are suitable, the or·ganism's distribution may be limited byhabitat selection behavior.

Although habitat selection is one oftheleast understood ofall ecological processes,some instances in insects have beenclosely studied. Female insects often de­posit eggs only in response to a very nar­row set of stimuli, which may restrictdistribution of the insects to certain hostplants. Larvae ofthe European corn borer,for example, can feed on a wide variety ofplants but are found almost exclusively on

corn because egg-laying females are at­tracted by odors produced by the cornplant. Habitat selection behavior clearlyrestricts the plant species on which thecorn borer is found.

SOURCEBiotic Factors W, J. fletcher, Interd(tions among subtidal Australian sea urchins, gastropods, and algaeeffects of experimental removals, fcoIogiciJI Morographs 57:89-109 (1989),

!fbehavior does not limit the distributionof a species, our next question is whetherbiotic factors-that is, other species-are

_Qllf.iIiM Seaweed cover increased the most when both urchins and limpets were re­moved. How might you explain this result?

CHAPTER fifTY· TWO An Introduction to Ecology and the Biosphere 1153

seaweeds and other algae are common, large stands of sea­weeds do not become established. Thus, sea urchins appearto limit the local distribution of seaweeds. This kind of in­teraction can be tested by "removal and addition~ experi­

ments. In studies near Sydney, Australia, WI. J. Fletcher tested

the hypothesis that sea urchins are a biotic factor limitingseaweed distribution. Because there are often other herbi­

vores in the habitats where seaweeds may grow, Fletcher per­formed a series of manipulative field experiments to isolatethe influence of sea urchins on seaweeds in his study area(see Figure 52.8). By removing sea urchins from certain plotsand observing the dramatic increase in seaweed cover, heshowed that urchins limited the distribution of seaweeds.

In addition to predation and herbivory, the presence or ab­sence of food resources, parasites, pathogens, and competingorganisms can act as biotic limitations on species distribution.Some of the most striking cases of limitation occur when hu­mans accidentally or intentionally introduce exotic predators

or pathogens into new areas and wipe out native species. Youwill encounter examples ofthese impacts in Chapter 56, whichdiscusses conservation ecology.

Abiotic Factors

The last question in the flowchart in Figure 52.6 considerswhether abiotic factors, such as temperature, water, salinity,

sunlight, or soil, might be limiting a species' distribution. If thephysical conditions at a site do not allow a species to surviveand reproduce, then the species wm not be found there.Throughout this discussion, keep in mind that the environ­ment is characterized by both spatial heterogeneity andtemporal heterogeneity; that is, most abiotic factors vary inspace and time. Although two regions of Earth may experi­ence different conditions at any given time, daily and annualfluctuations of abiotic factors may either blur or accentuate

regional distinctions. Furthermore, organisms can avoidsome stressful conditions temporarily through behaviors suchas dormancy or hibernation.

Temperature

Environmental temperature is an important factor in the dis­tribution of organisms be<ause of its effect on biological

processes. Cells may rupture ifthe water they contain freezes (attemperatures below (fC), and the proteins of most organismsdenature at temperatures above 45'C. In addition, feworgan­

isms can maintain an active metabolism at very low orvery hightemperatures, though extraordinary adaptations enable someorganisms, such as thermophilic prokaryotes (see Chapter 27),to live outside the temperature range habitable by other life.Most organisms function best within a specific range of envi­ronmental temperature. Temperatures outside that range mayforce some animals to expend energy regulating their internaltemperature, as mammals and birds do (see Chapter 40).

1154 UNIT EIGHT Ecology

Water

The dramatic variation in water availability among habitats isanother important factor in species distribution. Species liv­

ing at the seashore or in tidal wetlands Can desiccate (dry out)as the tide recedes. Terrestrial organisms face a nearly con­stant threat of desiccation, and the distribution of terrestrialspecies reflects their ability to obtain and conserve water.Desert organisms, for example, exhibit a variety of adapta­tions for acquiring and conserving water in dry environments,

as described in Chapter 44.

Salinity

As you learned in Chapter 7, the salt concentration ofwater inthe environment affects the water balance of organisms

through osmosis. Most aquatic organisms are restricted to ei­ther freshwater or saltwater habitats by their limited ability toosmoregulate (see Chapter 44). Although many terrestrial or­ganisms can excrete excess salts from specialized glands or infeces, salt flats and other high-salinity habitats typically havefew species of plants or animals.

Sunlight

Sunlight absorbed by photosynthetic organisms provides theenergy that drives most ecosystems, and too little sunlight canlimit the distribution of photosynthetic species. In forests,shading by leaves in the treetops makes competition for lightespecially intense, particularly for seedlings growing on theforest floor. In aquatic environments, every meter of waterdepth selectively absorbs about 45% of the red light and about2% of the blue light passing through it. As a result, most pho­tosynthesis in aquatic environments occurs relatively near thesurface.

Too much light can also limit the survival oforganisms. The

atmosphere is thinner at higher elevations, absorbing lessultraviolet radiation, so the sun's rays are more likely to damageDNA and proteins in alpine environments (Figure 52,9). In

other ecosystems, such as deserts, high light levels can increasetemperature stress if animals are unable to avoid the light or tocool themselves through evaporation (see Chapter 40).

Rocks and Soil

The pH, mineral composition, and physical structure ofrocks and soil limit the distribution of plants and thus ofthe animals that feed on them, contributing to the patchi­ness of terrestrial ecosystems. The pH of soil and water canlimit the distribution of organisms directly, through ex­treme acidic or basic conditions, or indirectly, through thesolubility of nutrients and toxins. In streams and rivers,the composition of the substrate (bottom surface) can af­fect water chemistry, which in turn influences the residentorganisms. In freshwater and marine environments, the

.. Figure 52.11 The great ocean conveyor belt. Water is warmed at the equator and flows along theocean surface to the North Atlantic, where it cools, becomes denser, and sinks thousands of meters. The deep,cold water may not return to the ocean surface for as long as 1,000 years.

Climate

o )'

water ..

Bodies of Water Ocean currents influence climate alongthe coasts of continents by heating or cooling overlying airmasses, which may then pass across the land. Coastal regionsare also generally moister than inland areas at the same lati­

tude. The cool, misty climate produced by the cold CaJiforniacurrent that flows southward along the western United Statessupports a coniferous rain forest ecosystem in the PacificNorthwest and large redwood groves farther south. Similarly,the west coast of northern Europe has a mild climate becausethe Gulf Stream carries warm water from the equator to theNorth Atlantic, driven in part by the "great ocean conveyorbelt" (Figure 52.11). As a result, northwest Europe is warmerduring winter than New England, which is farther south but iscooled by the Labrador Current flowing south from the coastof Greenland.

Because of the high specific heat ofwater (see Chapter 3),

oceans and large lakes tend to moderate the climate ofnearby land. Duringa hot day, when the land is warmer thanthe nearby body of water, air over the land heats up and

The sun's warming effect on the atmosphere, land, and water es­tablishes the temperature variations, cycles of air movement,

and evaporation of water that are responsible for dramatic lati­tudinaJ variations in climate. Figure 52.10, on the next WiO

pages, summarizes Earth's climate patterns and how they are

formed.

Regional, Local, and Seasonal Effects on Climate

Proximity to bodies of water and topographic features suchas mountain ranges create regional climatic variations, andsmaller features of the landscape contribute to local climaticvariation. Seasonal variation is another influence on climate.

•

Gulfstream

Labradorcurrent

Earth's global climate patternsare determined largely by the in­put of solar energy and theplanet's movement in space.

Four abiotic factors-temperature, precipitation, sunlight, andwind-are the major components of climate, the long-term,prevailing weather conditions in a particular area. Climatic fac­tors, particularly temperatureand water availability, have amajor influence on the distribu­tion ofterrestrial organisms. Wecan describe climate patterns ontwo scaJes: macroclimate, pat­terns on the global, regional, andlocal level; and microclimate,

very fine patterns, such as thoseencountered by the communityof organisms that live beneatha fallen log. First let's considerEarth's macroclimate.

Global Climate Patterns

structure of the substrate determines the organisms thatcan attach to it or burrow into it.

Now that we have surveyed some ofthe abiotic factors thataffect the distribution of organisms, let's focus on how thosefactors vary with climate, as we consider the major role thatclimate plays in determining species distribution.

.. Figure 52.9 Alpine tree. Organisms living at high elevations areexposed to high levels of ultraviolet radiation They face otherchallenges as well. including freezing temperatures and strong winds,which increase water loss and inhibit the growth of limbs on thewindward side of trees.

(HfJ,PTER fifTY· TWO An lntrodu(tion to Ecology and the Biosphere 1155

• FIguro 52.10

Exploring Global Climate Patterns

Latitudinal Variation in Sunlight Intensity

Earth's curved shape causes latitudinalvariation in the intensity of sunlight.Because sunlight strikes the tropics(those regions that lie between 23.5"north latitude and 23.5" south latitude)most directly, more heat and light perunit of surface area are delivered there.At higher latitudes, sunlight strikesEarth at an oblique angle, and thus thelight energy is more diffuse on Earth'ssurface.

low angle of lncomll19 S1Jnlight

Seasonal Variation in Sunlight Intensity

December solstice: NorthernHemisphere tilts iIWay from sunand has shortest day and longestnight; Southern HemISPhere liltstoward sun and has longest dayand shortest night.

March equinox: Equator faces sun dIrectly:neither pole tilts toward sun; all regions on Earthe~penence 12 hours of daylight and 12 hours ofdarkness.

'!II.lb.~10° (equator)

~o.sJune solstice: NorthernHemisphere lilts toward sunand has longest day andshortest night; SouthernHemisphere tilts away fromsun and has shortest dayand longest night.

September equinox: Equator faces sundirectly; neither pole tilts toward sun; allre(jlons on Earth eXpener1ce 12 hours ofdaylight and 12 hours of darkness.

Earth's tilt causes seasonal variation in the intensity of solar radiation. Because the planet is tiltedon its axis by 23S relative to its plane oforbit around the sun, the tropics experience the greatestannual input of solar rndiation and the least seasonal variation. The seasonal variations of light andtemperature increase toward the poles.

1156 UNIT UGHT Ecology

Global Air Circulation and Precipitation Patterns

300 NDescending Descendingdry air dry air

0° (equator) absorbs Ascending absorbsmoisture moist air moisture

releases30°5 moisture

23Y 0' 23.5° 30030"

Arid Tropics Aridzone zone

Intense solar radiation near the equator initiates a global pattern of air circulation and

precipitation. High temperatures in the tropics evaporate water from Earth's surface and cause

warm, wet air masses to rise (blue arrows) and flow toward the poles. The rising air masses

release much of their water content, creating abundant precipitation in tropical regions. The

high-altitude air masses, now dry, descend (brown arrows) toward Earth, absorbing moisture

from the land and creating an arid climate conducive to the development of the deserts that are

common at latitudes around 30' north and south. Some of the descending air then flows toward

the poles. At latitudes around 60' north and south, the air masses again rise and release abundant

precipitation (though less than in the tropics). Some of the cold, dry rising air then flows to the

poles, where it descends and flows back toward the equator, absorbing moisture and creating the

comparatively rainless and bitterly cold climates of the polar regions.

Global Wind Patterns

Westerlies. ~-"",,,,,'- ..//

Air flowing close to Earth's surface

creates predictable global wind patterns.

As Earth rotates on its axis, land near

the equator moves faster than that at

the poles, deflecting the winds from the

vertical paths shown above and creating

more easterly and westerly flows.

Cooling trade winds blow from east to

west in the tropics; prevailing westerlies

blow from west to east in the temperate

zones, defined as the regions between

the Tropic of Cancer and the Arctic

Circle and between the Tropic of

Capricorn and the Antarctic Circle.

C~"'PH~ fIHY·TWO An Introduction to Ecology and the Biosphere 1157

.4 Figure 52.12 Moderating effects of a large body of wateron climate. This figure illustrates what happens on a hot summer day_

(--------=;.;?--J

Many features in the environment influence microclimates bycasting shade, affecting evaporation from soil, or changingwind patterns. For example, forest trees frequently moderatethe microclimate below them. Consequently, cleared areasgenerally experience greater temperature extremes than the

Microclimate

When warm, moist air approaches a mountain, the air risesand cools, releasing moisture on the windward side ofthe peak(Figure 52.13). On the leeward side, cooler, dry air descends,absorbing moisture and prodUcing a "rain shadow," Deserts

commonly occur on the leeward side of mountain ranges, aphenomenon evident in the Great Basin and the MojaveDesert of western North America, the Gobi Desert of Asia,

and the small deserts found in the southwest corners of someCaribbean islands.

Seasonality As described earlier, Earth's tilted axis of rota­tion and its annual passage around the sun cause strong sea­sonal cycles in middle to high latitudes (see Figure 52.10). Inaddition to these global changes in day length, solar radia­tion, and temperature, the changing angle ofthe sun over thecourse of the year affects local environments. For example,

the belts ofwet and dry air on either side ofthe equator moveslightly northward and southward with the changing angle ofthe sun, producing marked wet and dry seasons around 20'north and 20· south latitude, where many tropical deciduous

forests grow. In addition, seasonal changes in wind patternsproduce variations in ocean currents, sometimes causing theupwelling of cold water from deep ocean layers. This nutri­ent-rich water stimulates the growth of surface-dwellingphytoplankton and the organisms that feed on them.

OWarmairover land rises.

e Air cools athigh elevation.

o Cool air over watermoves inland, replacingrising warm air over land.

o Coolerair sinksover water.

rises, drawing a cool breeze from the water across the land(Figure 52,12), At night, air over the now warmer waterrises, drawing cooler air from the land back out over the wa­

ter, replacing it with warmer air from offshore. The modera­tion of climate may be limited to the coast itself, however. Incertain regions, such as southern California, cool, dry oceanbreezes in summer are warmed when they contact the land,absorbing moisture and creating a hot, rainless climate just afew miles inland (see Figure 3.5). This climate pattern also oc­

curs around the Mediterranean Sea, which gives it the nameMediterranean climate.

Mountains Mountains affect the amountof sunlight reaching an area and conse­quently the local temperature and rainfall.South-facing slopes in the Northern Hemi­sphere receive more sunlight than nearbynorth-facing slopes and are thereforewarmer and drier. These abiotic differencesinfluence species distribution; for example,in many mountains of western NorthAmerica, spruce and other conifers occupy

the cooler north-facing slopes, whereasshrubby, drought-resistant plants inhabitthe south-facing slopes. In addition, everyl,{X)}.m increase in elevation produces a

temperature drop of approximately 6·C,equivalent to that produced by an 88O-kmincrease in latitude. This is one reason thebiological communities of mountains aresimilar to those at lower elevations but far­ther from the equator.

o As moist air moves in offthe ocean and encountersmountains, it flows upward, coolsat higher altitudes, and drops alarge amount of water as precipitation.

Ocean

.4 Figure 52.13 How mountains affect rainfall.

e On the leeward side ofthe mountains, there is littleprecipitation. As a result ofthis rain shadow, a desert isoften present.1-

Leeward sideof mountain

Mountainrange

1158 U"IT EIG~T Ecology

forest interior because ofgreater solar radiation and wind cur­rents that are established by the rapid heating and cooling ofopen land. Within a forest, low·lying ground is usually wetterthan high ground and tends to be occupied by differentspecies of trees. A log or large stone can shelter organismssuch as salamanders, worms, and insects, buffering themfrom the extremes of temperature and moisture. Everyenvi­ronment on Earth is similarly characterized by a mosaic ofsmall-scale differences in the abiotic factors that influence thelocal distributions oforganisms.

• Currentrange

D Predictedrange

o Overlap

Long-Term Climate Change(a)4,SOC warming over

next century(b)6'soC warming over

next century

sistance in moving into new ranges where they can sur­vive as the climate warms, species such as the Americanbeech may have much smaller ranges and may even be­come extinct.

... Figure 52.14 Current range and predicted range for theAmerican beech (Fagus grandifolia) under two scenariosof climate change.

D The predicted range in each scenario is based on climate factorsalone. Whal other factors might alter the distribution of this species?

rz~~:~r;b~:~~s are diverseand dynamic systems thatcover most of Earth

We have seen how both biotic and abiotic factors influence thedistribution of organisms on Earth, Combinations ofthese fac­tors determine the nature of Earth's many biomes, major ter­restrial or aquatic life zones, characterized by vegetation type interrestrial biomes or the physical environment in aquatic bi­omes. Well begin by examining Earth's aquatic biomes.

Aquatic biomes account for the largest part of the bio­sphere in terms of area, and all types are found around the

52.2CONCEPT CHECK

I, Give examples of human actions that could expand aspecies' distribution by changing its (a) dispersal or(b) biotic interactions.

2. Explain how the sun's unequal heating of Earth's sur­face influences global climate patterns.

3. _','!ifni. You suspect that deer are restricting thedistribution of a tree species by preferentially eatingthe seedlings of the tree. How might you test thathypothesis?

For suggested answers, see AppendiK A.

If temperature and moisture are the most important factorslimiting the geographic ranges of plants and animals, then theglobal climate change currently under way will profoundly af­fect the biosphere (see Chapter 55). One way to predict thepossible effects of climate change is to look back at thechanges that have occurred in temperate regions since the lastice age ended.

Until about 16,000 years ago, continental glaciers cov­ered much of North America and Eurasia. As the climatewarmed and the glaciers retreated, tree distributions ex­panded northward. A detailed record of these migrationsis captured in fossil pollen deposited in lakes and ponds. (Itmay seem odd to think of trees "migrating;' but recall fromChapter 38 that wind and animals can disperse seeds,sometimes over great distances.) 1£ researchers can deter­mine the climatic limits of current geographic distribu­tions for organisms, they can make predictions about howdistributions will change with climatic warming. A majorquestion when applying this approach to plants is whetherseed dispersal is rapid enough to sustain the migration ofeach species as climate changes. For example, fossils sug­gest that the eastern hemlock was delayed nearly 2,500years in its movement north at the end of the last ice age.This delay in seed dispersal was partly attributable to thelack of"wings n on the seeds, causing the seeds to fall closeto their parent tree.

Let's look at a specific case of how the fossil record ofpast tree migrations can inform predictions about the bi­ological impact of the current global warming trend.Figure 52.14 shows the current and predicted geographicranges of the American beech (Fagus grandifolia) undertwo different climate-change models. These models pre­dict that the northern limit of the beech's range will move700-900 km northward in the next century, and its south­ern range limit will move northward an even greater dis­tance. If these predictions are even approximately correct,the beech must move 7-9 km per year northward to keeppace with the warming climate. However, since the end ofthe last ice age, the beech has migrated into its presentrange at a rate ofonly 0.2 km per year. Without human as-

CHAPTER fifTY· TWO An Introduction to Ecology and the Biosphere 1159

globe (Figure 52.15), Ecologists distinguish between fresh·

water biomes and marine biomes on the basis of physical andchemical differences. For example, marine biomes generallyhave salt concentrations that average 3%, whereas freshwater

biomes are usually characterized by a salt concentration oflessthan 0.1%.

The oceans make up the largest marine biome, coveringabout 75% of Earth's surface. Because of their vast size, theyhave an enormous impact on the biosphere, The evaporationof water from the oceans provides most of the planet's rain­

fall, and ocean temperatures have a major effect on world cli­

mate and wind patterns. In addition, marine algae and

k--30'N

Tropic ofCancer

f--7,-Equator---{<;,%,-r.,,,-----i!.. , Tropic 01

____~ S.a[lric.?~n _

\---30'5-------1

• lakes

• Coral reefs

/"' Rivers

Oceanic pelagicand benthic zones

• Estuaries

Intertidal zones

• •

... Figure 52.15 The distribution of major aquatic biomes.

Intertidal zone

Pelagizone

Aph,'" jzone

•Oceanic zone

• ••

Continentalshelf

Photic zone200 m----cl-'\"~-'<~""::::::.=cn

2,000-6,000 m------;~:::;~:J.:::bAbyssal zone

(deepest regions of ocean floor)

Pelagiczone

Aphoticzone

Photiczone

(a) Zonation in a lake. The lake environment IS generally classifiedon the basis of three physical CrIteria: light penetration (photicand aphotic zones), distance from shore and water depth (littoraland limnetic zones), and whether it is open water (pelagic zone)or bottom (benthic zone),

(b) Marine zonation. like lakes. the marine environment isgenerally claSSIfied on the basis of light penetration (photicand aphotic zones), distance Irom shore and water depth(intertidal, neritic, and oceanic zones), and whether it is openwater (pelagic zone) or bottom (benthic and abyssal zones).

... Figure 52.16 Zonation in aquatic environments.

1160 U"IT EIG~T Ecology

photosynthetic bacteria supply a substantial portion of theworld's oxygen and consume large amounts of atmosphericcarbon dioxide.

Freshwater biomes are closely linked to the soils and biotic

components of the terrestrial biomes through which they

pass or in which they are situated. The particular characteris­tics of a freshwater biome are also influenced by the patternsand speed of water flow and the climate to which the biomeis exposed.

Winter

,.,.,.

o In winter. the coldestwater In the lake (O"()lies Just below thesurface ice; water isprogressively warmer atdeeper levels of thelake, typically 4"( at thebottom.

... Figure S2.17 Seasonal turnover in lakes with winter icecover. Because of the seasonal turnover shown here, lake waters arewell oxygenated at all depths in spring and autumn; in winter andsummer, when the lake IS stratified by temperature. oxygenconcentrations are lower in deeper waters and higher near the surfaceof the lake.

ocean is so deep, most of the ocean volume is virtually devoidof light (the aphotic zone) and harbors relatively little life, ex­cept for microorganisms and relatively sparse populations offishes and invertebrates. Similar factors limit species distribu­tion in deep lakes as well.

Figure 52.18, on the next four pages, surveys the majoraquatic biomes.

f) In spring. as the sunmelts the ice. the surfacewater warms to 4"( andsinks below the coolerlayers immediately below,eliminating the thermalstratification. Spring windsmix the water to greatdepth, bringing oxygento the bottom waters andnutrients to the surface.

f) In summer, the lakeregains adistindivethermal profile, withwarm surface waterseparated from coldbottom water by anarrow vertical zone ofabrupt temperaturechange, called athermocline.

o In autumn, as surfacewater cools rapidly, itSinks below theunderlying layers.remixing the water untilthe surface begins tofreeze and the wintertemperature profile isreestablished.

,

() ,f'

() ,./'

Spring

Summer

Autumn

Thermocline

Many aquatic biomes are physically and chemically stratified(layered), as illustrated for both a lake and a marine environ­ment in Figure 52.16, on the facing page. Light is absorbed byboth the water itself and the photosynthetic organisms in it,

so its intensity decreases rapidly with depth, as mentionedearlier. Ecologists distinguish between the upper photic zone,where there is sufficient light for photosynthesis, and thelower aphotic zone, where little light penetrates. At the bot­tom of all aquatic biomes, the substrate is called the benthiczone. Made up of sand and organic and inorganic sediments,the benthic zone is occupied by communities of organismscollectively called the benthos. A major source of food formany benthic species is dead organic matter called detritus,which "rains" down from the productive surface waters of thephotic zone. In the ocean, the part of the benthic zone that liesbetween 2,000 and 6,000 m below the surface is known as the

abyssal zone.Thermal energy from sunlight warms surface waters to

whatever depth the sunlight penetrates, but the deeper wa­ters remain quite cold. In the ocean and in most lakes, a nar­

row layer ofabrupt temperature change called a thermoclineseparates the more uniformly warm upper layer from moreuniformly cold deeper waters. Lakes tend to be particu­larly layered with respect to temperature, especially duringsummer and winter, but many temperate lakes undergo asemiannual mixing of their waters as a result of chang­ing temperature profiles (Figure 52.17). This turnover, asit is called, brings oxygenated water from a lake's surfaceto the bottom and nutrienHich water from the bottom tothe surface in both spring and autumn. These cyclicchanges in the abiotic properties of lakes are essential for

the survival and growth of organisms at all levels withinthis ecosystem.

In both freshwater and marine environments, communi­

ties are distributed according to water depth, degree of lightpenetration, distance from shore, and whether they arefound in open water or near the bottom. Marine communi­ties, in particular, illustrate the limitations on species distribu­tion that result from these abiotic factors. Plankton and manyfish species occur in the relatively shallow photic zone (seeFigure 52.16b). Because water absorbs light so well and the

Stratification of Aquatic Biomes

CHAPTER fifTY· TWO An Introduction to Ecology and the Biosphere 1161

• Figure 52.18

•• • Aquatic Biomes

Lakes

Physical Environment Standing bodies ofwater range from ponds afew square meters in area to lakes covering thousands of square kilometers.Light decreases with depth, creating stratification (see Figure 52.16<1).Temperate lakes may have a seasonal thermocline (see Figure 52.17); tropicallowland lakes have a themlocline year-round.

An oligotrophic lake in Grand TetonNational Park, Wyoming

A eutrophIC lake In the OkavangoDelta, Botswana

Chemical Environment The salinity, oxygenconcentration, and nutrient content differ greatly amonglakes and can vary with season. Oligotrophic lakes arenutrient-poor and generally oxygen-rich: eutrophi<:lakes are nutrient-rich and often depleted of oxygen inthe deepest zone in summer and if ice covered in winter.The amount of decomposable organic matter in bottomsediments is low in oligotrophic lakes and high ineutrophic lakes; high rates of decomposition in deeperlayers of eutrophic lakes cause periodic oxygen depletion.

Geologic Features Oligotrophic lakes may becomemore eutrophic over time as runoff adds sediments andnutrients. They tend to have less surface area relative totheir depth than eutrophic lakes have.

Photosynthetic Organisms Rooted and floatingaquatic plants live in the littoral zone, the shallow, well­lighted waters close to shore. Farther from shore, wherewater is too deep to support rooted aquatic plants, thelimnetic zone is inhabited by a variety of phytoplanktonand cyanobacteria.

Heterotrophs In the limnetic zone, small driftingheterotrophs, or zooplankton, graze on the phytoplankton.The benthic zone is inhabited by assorted invertebrateswhose species composition depends partly on oxygenlevels. Fishes live in all zones with sufficient oxygen.

Human Impact Runofffrom fertilized land anddumping of wastes leads to nutrient enrichment, whichcan produce algal blooms, oxygen depletion, and fish kills.

Ph)'5ical Environment A wetland is a habitat that is inundated bywater at least some of the time and that supports plants adapted towater-saturated soil. Some wetlands are inundated at all times,whereas others flood infrequently.

cnemical Environment Becauseofhigh organic production by plantsand decomposition by microbes and other organisms, both the water andthe soils are periodically low in dissolved oxygen. Wetlands have a highcapacity to filter dissolved nutrients and chemical pollutants.

Geologic Features Basin wetlands develop in shallow basins, rangingfrom upland depressions to filled-in lakes and ponds. Riverine wetlandsdevelop along shallow and periodically flooded banks of rivers andstreams. Fringe wetlands occur along the coasts of large lakes and seas,where water flows back and forth because of rising lake levels or tidalaction. Thus, fringe wetlands include both freshwater and marine biomes.

Photosynthetic Organisms Wetlands are among the mostproductive biomes on Earth. Their water-saturated soils favor thegrowth of plants such as floating pond lilies and emergent cattails,many sedges, tamarack, and black spruce, which have adaptationsenabling them to grow in water or in soil that is periodically anaerobicowing to the presence of unaerated water. Woody plants dominate thevegetation of swamps, while bogs are dominated by sphagnum mosses.

Heterotrophs Wetlands are home to a diverse community ofinvertebrates, which in tum support a wide variety ofbirds. Herbivores,

1162 U"IT EIGHT Ecology

Okefenokee National Wetland Reserve in Georgia

from crustaceans and aquatic insect larvae to muskrats, consumealgae, detritus, and plants. Carnivores are also varied and mayinclude dragonflies, otters, alligators, and owls.

Human Impact Draining and filling have destroyed up to 90% ofwetlands, which help purify water and reduce peak flooding.

Streams and Rivers

Human Impact Municipal, agricultural, and industrial pollutiondegrade water quality and kill aquatic organisms. Damming andflood control impair the natural functioning of stream and rivere<:osystems and threaten migratory species such as salmon.

Physical Environment The most prominent physicalcharacteristic of streams and rivers is their current. Headwaterstreams are generally cold, clear, turbulent, and swift. Fartherdownstream, where numerous tributaries may have joined,forming a river, the water is generally warmer and more turbidbecause of suspended sediment. Streams and rivers arestratified into vertical zones.

Chemical Environment The salt and nutrient content ofstreams and rivers increases from the headwaters to the mouth.Headwaters are generally rich in oxygen. Downstream watermay also contain substantial oxygen, except where there hasbeen organic enrichment. A large fraction of the organic matterin rivers consists of dissolved or highly fragmented materialthat is carried by the current from forested streams.

Geologic Features Headwater stream channels are oftennarrow, have a rocky bottom, and alternate between shallowsections and deeper pools. The downstream stretches of riversare generally wide and meandering. River bottoms are often siltyfrom sediments deposited over long periods of time.

Photosynthetic Organisms Headwater streams that flowthrough grasslands or deserts may be rich in phytoplankton orrooted aquatic plants.

Heterotrophs A great diversity of fishes and invertebratesinhabit unpolluted rivers and streams, distributed according to,and throughout, the vertical wnes. In streams flowing throughtemperate or tropical forests, organic matter from terrestrialvegetation is the primary source of food for aquatic consumers.

A headwater stream in the GreatSmoky Mountains

The Mississippi River far from itsheadwaters

Human Impact Pollution from upstream, and also filling anddredging, have disrupted estuaries worldwide.

Continued on next page

Physical Environment An estuary is a transition area betweenriver and sea. Seawater flows up the estuary channel during arising tide and flows back down during the falling tide. Often,higher-density seawater occupies the bottom of the channel andmixes little with the lower-density river water at the surface.

Chemical Environment Salinity varies spatially within estuaries,from nearly that of fresh water to that of seawater. Salinity alsovaries with the rise and fall of the tides. Nutrients from the rivermake estuaries, like wetlands, among the most productive biomes.

Geologic Features Estuarine flow patterns combined with thesediments carried by river and tidal waters create a complexnetwork of tidal channels, islands, natural levees, and mudflats.

Photosynthetic Organisms Saltmarsh grasses and algae,including phytoplankton, are the major producers in estuaries.

Heterotrophs Estuaries support an abundance of worms,oysters, crabs, and many fish spe<:ies that humans consume, Manymarine invertebrates and fishes use estuaries as a breeding groundor migrate through them to freshwater habitats upstream, Estuariesare also crucial feeding areas for waterfowl and some marinemammals.

...

An estuary in a low coastal plain 01 Georgia

--

C~"'PH~ flfTY·rwo An Introduction to Ecology and the Biosphere 1163

• Figure 52.18 (continued)

•• • Aquatic Biomes

Rocky intertidal zone on the Oregon coast

Ph)'5ical Environment An intertidal zone is periodicallysubmerged and exposed by the tides, twice daily on most marineshores. Upper zones experience longer exposures to air andgreater variations in temperature and salinity. Changes inphysical conditions from the upper to the lower intertidal zoneslimit the distributions of many organisms to particular strata, asshown in the photograph.

Chemical Environment Oxygen and nutrient levels aregenerally high and are renewed with each turn of the tides.

Geologic Features The substrates of intertidal zones,which are generally either rocky or sandy, select forparticular behavior and anatomy among intertidalorganisms. The configuration of bays or coastlinesinfluences the magnitude of tides and the relative exposureof intertidal organisms to wave action.

Pnotosyntnetic Organisms A high diversity and biomassof attached marine algae inhabit rocky intertidal zones,especially in the lower zone. Sandy intertidal zones exposedto vigorous wave action generally lack attached plants oralgae, while sandy intertidal zones in protected bays orlagoons often support rich beds of sea grass and algae.

Heterotropns Many of the animals in rocky intertidalenvironments have structural adaptations that enable them toattach to the hard substrate. The composition, density, anddiversity of animals change markedly from the upper to thelower intertidal wnes. Many of the animals in sandy or muddyintertidal zones, such as worms, clams, and predatorycrustaceans, bury themselves and feed as the tides bringsources of food. Other common animals are sponges, seaanemones, echinoderms, and small fishes.

Human Impact Oil pollution has disrupted many intertidalareas.

Oceanic Pelagic Zone

Open ocean off the island of Hawaii

Physical Environment The oceanic pelagic zone is a vastrealm of open blue water, constantly mixed by wind-drivenoceanic currents. Because of higher water clarity, the photic zoneextends to greater depths than in coastal marine waters.

Chemical Environment Oxygen levels are generally high.Nutrient concentrations are generally lower than in coastal waters.Because they are thermally stratified year-round, some tropicalareas of the oceanic pelagic lOne have lower nutrientconcentrations than temperate oceans. Turnoverbetween fall and spring renews nutrients in the photiczones of temperate and high-latitude ocean areas.

Geologic Features This biome covers approximately70% of Earth's surface and has an average depth ofnearly 4,000 m. The deepest point in the ocean is morethan 10,000 m beneath the surface.

Photosynthetic Organisms The dominantphotosynthetic organisms are phytoplankton,including photosynthetic bacteria, that drift with theoceanic currents. Spring turnover and renewal ofnutrients in temperate oceans produces a surge ofphytoplankton growth. Because of the large extentof this biome, photosynthetic plankton account forabout half of the photosynthetic activity on Earth.

Heterotrophs The most abundant heterotrophs in this biomeare zooplankton. These protists, worms, copepods, shrimp-likekrill, jellies, and the small larvae of invertebrates and fishes grazeon photosynthetic plankton. The oceanic pelagic zone alsoincludes free-swimming animals, such as large squids, fishes, seaturtles, and marine mammals.

Human Impact Overfishing has depleted fish stocks in allEarth's oceans, which have also been polluted by waste dumping.

1164 U"IT EIGHT Ecology

Physical Environment Coral reefs are formed largely fromthe calcium carbonate skeletons of corals. Shallow reef-buildingcorals live in the photic zone of relatively stable tropicalmarine environments with high water clarity, primarily onislands and along the edge of some continents. They aresensitive to temperatures below about 18-20'C and above30·C. Deep-sea coral reefs, found between 200 and 1,500 mdeep, are less known than their shallow counterparts butharbor as much diversity as many shallow reefs do.

Chemical Environment Corals require high oxygen levels andare excluded by high inputs of fresh water and nutrients.

Geologic Filatures Corals require a solid substrate forattachment. A typiCll coral reefbegins as afringing reefon a young,high island, forming an offshore harrier reef later in the history ofthe island and becoming a coral atoll as the older island submerges.

Photosynthetic Organisms Unicellular algae live within thetissues of the corals, forming a mutualistic relationship thatprovides the corals with organic molecules. Diversemulticellular red and green algae growing on the reef alsocontribute substantial amounts of photosynthesis.

Heterotrophs Corals, a diverse group of cnidarians (seeChapter 33), are themselves the predominant animals on coral reefs.However, fish and invertebrate diversity is exceptionally high.Overall animal diversity on coral reefs rivals that of tropical forests.

Acoral reef in the Red Sea

Human Impact Collecting of coral skeletons and overfishinghave reduced populations of corals and reef fishes. Globalwarming and pollution may be contributing to large-scale coraldeath. Development of coastal mangroves for aquaculture hasalso reduced spawning grounds for many species of reef fishes.

Marine Benthic Zone

Physical Environment The marine benUtic lOne consists oftheseafloor below the surface waters ofthe coastal, or neritic, zone andthe offshore, pelagic zone (see Figure 52.16b). Except for shallow,near-coastal areas, the marine benthic zone receives no sunlight.Water temperature declines with depth, while pressure increases. As aresult, organisms in the verydecp benthic, or abyssal, zone areadapted to continuous cold {about 3'C) and very high water pressure.

Chemical Environment Except in some areas of organicenrichment, oxygen is present at sufficient concentrations tosupport a diversity of animals.

Adeep-sea hydrothermal vent community

Geologic Features Soft sediments cover most of the benthiclone. However, there are areas of rocky substrate on reefs,submarine mountains, and new oceanic crust.

Autotrophs Photosynthetic organisms, mainly seaweeds andfilamentous algae, are limited to shallow benthic areas with sufficientlight to support them. Unique assemblages oforganisms, such asthose shown in the photo, are found near deep-sea hydrothennalvents on mid-ocean ridges. In these dark, hot environments, the foodproducers are chemoautotrophic prokaryotes {see Chapter 27) thatobtain energy by oxidizingH~ formed by a reaction ofthe hot water

with dissolved sulfate (50/-).

Heterotrophs Neritic benthic communities includenumerous invertebrates and fishes. Beyond the photiczone, most consumers depend entirely on organicmatter raining down from above. Among the animals ofthe deep-sea hydrothermal vent communities are gianttube worms (pictured at left), some more than I m long.They are nourished by chemoautotrophic prokaryotesthat live as symbionts within their bodies. Many otherinvertebrates, including arthropods and echinoderms,are also abundant around the hydrothermal vents.

Human Impact Overfishing has decimated importantbenthic fish populations, such as the cod of the GrandBanks off Newfoundland. Dumping of organic wastes hascreated oxygen-deprived benthic areas.

C~"'PH~ fIHY·TWO An Introduction to Ecology and the Biosphere 1165

r;~::~;:c~~~~nd distribution ofterrestrial biomes are controlledby climate and disturbance

All the abiotic factors discussed in this chapter, but especiallyclimate, are important in determining why a particular terres­trial biome is found in a certain area. Because there are latitu­dinal patterns ofclimate over Earth's surface (see Figure 52.10),there are also latitudinal patterns of biome distribution(Figure 52.19). These biome patterns in turn are modified bydisturbance, an event (such as a storm, fire, or human activ-

The first two questions refer to Figure 52.18.1. Many organisms living in estuaries experience fresh­

and saltwater conditions each day with the rising andfalling of tides. What challenge does this pose for thephysiology of the organisms?

2. Why are phytoplankton, and not benthic algae orrooted aquatic plants, the dominant photosyntheticorganisms of the oceanic pelagic zone?

3. -'*,.)114 Water leaving a reservoir behind a dam

is often taken from deep layers of the reservoir.Would you expect fish found in a river below a dam insummer to be species that prefer colder or warmerwater than fish found in an undammed river? Explain.

For suggested answers. see Appendix A

CONCEPT CHECI( 52.3 ity) that changes a community, removing organisms from itand altering resource availability. Frequent fires, for instance,can kill woody plants and keep a savanna from becoming thewoodland that climate alone would otherwise support.

Climate and Terrestrial Biomes

We can see the great impact of climate on the distribution oforganisms by constructing a climograph, a plot of the tem­perature and precipitation in a particular region. For example,Figure 52.20 is a dimograph of annual mean temperatureand precipitation for some of the biomes found in NorthAmerica. Notice that the range of precipitation in northernconiferous forests is similar to that in temperate forests, butthe temperature ranges are different. Grasslands are generallydrier than either kind of forest, and deserts are drier still.

Factors other than mean temperature and precipitationalso playa role in determining where biomes exist. For exam­ple, certain areas in North America with a particular combi­nation of temperature and precipitation support a temperatebroadleaf forest, but other areas with similar values for these

variables support a coniferous forest. How do we explain thisvariation? First, remember that the climograph is based on an­nual averages. Often, however, the pattern of climatic varia­tion is as important as the average climate. For example, someareas may receive regular precipitation throughout the year,whereas other areas with the same annual precipitation havedistinct wet and dry seasons. A similar phenomenon may oc­

cur with respect to temperature. Other environmental char­acteristics, such as the type of bedrock in an area, may greatlyaffect mineral nutrient availability and soil structure, which inturn affect the kind of vegetation that can grow.

".....

Tropic of_______ ~~e~c9~ _

\--30'5-----iI\/ ;--------\:';t?---=-----i:7'\)~'l-_:i

--- .. r-----.---• Tropic of

Cancer

!----Equator----{f

.. Figure 52.19 The distribution of major terrestrial biomes. Although biomes are mappedhere with >harp boundaries. biomes actually grade into one another, sometimes over large areas

1166 U"IT EIG~T Ecology

General Features ofTerrestrial Biomesand the Role of Disturbance

... Figure 52.20 A c1imograph for some major types ofbiomes in North America. The areas plotted here encompass therange of annual mean temperature and precipitation in the biomes.

CONCEPT CHECK 52.4I. Based on the climograph in Figure 52.20, what mainly

differentiates dry tundra and deserts?2. Identify the natural biome in which you Jive and sum­

marize its abiotic and biotic characteristics. Do thesereflect your actual surroundings? Explain.

3. -','IlfUI• If global warming increases averagetemperatures on Earth by 4'C in this century, predictwhich biome is most likely to replace tundra in somelocations as a result. Explain your answer.

For suggested answers, see Appendix A.

ous forest (taiga) of North America, red spruce is common inthe east but does not occur in most other areas, where blackspruce and white spruce are abundant. In an example of con·vergent evolution (see Figure 26.7), cacti living in North Amer­

ican deserts appear very similar to plants called euphorbs

found in African deserts, although cacti and euphorbs belongto different evolutionary lineages.

Biomes are dynamic, and disturbance rather than stabil­ity tends to be the rule. For example, hurricanes createopenings for new species in tropical and temperate forests.In northern coniferous forests, gaps are produced when oldtrees die and fall over or when snowfall breaks branches.These gaps allow deciduous species, such as aspen andbirch, to grow. As a result, biomes usually exhibit extensivepatchiness, with several different communities representedin any particular area.

In many biomes, the dominant plants depend on periodicdisturbance. For example, natural wildfires are an integral

component of grasslands, savannas, chaparral, and manyconiferous forests. However, fires are no longer common

across much of the Great Plains because tallgrass prairieecosystems have been converted to agricultural fields thatrarely burn. Before agricultural and urban development,much of the southeastern United States was dominated by asingle conifer species, the longleaf pine. Without periodicburning, broadleaf trees tended to replace the pines. Forestmanagers now use fire as a tool to help maintain manyconiferous forests.

Figure 52.21, on the next four pages, summarizes the maojor features of terrestrial biomes. As you read about the char­acteristics ofeach biome, remember that humans have alteredmuch of Earth's surface, replacing original biomes with urban

and agricultural ones. Most of the eastern United States, forexample, is classified as temperate broadleafforest, but little ofthat original forest remains.

Throughout this chapter, you have seen how the distribu­tions of organisms and biomes depend on both abiotic and bi­otic factors. In the next chapter, we will begin to work our waydown the hierarchy outlined in Figure 52.2, focusing on howabiotic and biotic factors influence the ecology of populations.

Tropical forestTemperate grassland

Annual mean precipitation (em)

Temperate Rbroadleafforest

Northern IIconiferousforest

Arctic and

~alpinetundra

100 200 300 400

Desert

30

2•,,• IS~

E•c~E 0~c~

Most terrestrial biomes are named for major physical or cli­matic features and for their predominant vegetation. Temper­ate grasslands, for instance, are generally found in middlelatitudes, where the climate is more moderate than in thetropics or polar regions, and are dominated by various grass

species (see Figure 52.19). Each biome is also characterized bymicroorganisms, fungi, and animals adapted to that particu­lar environment. For example, temperate grasslands are morelikely than forests to be populated by large grazing mammals.

Although Figure 52.19 shows distinct boundaries betweenthe biomes, in actuality, terrestrial biomes usually grade intoeach other without sharp boundaries. The area of intergrada­tion, called an ecotone, may be wide or narrow.

Vertical layering isan important feature ofterrestrial biomes,and the shapes and sizes ofplants largely define that layering. Inmany forests, for example, the layers from top to bottom consistof the upper canopy, the low-tree layer, the shrub understory,

the ground layer of herbaceous plants, the forest floor (litterlayer), and the root layer. Nonforest biomes have similar, though

usually less pronounced, layers. Grasslands have an herbaceouslayer of grasses and forbs (small broadleaf plants), a litter layer,and a root layer. Layering of vegetation provides many differenthabitats for animals, which often occupy well-defined feedinggroups, from the insectivorous birds and bats that feed abovecanopies to the small mammals, numerous worms, and arthro­pods that search for food in the litter and root layers.

The species composition ofeach kind ofbiome varies fromone location to another. For instance, in the northern conifer-

CHAPTER fifTY· TWO An Introduction to Ecology and the Biosphere 1167

• Figure 52.21

•• • Terrestrial Biomes

Tropical Forest

Distribution Equatorial andsubequatorial regions.

Precipitation In tropical rainforests, minfJll is relatively con­stant, about 200-4OCl cm annu­ally. In tropical dry forests,precipitation is highly seasonal,about 150-200 cm annually, witha six- to seven-month dry season.

Temperature Air tempera­tures are high year-round, aver­aging 25-29·C with littleseasonal variation.

Plants Tropical forests are ver­tically layered, and competitionfor light is intense. Layers in rainforests include emergent treesthat grow above a closed canopy,the canopy trees, one or m'o lay­ers of subcanopy trees, andshrub and herb layers. There aregenerally fewer layers in tropicaldry forests. Broadleaf evergreentrees are dominant in tropicalmin forests, whereas tropical dryforest trees drop their leaves dur­ing the dry season. Epiphytes

such as bromeliads and orchidsgenerally cover tropical foresttrees but are less abundant in dryforests. Thorny shrubs and suc­culent plants are common insome tropical dry forests.

Animals Earth's tropicalforests are home to millions ofspecies, including an estimated5-30 million still undescribedspecies of insects, spiders, andother arthropods. In fJct, ani­mal diversity is higher in tropi­cal forests than in any otherterrestrial biome. The animals,including amphibians, birds andother reptiles, mammals, andarthropods, are adapted to thevertically layered environmentand are often inconspicuous.

Human Impact Humans longago established thriving com­munities in tropical forests.Rapid population growth lead­ing to agriculture and develop­ment is now destroying sometropical forests.

Desert

Distribution Deserts occur inbands near 30· north and southlatitude or at other latitudes inthe interior of continents (for in­stance, the Gobi Desert of northcentral Asia).

Precipitation Precipitation islow and highly variable, generallyless than 30 cm per year.

Temperature Temperature isvariable seasonally and daily.Maximum air temperature in hotdeserts may exceed 50"C; in colddeserts air temperature may fallbelow -30T.

Plants Desert landscapes aredominated by low, widel}'scatteredvegetation: the proportion ofbareground is high compared with otherterrestrial biomes. The plants in­clude succulents such as cacti,deeply rooted shrubs, and herbsthat grow during the infrequentmoist periods. Desert plant adapta­tions include heat and desiccation

tolerance, water storage, and re­duced leafsurface area. Physical de­fenses, such as spines, and chemicaldefenses, such as toxins in the leavesofshrubs, are common. Many oftheplants exhibit C4 or CAM photo-­synthesis (see Chapter 10).

Animals Common desert ani­mals include many kinds ofsnakes and lizards, scorpions,ants, beetles, migratory and resi­dent birds, and seed-eatingrodents. Many species arenocturnal. Water conservation isa common adaptation, with somespecies surviving on water frommetabolic breakdown of carbo­hydrates in seeds.

Human Impact Long-distancetransport ofwater and deepgroundwater wells have allowedhumans to maintain substantialpopulations in deserts. Conversionto irrigated agriculture and urban­ization have reduced the naturalbiodiversity ofsome deserts.

1168 U"IT EIGHT Ecology

Savanna

Distribution Equatorial andsubequatorial regions,

Precipitation Rainfall, whichis seasonal, averages 30-50 cmper )'ear. The dl'}' season can lastup to eight or nine months.

Temperature The U\"anna iswarm year.round, averaging24-29"C, but with somewhatmore seasonal variation than intropical forests.

Plants The scattered treesfound at diff~t densities in thesavanna often are thorny andhave smalllea\'es, an apparentadaptation to the re!ati\'ely dryconditions. Fires are common inthe dry season, and the dominantplant species are fire-adapted andtolerant of seasonal drought.Grasses and forbs, which makeup most of the ground cover.

grow rapidly in response to sea·sonal rains and are tolerant ofgrazing b)' large mammals andother herbivores.

Animals large plant·eatingmammals. such as wildebeestsand bison. and predators, includ·ing lions and hyenas, are com­mon inhabitants. However, thedominant herbivores are actuallyinsects, especially termites. Our·ing seasonal droughts. gratingmammals often migrate to partsofthe savanna with more forageand scattered watering holes.

Human Impact There is evi·denee that the earliest humanslived in savannas. Fires set b}' hu·mans may help maintain thisbiome. Calde ranching and over­hunting hm'!ed to declines inlarge-mammal populations.

A savaTla 11 Kenya

.....

Chaparral

Distribution This biome oc­curs in midlatitude coastal re­gions on several continents, andits many names reflect its far­flung distribution: chaparral inNorth America, matorral inSpain and Chile, ganglle andmaquis in southern France, andfynbos in South Africa.

Precipitation Precipitation ishighly seasonal, with rainy wintersand long, dry summers. Annualprecipitation generally falls withinthe range of30-50cm.

Temperature Fall, winter, andspring are cooL with averagetemperatures in the range ofIo-ITC. Average summer tem­perature can reach 3O"C, anddaytime maximum temperaturecan exceed 40"<:.

Plants 01aparraI isdominatedb)' shrubs and smaU trees, alongwith a man)' kinds ofgrasses andherbs. Pbnt~t)' is high, ...ithnuny species confmed 10 a specifIC.

relatively small geographic area.Adaptations to drought include thetough evergreen leaves ofwoodyplants, which reduce ",,-ater loss.Adaptations to fire are also promi­nent. Some ofthe shrubs produceseeds that will genninate only aftera hot fire; food reserves stOTed intheir fire-resistant roots enablethem to resprout quickly and usenutrients released by the fire.

Animals Native mammals in­clude browsers. such as deer andgoats, that feed on twigs andbuds of woody vegetation, and ahigh diversity of small mammals.Chaparral areas also supportmany species of amphibians,birds and other reptiles, andinsects.

Human Impact Chaparralareas have been heavily settledand reduced through conversionto agriculture and urbaniution.Humans contribute to the firesthat sweep across the chaparraL Continued on next page

(H.unl f."Y_TWO An Introduction to Ecology and the Biosphere 1169

• Figure 52.21 (continued)

•• • Terrestrial Biomes

Temperate Grassland

Distribution The veldts ofSouth Africa, the puSZla of Hun­gary, the pampas of Argentinaand Uruguay, the steppes of Rus­sia, and the plains and prairies ofcentral North America are alltemperate grasslands.

Precipitation Precipitation isoften highly seasonal, with rela­tively dry winters and wet sum­mers. Annual precipitationgenerally averages between 30and 100 em. Periodic drought iscommon.

Temperature Winters are gen­erally cold, with average temJ}Cra­tures frequently falling well below-W·e. Summers, with averagetemJ}Cratures often approaching3Q'e, are hot.

Plants The dominant plantsare grasses and forbs, which varyin height from a few centimetersto 2 m in lallgrass prairie. Many

have adaptations that help themsurvive periodic, protracteddroughts and fire: For example,grasses can sprout qUickly follow­ing fire. Grazing by large mam­mals helps prevent establishmentof woody shrubs and trees.

Animals Native mammals in­dude large grazers such as bisonand wild horses. Temperategrasslands are also inhabited by awide variety of burrowing mam­mals, such as prairie dogs inNorth America.

Human Impact Deep, fertilesoils make temperate grasslandsideal places for agriculture, espe­cially for growing grains. As aconsequence, most grassland inNorth America and much ofEurasia has been converted 10

farmland. In some drier grass­lands, cattle and other grazershave helped change parts of thebiome into desert.

Northern Coniferous Forest

Distribution Extending in abroad band across northernNorth America and Eurasia tothe edge of the arctic tundra, thenorthern coniferous forest, ortaiga, is the largest terrestrialbiome on Earth.

Precipitation Annual precipi­tation generally ranges from 30to 70 em, and periodic droughtsare common. However, somecoastal coniferous forests of theU.S. Pacific Northwest are tem­perate rain forests that may re­ceive over 3()() em of annualprecipitation.

Temperature Winters are usu­ally cold and long; summers maybe hot. Some areas of coniferousforest in Siberia typically range intemperature from -50'C in win­ter to over 20'C in summer.

Plants Cone-bearing trees, suchas pine, spruce, fir, and hemlock,

dominate northern coniferousforests. The conical shape of manyconifers prevents too much snowfrom accumulating and breakingtheir branches. The diversity ofplants in the shrub and herb layersof these forests is lower than intemperate broadleafforests.

Animals While many migra­tory birds nest in northern conif­erous foft'sts, other species residethere year-round. The mammalsof this biome, which includemoose, brown bears, and Siberiantigers, are diverse. Periodic out­breaks of insecIs thai feed on thedominant trees can kill vast tractsoftrees.

Human Impact Although theyhave not been heavily settled byhuman populations, northernconiferous fore~ts aft' being loggedat an alarming rate, and the old­growth stands of these trees maysoon disappear.

1170 U"IT EIGHT Ecology

Temperate Broadleaf Forest

Distribution Found mainly atmidlatiludes in the NorthernHemisphere, with smaller areasin New Zealand and Australia.

Precipttation Precipitation cana\-erage from about 70 IOO\"er200em annually. Significant amountsfaU during aU seuons. includingsummer r1lin and. in some foresu.....inter snow.

Temperature Winter temper­atures average around O'c.Summers, with maximum tem­

pt'r2tures near 35"<:. a~ hot andhumid.

Plants A mature tempcl'llltcbroadleafforesl has distinct '"eT­ticallarers. including a dosedcanopy, one or tv.·o strata of un­derstor)'trtes, a shrub larer, andan herbaceous stratum. There arefew epiphytes. The dominantplants in the Northern Hemi­sphere are de<:iduous tTee$,

which drop their leaves beforewinter, when low temperatureswould reduce photosynthesis andmake 'A'ilter uptake from frozensoil difficult. In Austr.dia, ever­green eucalyptus dominate theseforests.

Animals In the NorthernHemisphere, many nu.mnu.ls hi­bernate in winter, ....'hile man}'bird species migrate to .....armerdimates, The mammills, birds,ilnd insects make use of ill verti·allarers ofthe focest.

Human Impact TempeT'iltebroildleafforest hilS been heilvilysettled on illl continents, Loggingilnd land clearing for agricultureilnd urban development de­strored virtuilly :ill the originaldeciduous forests in NorthAmeriCll. Ho.....ever, owing totheir Cllpildty for recovery, theseforests are returning over muchof their former range.

Tundra

Distribution Tundra coversexpansive areas of the Arctic,amounting to 20% of Earth's landsurface. High winds and low tem­peratures create similar planlcommunities, called alpine tun­dra, on very high mountaintopsat all latitudes, including thetropics.

Precipitation Precipitation av­erages from 20 to 60 cm annuallyin arctic tundra but may exceed100 cm in alpine tundra.

Temperature Winters arelong and cold, with averages insome areas below - 3O'C. Sum­men are short with low tempera­tures, generally averaging lessthan Hrc.

Plants The vegetation oftundrais mostly hemaceous, consisting ofa mixture of mosses, grasses, andforbs, along with some dwarfshrubs and trees and lichens. Apermanently frozen layer of soilcalled permafrost restricts thegro'Nth of plant roots.

Animals large grazing muskoxen are resident, while caribouand reindeer are migratory.Predators include bears, wolves,and foxes. Many bird species mi·grate to the tundra for summernesting.

Human Impact Tundra issparsely settled but has become thefocus ofsigniflCllllt mineral and oilextraction in recent rears.

(H ... 'T£I f.'TY·TWO An Introduction to Ecology and the Biosphere 1171

-£.Ii ]f.- Go to the Study Area at www.masteringbio.com for 6ioFlix3-D Animations, MP3 Tutors, Videos. Practice Tests, an eBook. and more.

SUMMARY OF KEY CONCEPTS

_i,liiiii_ 52.1Ecology integrates all areas of biological research andinforms environmental decision making (pp. 1148-1151)

.. Linking Ecology and Evolutionary Biology Events that oc-cur in ecological time affect life in evolutionary time.

.. Ecology and Environmental Issues Ecologists distinguishhetween the science of ecology and environmental advocacy.Ecology provides a scientific basis for solving environmentalproblems, but policymakers must also balance social, eco­nomic, and political factors in reaching their decisions.

ACllvity Science. Te<:hnology, and Society: DDT

_i,liiiii- 52.2Interactions between organisms and the environmentlimit the distribution of species (pp. 1151-1159)

Why iSlp('oes X arn.ent from an area?

+

-ilili"- 52.3Aquatic biomes are diverse and dynamic systems thatcover most of Earth (pp. 1159-1166)

.. Stratification of Aquatic Biomes Aquatic biomes accountfor the largest part of the biosphere in terms of area and aregenerally stratified (layered) with regard to light penetration,temperature, and community structure. Marine biomes have ahigher salt concentration than freshwater biomes.

-$1401',·Activity Aquatic Biomes

-i liliii- 52.4The structure and distribution of terrestrial biomes arecontrolled by climate and disturbance (pp. 1166-1171)

.. Climate and Terrestrial Biomes Climographs show thattemperature and precipitation are correlated with biomes, butbecause biomes overlap, other abiotic factors playa role inbiome location.

.. Ceneral Features ofTerreslrial Biomes and the Role ofDisturbance Terrestrial biomes are often named for majorphysical or climatic factors and for their predominant vegeta­tion. Vertical layering is an important feature of terrestrialbiomes. Disturbance, both natural and human induced, influ­ences the type of vegetation found in biomes.

-$1401',·Activity Terrestrial Biomes

AClivity Adaptations to Biotic and Abiotic Factor!;

Innstlgatlon How Do Abiotic Factors Affe<:t Distribution of Organisms?

.. Climate Global climate patterns are largely determined bythe input of solar energy and Earth's revolution around thesun. Bodies of water, mountains, and the changing angle ofthe sun over the year exert regional, local, and seasonal effectson climate. Fine-scale differences in abiotic factors determinemicroclimates.

Does di'>Pffi<ll ~m't ,ts dlstnbution?

Does behavior ~ml1l11 distnbution?

Do biotic filCtors (other spe<:ie<;)I,mlt It5 distribution)

Do abiotic filCtorsliml1,ts dimibullOn)

",--"''--'•• Habitat selection

Yes ~ Predat'on. paraSItism.compe1ltion, disease

ChemICal

-CfilCtors Water. oXJ'gen. saI,nlty, pH,

soil nutnents, etc.

Temperature. light, soilf'hyslCal struaure, lire. mOisture. etc.factors

TESTING YOUR KNOWLEDGE

SElF·QUIZ

I. Which of the following areas of study focuses on the exchangeof energy, organisms, and materials between ecosystems?a. population ecology d. ecosystem ecologyb. organismal ecology e. community ecologyc. landscape ecology

2. -'w,nIM If Earth's axis of rotation suddenly became per­pendicular to the plane ofits orbit, the most predictable effect

would bea. no more night and day.b. a big change in the length of the year.c. a cooling of the equator.d. a loss of seasonal variation at high latitudes.e. the elimination of ocean currents.

3. When climbing a mountain, we can observe transitions in bio­logical communities that are analogous to the changesa. in biomes at different latitudes.b. at different depths in the ocean.c. in a community through different seasons.d. in an ecosystem as it evolves over time.e. across the United States from east to west.

1172 UNIT EIGHT Ecology

4. The oceans affect the biosphere in all of the following ways

excepta. producing a substantial amount of the biosphere's oxygen.

b. removing carbon dioxide from the atmosphere.

c. moderating the climate of terrestrial biomes.

d. regulating the pH of freshwater biomes and terrestrial

groundwater.e. being the source of most of Earth's rainfalL

5. Which lake zone would be absent in a very shallow lake?

a. benthic zone d. littoral zone

b. aphotic zone e. limnetic zone

c. pelagic zone

that shows how otter density depends on kelp abundance,

using the data shown below. Then formulate a hypothesis to

explain the pattern you observed.

Kelp Abundance Otter DensitySite (% cover) (# sightings per day)

75 98

2 15 18

3 60 85

4 25 36

6. Which of the following is true with respect to oligotrophic

lakes and eutrophic lakes?

a. Oligotrophic lakes are more subject to oxygen depletion.b. Rates of photosynthesis are lower in eutrophic lakes.

c. Eutrophic lake water contains lower concentrations of

nutrients.

d. Eutrophic lakes are richer in nutrients.

e. Sediments in oligotrophic lakes contain larger amounts of

decomposable organic matter.

7. Which of the following is characteristic of most terrestrial

biomes?

a. annual average rainfall in excess of250 cm

b. a distribution predicted almost entirely by rock and soil

patterns

c. clear boundaries between adjacent biomes

d. vegetation demonstrating stratification

e. cold winter months

For Self-Quiz Q"swers, see Appe"dix A.

MM#,jf._ Visit the Study Area al www.masteringbio.com for aPractice Test.

EVOLUTION CONNECTION

II. Discuss how the concept of timI' applies to ecological situations

and evolutionary changes. Do ecological time and evolutionarytime ever overlap? If so, what are some examples?

SCIENTIFIC INQUIRY

12. lens Clausen and colleagues, at the Carnegie Institution ofWash­

ington, studied howthe size ofyarrow plants (Achillea lanulosa)growing on the slopes ofthe Sierra Nevada varied with elevation.

They found that plants from low elevations were generally taller

than plants from high elevations, as shown below:

d. temperate broadleaf forest

e. temperate grassland

100

0

3,000:g

2,000•~ Sierra Nevada,

1,000 Great 8asin•;; Plateau0

Seed collection sites

Source' J Clausen et .11. Experimental studies on the nature of species. III.Environmental responses of climatic races of Achillea, CarnegieInstitution of Washington Publication No. 581 (1948).

8. \Vhich ofthe following biomes is correctly paired with the de­

scription ofits climate?a. savanna-low temperature, precipitation uniform during

the yearb. tundra-long summers, mild winters

c. temperate broadleaf forest-relatively short growingseason, mild winters

d. temperate grasslands-relatively warm winters, most

rainfall in summer

e. tropical forests-nearly constant day length and

temperature

9. Suppose that the number of bird species is determined mainly

by the number of vertical strata found in the environment. If

so, in which of the following biomes would you find the great­

est number of bird species?

a. tropical rain forest

b. savanna

c. desert

10. 1.@N'iI After reading the experiment ofW J. Fletcher de­

scribed in Figure 52.8, you decide to study feeding relationships

among sea otters, sea urchins, and kelp on your own. You know

that sea otters prey on sea urchins and that urchins eat kelp. At

four coastal sites, you measure kelp abundance. Then you

spend one day at each site and mark whether otters are present

or absent every 5 minutes during daylight hours. Make a graph

dausen and colleagues proposed two hypotheses to explain thisv.uiation within a species: (I) There are genetic differences be­m'een populations of plants found at different elev<1tions. (2)

The species has developmentall1exibility and can assume tall or

short growth forms, depending on local abiotic factors. Ifyouhad seeds from yarrow plants found at low and high elevations,

what experiments would you perform to test these hypotheses?

CHfJ,PTER fifTY· TWO An Introduction to Ecology and the Biosphere 1173