Chapter 24 Population Ecology

8/13/2019 Chapter 24 Population Ecology

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493

Why do tropical songbirds lay fewereggs?

Sometimes odd generalizations in science lead to unex-pected places. Take, for example, a long obscure mono-

graph published in 1944 by British ornithologist (bird ex-pert) Reginald Moreau in the journal Ibis on bird eggs.Moreau had worked in Africa for many years before mov-ing to a professorship in England in the early 1940s. Hewas not in England long before noting that the Britishsongbirds seemed to lay more eggs than he was accustomedto seeing in nests in Africa. He set out to gather informa-tion on songbird clutch size (that is, the number of eggs ina nest) all over the world.

Wading through a mountain of data (hisIbispaper is 51pages long!), Moreau came to one of these odd generaliza-tions: songbirds in the tropics lay fewer eggs than theircounterparts at higher latitudes (see above right). Tropicalsongbirds typically lay a clutch of 2 or 3 eggs, on average,while songbirds in temperate and subarctic regions gener-ally lay clutches of 4 to 6 eggs, and some species as many as10. The trend is general, affecting all groups of songbirdsin all regions of the world.

What is a biologist to make of such a generalization? At

first glance, we would expect natural selection to maximizeevolutionary fitnessthat is, songbirds the world overshould have evolved to produce as many eggs as possible.Clearly, the birds living in the tropics have not read Dar-win, as they are producing only half as many eggs as theyare capable of doing.

A way out of this quandary was proposed by ornitholo-gist Alexander Skutch in 1949. He argued that birds pro-duced just enough offspring to offset deaths in the popula-

tion. Any extra offspring would be wasteful of individuals,and so minimized by natural selection. An interesting idea,but it didnt hold water. Bird populations are not smaller inthe tropics, or related to the size of the populations there.

A second idea, put forward a few years earlier in 1947by a colleague of Moreaus, David Lack, was morepromising. Lack, one of the twentieth-centurys great bi-ologists, argued that few if any birds ever produce asmany eggs as they might under ideal conditions, for thesimple reason that conditions in nature are rarely ideal.

Natural selection will indeed tend to maximize reproduc-

tive rate (that is, the number of eggs laid in clutches) as

Darwin predicted, but only to the greatest level possiblewithin the limits of resources. There is nothing here thatwould have surprised Darwin. Birds lay fewer eggs in thetropics simply because parents can gather fewer resourcesto provide their young therecompetition is just toofierce, resources too scanty.

Lack went on to construct a general theory of clutch sizein birds. He started with the sensible assumption that in aresource-limited environment birds can supply only so

much food to their young. Thus, the more offspring theyhave, the less they can feed each nestling. As a result, Lackproposed that natural selection will favor a compromise be-tween offspring number and investment in each offspring,which maximizes the number of offspring which are fedenough to survive to maturity.

The driving force behind Lacks theory of optimalclutch size is his idea that broods with too many offspringwould be undernourished, reducing the probability that thechicks would survive. In Lacks own words:

The average clutch-size is ultimately determined by the av-erage maximum number of young which the parents can success-fully raise in the region and at the season in question, i.e. ... nat-ural selection eliminates a disproportionately large number ofyoung in those clutches which are higher than the average,through the inability of the parents to get enough food for theiryoung, so that some or all of the brood die before or soon afterfledging (leaving the nest), with the result that few or no descen-dants are left with their parents propensity to lay a larger

clutch.

Part

VII

Ecology and Behavior

This Kentucky warbler is tending her nest of eggs.A similarspecies in the tropics would lay fewer eggs. Why?


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494 Part VII Ecology and Behavior

The ExperimentLacks theory is attractive because of its simplicity andcommon sensebut is it right? Many studies have beenconducted to examine this hypothesis. Typically, experi-menters would remove eggs from nests, and look to see ifthis improved the survivorship of the remaining off-spring. If Lack is right, then it should, as the remainingoffspring will have access to a larger share of what theparents can provide. Usually, however, removal of eggs

did not seem to make any difference. Parents just ad-justed down the amount of food they provided. The situ-ation was clearly more complicated than Lacks simpletheory envisioned.

One can always argue with tests such as these, how-ever, as they involve direct interference with the nests,potentially having a major influence on how the birds be-have. It is hard to believe that a bird caring for a nest ofsix eggs would not notice when one turned up missing. Aclear test of Lacks theory would require avoiding all

intervention.Just such a test was completed in 1987 in the woods near

Oxford, England. Over many years, Oxford University re-searchers led by Professor Mark Boyce (now at the Univer-sity of Wyoming, Laramie) carefully monitored nests of asongbird, the greater tit, very common in the Englishcountryside. They counted the number of eggs laid in eachnest (the clutch size) and then watched to see how many ofthe offspring survived to fly away from the nest. Nothing

was done to interfere with the birds. Over 22 years, theypatiently examined 4489 nests.

The ResultsThe Oxford researchers found that the average clutch sizewas 8 eggs, but that nests with the greatest number of sur-viving offspring had not 8 but 12 eggs in them! Clearly,Lacks theory is wrong. These birds are not producing asmany offspring as natural selection to maximize fitness(that is, number of surviving offspring) would predict (seeabove left).

Lacks proposal had seemed eminently sensible. What

was wrong? In 1966 the evolutionary theorist GeorgeWilliams suggested the problem was that Lacks theory ig-nores the cost of reproduction (see above). If a bird spendstoo much energy feeding one brood, then it may not sur-vive to raise another. Looking after a large clutch may ex-tract too high a price in terms of future reproductive suc-cess of the parent. The clutch size actually favored bynatural selection is adjusted for the wear-and-tear on theparents, so that it is almost always smaller than the numberwhich would produce the most offspring in that nestjust

what the Oxford researchers observed.However, even Williams cost-of-reproduction is notenough to completely explain Boyces greater tit data.There were marked fluctuations in the weather over theyears that the Oxford researchers gathered their data, andthey observed that harsh years decreased survival of theyoung in large nests more than in small ones. This bad-year effect reduces the fitness of individuals laying largerclutches, and Boyce argues that it probably contributes at

least as much as cost-of-reproduction in making it moreadvantageous, in the long term, for birds to lay clutchessmaller than the Lack optimum.

Testing Lacks theory of optimum clutch size. In this studyfrom woods near Oxford, England, researchers found that themost common clutch size was 8, even though clutches of 12 pro-duced the greatest number of surviving offspring. (After Boyceand Perrins, 1987.)

Two theories of optimum clutch size. David Lacks theory pre-dicts that optimum clutch size will be where reproductive successof the clutch is greatest. George Williamss theory predicts thatoptimum clutch size will be where the netbenefit is greatestthatis, where the difference between the cost of reproduction and thereproductive success of the clutch is greatest.


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495

24

Population Ecology

Concept Outline

24.1 Populations are individuals of the same speciesthat live together.

Population Ecology. The borders of populations aredetermined by areas in which individuals cannot surviveand reproduce. Population ranges expand and contractthrough time as conditions change.Population Dispersion. The distribution of individuals ina population can be clumped, random, or even.

Metapopulations. Sometimes, populations are arranged innetworks connected by the exchange of individuals.

24.2 Population dynamics depend critically upon agedistribution.

Demography. The growth rate of a population is asensitive function of its age structure; populations withmany young individuals grow rapidly as these individualsenter reproductive age.

24.3 Life histories often reflect trade-offs betweenreproduction and survival.

The Cost of Reproduction. Evolutionary success is atrade-off between investment in current reproduction andin growth that promotes future reproduction.

24.4 Population growth is limited by the environment.

Biotic Potential. Populations grow if the birthrate exceedsthe death rate until they reach the carrying capacity of theirenvironment.

The Influence of Population Density. Some of thefactors that regulate a populations growth depend upon thesize of the population; others do not.Population Growth Rates and Life History Models. Somespecies have adaptations for rapid, exponential populationgrowth, whereas other species exhibit slower populationgrowth and have intense competition for resources.

24.5 The human population has grown explosively inthe last three centuries.

The Advent of Exponential Growth. Human populationshave been growing exponentially since the 1700s and willcontinue to grow in developing countries because of thenumber of young people entering their reproductive years.

Ecology, the study of how organisms relate to one an-other and to their environments, is a complex and

fascinating area of biology that has important implicationsfor each of us. In our exploration of ecological principles,we will first consider the properties of populations, em-phasizing population dynamics (figure 24.1). In chapter25, we will discuss communities and the interactions thatoccur in them. Chapter 26 moves on to focus on animalsand how and why they behave as they do. Chapter 27 thendeals with behavior in an environmental context, the ex-tent to which natural selection has molded behaviorsadaptively.

FIGURE 24.1Life takes place in populations.This population of gannets issubject to the rigorous effects of reproductive strategy,competition, predation, and other limiting factors.


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Population Distributions

No population, not even of humans, occurs in all habitats

throughout the world. Most species, in fact, have relativelylimited geographic ranges. The Devils Hole pupfish, forexample, lives in a single hot water spring in southernNevada, and the Socorro isopod is known from a singlespring system in Socorro, New Mexico (figure 24.2). At theother extreme, some species are widely distributed. Popula-tions of some whales, for example, are found throughout allof the oceans of the northern or southern hemisphere.

In chapter 29 we will discuss the variety of environmentalchallenges facing organisms. Suffice it to say for now that nopopulation contains individuals adapted to live in all of thedifferent environments on the earth. Polar bears are exqui-sitely adapted to survive the cold of the Arctic, but you wontfind them in the tropical rain forest. Certain bacteria can livein the near boiling waters of Yellowstones geysers, but theydo not occur in cooler streams that are nearby. Each popula-tion has its own requirementstemperature, humidity, cer-tain types of food, and a host of other factorsthat deter-mine where it can live and reproduce and where it cant. In

addition, in places that are otherwise suitable, the presenceof predators, competitors, or parasites may prevent a popula-tion from occupying an area.


Population Ecology

Organisms live as members of populations, groups of indi-

viduals of a species that live together. In this chapter, wewill consider the properties of populations, focusing on ele-ments that influence whether a population will grow orshrink, and at what rate. The explosive growth of theworlds human population in the last few centuries providesa focus for our inquiry.

A population consists of the individuals of a givenspecies that occur together at one place and time. This flex-ible definition allows us to speak in similar terms of the

worlds human population, the population of protozoa inthe gut of an individual termite, or the population of deerthat inhabit a forest. Sometimes the boundaries defining aboundary are sharp, such as the edge of an isolated moun-tain lake for trout, and sometimes they are more fuzzy,such as when individuals readily move back and forth be-tween areas, like deer in two forests separated by a corn-field.

Three aspects of populations are particularly important:the range throughout which a population occurs, the dis-persion of individuals within that range, and the size a pop-ulation attains.

24.1 Populations are individuals of the same species that live together.

Iriomote cat

Northern white rhinoceros

New Guineatree kangaroo

IiwiHawaiian bird

Pupfish

Catalina Islandmahogany tree

FIGURE 24.2Species that occur in only one place.These species, and many others, are only found in a single population. All are endangered species,and should anything happen to their single habitat, the populationand the specieswould go extinct.


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Range Expansions andContractions

Population ranges are not static, but,rather, change through time. Thesechanges occur for two reasons. In somecases, the environment changes. Forexample, as the glaciers retreated at theend of the last ice age, approximately10,000 years ago, many North Ameri-can plant and animal populations ex-panded northward. At the same time,

as climates have warmed, species haveexperienced shifts in the elevation atwhich they are found on mountains(figure 24.3).

In addition, populations can ex-pand their ranges when they are ableto circumvent inhospitable habitat tocolonize suitable, previously unoccu-pied areas. For example, the cattle

egret is native to Africa. Some timein the late 1800s, these birds ap-peared in northern South America,having made the nearly 2000-miletransatlantic crossing, perhaps aidedby strong winds. Since then, theyhave steadily expanded their rangesuch that they now can be foundthroughout most of the United States(figure 24.4).

Chapter 24 Population Ecology 497

Present

Alpine tundra

Spruce-fir forests

Mixed conifer forestWoodlands

Woodlands Grassland,chaparral, and

desert scrubGrassland, chaparral,and desert scrub

15,000 years ago

Alpine tundra

Spruce-fir forests

Mixed conifer forest

Elevation(km)

0 km

2 km

3 km

1 km

FIGURE 24.3Altitudinal shifts in population ranges. During the glacial period 15,000 years ago, conditions were cooler than they are now. As theclimate has warmed, tree species that require colder temperatures have shifted their distributional range upward in altitude so that they livein the climatic conditions to which they are adapted.

Equator 1937194319511958

1961

196019651964

1966 1970

1970

1956

Immigrationfrom Africa

FIGURE 24.4Range expansion of the cattle egret.Although the cattle egretso-named because itfollows cattle and other hoofed animals, catching any insects or small vertebrates that theydisturbfirst arrived in South America in the late 1800s, the oldest preserved specimen datesfrom the 1930s. Since then, the range expansion of this species has been well documented, asit has moved westward and up into much of North America, as well as down the western sideof the Andes to near the southern tip of South America.


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Population Dispersion

Another key characteristic of population structure is theway in which individuals of a population are arranged.They may be randomly spaced, uniformly spaced, or

clumped (figure 24.5).

Randomly spaced

Individuals are randomly spaced within populations whenthey do not interact strongly with one another or withnonuniform aspects of their microenvironment. Randomdistributions are not common in nature. Some species oftrees, however, appear to exhibit random distributions inAmazonian rain forests.

Uniformly spaced

Individuals often are uniformly spaced within a population.This spacing may often, but not always, result from compe-tition for resources. The means by which it is accom-plished, however, varies.

In animals, uniform spacing often results from behav-ioral interactions, which we will discuss in chapter 27. In

many species, individuals of one or both sexes defend a ter-ritory from which other individuals are excluded. Theseterritories serve to provide the owner with exclusive accessto resources such as food, water, hiding refuges, or matesand tend to space individuals evenly across the habitat.Even in nonterritorial species, individuals often maintain adefended space into which other animals are not allowed tointrude.

Among plants, uniform spacing also is a common resultof competition for resources. In this case, however, the

spacing results from direct competition for the resources.Closely spaced individual plants will contest for availablesunlight, nutrients, or water. These contests can be direct,such as one plant casting a shadow over another, or indi-rect, such as two plants competing to see which is moreefficient at extracting nutrients or water from a sharedarea. Only plants that are spaced an adequate distancefrom each other will be able to coexist, leading to uniformspacing.


Clumped

(a) Bacterial colonies

UniformRandom

(b) Random distribution of Brosimum alicastrum

(c) Uniform distribution of Coccoloba coronata

(d) Clumped distribution of Chamguava schippii

FIGURE 24.5Population dispersion. (a) Different arrangements of bacterialcolonies. The different patterns of dispersion are exhibited by

three different species of trees from the same locality in theAmazonian rain forest. (b) Brosimum alicastrum is randomlydispersed, (b) Coccoloba coronata is uniformly dispersed, and (d)Chamguava schippiiexhibits a clumped distribution.


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Clumped Spacing

Individuals clump into groups or clusters in response to un-even distribution of resources in their immediate environ-

ments. Clumped distributions are common in nature be-cause individual animals, plants, and microorganisms tendto prefer microhabitats defined by soil type, moisture, orcertain kinds of host trees.

Social interactions also can lead to clumped distribu-tions. Many species live and move around in large groups,which go by a variety of names (examples include herds ofantelope, flocks of birds, gaggles of geese, packs of wolves,prides of lions). Such groupings can provide many advan-tages, including increased awareness of and defenseagainst predators, decreased energetic cost of movingthrough air and water, and access to the knowledge of allgroup members.

At a broader scale, populations are often most denselypopulated in the interior of their range and less densely dis-tributed toward the edges. Such patterns usually resultfrom the manner in which the environment changes in dif-ferent areas. Populations are often best adapted to the con-ditions in the interior of their distribution. As environmen-

tal conditions change, individuals are less well adapted andthus densities decrease. Ultimately, the point is reached atwhich individuals cannot persist at all; this marks the edgeof a populations range.

The Human Effect

By altering the environment, we have allowed somespecies, such as coyotes, to expand their ranges, although,sadly, for most species the effect has been detrimental.Moreover, humans have served as an agent of dispersal formany species. Some of these transplants have been widelysuccessful. For example, 100 starlings were introduced

into New York City in 1896 in a misguided attempt to es-tablish every species of bird mentioned by Shakespeare.Their population steadily spread such that by 1980, theyoccurred throughout the United States. Similar stories

could be told for countless numbers of plants and animals,and the list increases every year. Unfortunately, the suc-cess of these invaders often comes at the expense of nativespecies.

Dispersal Mechanisms

Dispersal to new areas can occur in many ways. Lizards,for example, have colonized many distant islands, probably

by individuals or their eggs floating or drifting on vegeta-tion. Seeds of many plants are designed to disperse inmany ways (figure 24.6). Some seeds are aerodynamicallydesigned to be blown long distances by the wind. Othershave structures that stick to the fur or feathers of animals,so that they are carried long distances before falling to theground. Still others are enclosed in fruits. These seeds canpass through the digestive systems of mammals or birdsand then germinate at the spot upon which they are defe-cated. Finally, seeds of Arceuthobium are violently pro-

pelled from the base of the fruit in an explosive discharge.Although the probability of long-distance dispersal eventsoccurring and leading to successful establishment of newpopulations is slim, over millions of years, many such dis-persals have occurred.

A population is a group of individuals of the samespecies living together at the same place and time. The

range of a population is limited by ecologicallyinhospitable habitats, but through time, these rangeboundaries can change.


Solanum dulcamara Juniperus chinensis Rubus sp.

Windblownfruits

Adherentfruits

Fleshy

fruits

Asclepias syriaca Acer saccharum Terminalia calamansanai

Ranunculus muricatusBidens frondosaMedicago polycarpa

FIGURE 24.6Some of the manyadaptations of seeds tofacilitate dispersal. Seedshave evolved a number of

different means of movinglong distances from theirmaternal plant.


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Metapopulations

Species are often composed of a network of distinct popu-lations that interact with each other by exchanging individ-uals. Such networks are termed metapopulations and usu-ally occur in areas in which suitable habitat is patchilydistributed and separated by intervening stretches of un-suitable habitat.

To what degree populations within a metapopulation in-teract depends on the amount of dispersal and is often notsymmetrical: populations increasing in size may tend tosend out many dispersers, whereas populations at low levelswill tend to receive more immigrants than they send off. Inaddition, relatively isolated populations will tend to receiverelatively few arrivals.

Not all suitable habitats within a metapopulations areamay be occupied at any one time. For various reasons,some individual populations may go extinct, perhaps as aresult of an epidemic disease, a catastrophic fire, or in-breeding depression. However, because of dispersal fromother populations, such areas may eventually be recolo-nized. In some cases, the number of habitats occupied in ametapopulation may represent an equilibrium in which therate of extinction of existing populations is balanced by the

rate of colonization of empty habitats.A second type of metapopulation structure occurs inareas in which some habitats are suitable for long-termpopulation maintenance, whereas others are not. In thesesituations, termed source-sink metapopulations, thepopulations in the better areas (the sources) continuallysend out dispersers that bolster the populations in thepoorer habitats (the sinks). In the absence of suchcontinual replenishment, sink populations would have anegative growth rate and would eventually becomeextinct.

Metapopulations of butterflies have been studied partic-ularly intensively (figure 24.7). In one study, Ilkka Hanskiand colleagues at the University of Helsinki sampled pop-ulations of the glanville fritillary butterfly at 1600 mead-ows in southwestern Finland. On average, every year, 200populations became extinct, but 114 empty meadows werecolonized. A variety of factors seemed to increase the like-lihood of a populations extinction, including small popu-

lation size, isolation from sources of immigrants, low re-source availability (as indicated by the number of flowerson a meadow), and lack of genetic variation present withinthe population. The researchers attribute the greater num-ber of extinctions than colonizations to a string of very drysummers. Because none of the populations is large enoughto survive on its own, continued survival of the species insouthwestern Finland would appear to require the contin-ued existence of a metapopulation network in which new

populations are continually created and existing popula-tions are supplemented by emigrants. Continued badweather thus may doom the species, at least in this part ofits range.

Metapopulations, where they occur, can have two im-portant implications for the range of a species. First, bycontinual colonization of empty patches, they prevent

long-term extinction. If no such dispersal existed, theneach population might eventually perish, leading to disap-pearance of the species from the entire area. Moreover, insource-sink metapopulations, the species as a whole occu-pies a larger area than it otherwise might occupy. Forthese reasons, the study of metapopulations has becomevery important in conservation biology as natural habitatsbecome increasingly fragmented.

The distribution of individuals within a population canbe random, uniform, or clumped. Across broader areas,individuals may occur in populations that are looselyinterconnected, termed metapopulations.


10 km

Occupied habitat patch

Unoccupied habitat patch

Norway

Sweden

Finland

landIslands

FIGURE 24.7Metapopulations of butterflies.The glanville fritillary butterflyoccurs in metapopulations in southwestern Finland on the landIslands. None of the populations is large enough to survive forlong on its own, but continual emigration of individuals from

other populations allows some populations to survive. In addition,continual establishment of new populations tends to offsetextinction of established populations, although in recent years,extinctions have outnumbered colonizations.


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One of the important features of any population is itssize. Population size has a direct bearing on the ability ofa given population to survive: for a variety of reasons dis-

cussed in chapter 31, smaller populations are at a greaterrisk of disappearing than large populations. In addition,the interactions that occur between members of a popu-lation also depend critically on a populations size anddensity.

Demography

Demography(from the Greek demos, the people, +

graphos, measurement) is the statistical study of popula-tions. How the size of a population changes through timecan be studied at two levels. At the most inclusive level, wecan study the population as a whole to determine whetherit is increasing, decreasing, or remaining constant. Popula-tions grow if births outnumber deaths and shrink if deathsoutnumber births. Understanding these trends is often eas-ier if we break a population down into its constituent partsand analyze each separately.

Factors Affecting Population Growth Rates

The proportion of males and females in a population isits sex ratio. The number of births in a population isusually directly related to the number of females, but maynot be as closely related to the number of males inspecies in which a single male can mate with several fe-males. In many species, males compete for the opportu-nity to mate with females (a situation we discuss in chap-

ter 27); consequently, a few males get many matings,whereas many males do not mate at all. In such species, afemale-biased sex ratio would not affect populationgrowth rates; reduction in the number of males simplychanges the identities of the reproductive males withoutreducing the number of births. Among monogamousspecies like many birds, by contrast, in which pairs formlong-lasting reproductive relationships, a reduction in thenumber of males can directly reduce the number ofbirths.

Generation time, defined as the average interval be-tween the birth of an individual and the birth of its off-spring, can also affect population growth rates. Speciesdiffer greatly in generation time. Differences in body sizecan explain much of this variationmice go through ap-proximately 100 generations during the course of one ele-phant generationbut not all of it (figure 24.8). Newts,for example, are smaller than mice, but have considerablylonger generation times. Everything else equal, popula-

tions with shorter generations can increase in size morequickly than populations with long generations. Con-

versely, because generation time and life span are usuallyclosely correlated, populations with short generation

times may also diminish in size more rapidly if birthratessuddenly decrease.


24.2 Population dynamics depend critically upon age distribution.

1 m

10 m

100 m

1 mm

1 cm

10 cm

1 m

10 m

100 m

Generation time

1hour

1day

1week

1month

1year

10years

100years

B. aureusPseudomonas

E. coli

Spirochaeta

EuglenaTetrahymena

DidiniumParamecium

Stentor

DaphniaDrosophila

Housefly Clam

Horsefly

Bee Oyster

Snail Chameleon

Scallop Frog

MouseNewt

Turtle

Horseshoe crabCrabRat

SalamanderFox Beaver

SnakeMan

Deer

Elk Bear

ElephantRhino

DogwoodBalsam

BirchKelpWhale

Fir

Sequoia

Bodysize(length)

FIGURE 24.8

The relationship between body size and generation time. Ingeneral, larger animals have longer generation times, althoughthere are exceptions.


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Much can be learned from examination of life tables. Inthis particular case, we see that the probability of dyingincreases steadily with age, whereas the number of off-spring produced increases with age. By adding up thenumbers in the last column, we get the total number of

offspring produced per individual in the initial cohort.This number is almost 2, which means that for every orig-inal member of the cohort, on average two individualshave been produced. A figure of 1.0 would be the break-even number, the point at which the population was nei-ther growing nor shrinking. In this case, the populationappears to be growing rapidly.

In most cases, life table analysis is more complicatedthan this. First, except for organisms with short life spans,it is difficult to track the fate of a cohort from birth until

death of the last individual. An alternative approach is toconstruct a cross-sectional study, examining the fate of allcohorts over a single year. In addition, many factorssuch as offspring reproducing before all members of theirparental generations cohort have diedcomplicate theinterpretation of whether populations are growing orshrinking.

Survivorship Curves

One way to express some aspects of the age distributioncharacteristics of populations is through a survivorshipcurve. Survivorship is defined as the percentage of an orig-inal population that survives to a given age. Examples ofdifferent kinds of survivorship curves are shown in figure24.9. In hydra, animals related to jellyfish, individuals areequally likely to die at any age, as indicated by the straightsurvivorship curve (type II). Oysters, like plants, producevast numbers of offspring, only a few of which live to re-produce. However, once they become established and growinto reproductive individuals, their mortality rate is ex-tremely low (type III survivorship curve). Finally, eventhough human babies are susceptible to death at relativelyhigh rates, mortality rates in humans, as in many animalsand protists, rise steeply in the postreproductive years (typeI survivorship curve). Examination of the data for Poaannua reveals that it approximates a type II survivorshipcurve (figure 24.10).

The growth rate of a population is a sensitive functionof its age structure. The age structure of a populationand the manner in which mortality and birthrates varyamong different age cohorts determine whether apopulation will increase or decrease in size.


0 25

Survivalperthousand

1000

100

Human

(type I)

Hydra

(type II)

Oyster

(type III)10

1

50

Percent of maximum life span

10075

FIGURE 24.9Survivorship curves. By convention, survival (the vertical axis) isplotted on a log scale. Humans have a type I life cycle, the hydra(an animal related to jellyfish) type II, and oysters type III.

3

2345

10

20304050

100

200300400500

1000

6 9 12 15

Age (months)

Survivalperthousan

d

18 21 24 27

FIGURE 24.10Survivorship curve for a cohort of the meadow grass,Poa

annua.Mortality increases at a constant rate through time.


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Natural selection favors traits that maximize the numberof surviving offspring left in the next generation. Twofactors affect this quantity: how long an individual lives

and how many young it produces each year. Why doesntevery organism reproduce immediately after its ownbirth, produce large families of large offspring, care forthem intensively, and do this repeatedly throughout along life, while outcompeting others, escaping predators,and capturing food with ease? The answer is that no oneorganism can do all of thisthere are simply not enoughresources available. Consequently, organisms allocate re-sources either to current reproduction or to increasetheir prospects of surviving and reproducing at later lifestages.

The Cost of Reproduction

The complete life cycle of an organism constitutes its lifehistory. All life histories involve significant trade-offs.Because resources are limited, a change that increases re-production may decrease survival and reduce future re-production. Thus, a Douglas fir tree that produces more

cones increases its current reproductive success, but italso grows more slowly; because the number of conesproduced is a function of how large a tree is, this dimin-ished growth will decrease the number of cones it canproduce in the future. Similarly, birds that have moreoffspring each year have a higher probability of dyingduring that year or producing smaller clutches the fol-lowing year (figure 24.11). Conversely, individuals thatdelay reproduction may grow faster and larger, enhanc-

ing future reproduction.In one elegant experiment, researchers changed thenumber of eggs in nests of a bird, the collared flycatcher(figure 24.12). Birds whose clutch size (the number ofeggs produced in one breeding event) was decreased laidmore eggs the next year, whereas those given more eggsproduced fewer eggs the following year. Ecologists referto the reduction in future reproductive potential result-ing from current reproductive efforts as the cost ofreproduction.

Natural selection will favor the life history that maxi-mizes lifetime reproductive success. When the cost of re-production is low, individuals should invest in producing asmany offspring as possible because there is little cost. Lowcosts of reproduction may occur when resources are abun-dant, such that producing offspring does not impair sur-vival or the ability to produce many offspring in subsequentyears. Costs of reproduction will also be low when overallmortality rates are high. In such cases, individuals may be

unlikely to survive to the next breeding season anyway, sothe incremental effect of increased reproductive efforts maynot make a difference in future survival.

Alternatively, when costs of reproduction are high, life-time reproductive success may be maximized by deferringor minimizing current reproduction to enhance growth andsurvival rates. This may occur when costs of reproduction

significantly affect the ability of an individual to survive ordecrease the number of offspring that can be produced inthe future.


24.3 Life histories often reflect trade-offs between reproduction and survival.

1.00.50.2Annual adult mortality rate

Annualfecundityrate

0.10.05

5

2

1

0.5

0.2

0.1

FIGURE 24.11Reproduction has a price. Increased fecundity in birdscorrelates with higher mortality in several populations of birdsranging from albatross (low) to sparrow (high). Birds that raisemore offspring per year have a higher probability of dying duringthat year.

+2+10

Change in clutch size

C

lutchsizefollowingyear

12

7

6

5

FIGURE 24.12Reproductive events per lifetime.Adding eggs to nests ofcollared flycatchers (which increases the reproductive efforts ofthe female rearing the young) decreases clutch size the following

year; removing eggs from the nest increases the next years clutchsize. This experiment demonstrates the tradeoff between currentreproductive effort and future reproductive success.


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Investment per Offspring

In terms of natural selection, the number of offspring pro-duced is not as important as how many of those offspringthemselves survive to reproduce.

A key reproductive trade-off concerns how many re-

sources to invest in producing any single offspring. As-suming that the amount of energy to be invested in off-spring is limited, a trade-off must exist between thenumber of offspring produced and the size of each off-spring (figure 24.13). This trade-off has been experimen-tally demonstrated in the side-blotched lizard, Uta stans-buriana, which normally lays on average four and a halfeggs at a time. When some of the eggs are removed sur-gically early in the reproductive cycle, the female lizardproduces only 1 to 3 eggs, but supplies each of these eggswith greater amounts of yolk, producing eggs that aremuch larger than normal.

In many species, the size of offspring critically affectstheir survival prospectslarger offspring have a greaterchance of survival. Producing many offspring with littlechance of survival might not be the best strategy, but pro-ducing only a single, extraordinarily robust offspring alsowould not maximize the number of surviving offspring.Rather, an intermediate situation, in which several fairly

large offspring are produced, should maximize the numberof surviving offspring. This example is fundamentally thesame as the trade-off between clutch size and parental in-vestment discussed above; in this case, the parental invest-ment is simply how many resources can be invested in eachoffspring before they are born.

Reproductive Events per Lifetime

The trade-off between age and fecundity plays a key rolein many life histories. Annual plants and most insectsfocus all of their reproductive resources on a single largeevent and then die. This life history adaptation is calledsemelparity(from the Latin semel, once, parito, tobeget). Organisms that produce offspring several timesover many seasons exhibit a life history adaptation callediteroparity (from the Latin itero, to repeat). Speciesthat reproduce yearly must avoid overtaxing themselves inany one reproductive episode so that they will be able tosurvive and reproduce in the future. Semelparity, or bigbang reproduction, is usually found in short-lived speciesin which the probability of staying alive between broods islow, such as plants growing in harsh climates. Semelparityis also favored when fecundity entails large reproductivecost, as when Pacific salmon migrate upriver to theirspawning grounds. In these species, rather than investingsome resources in an unlikely bid to survive until the nextbreeding season, individuals place all their resources into

reproduction.

Age at First Reproduction

Among mammals and many other animals, longer-l ived

species reproduce later (figure 24.14). Birds, for example,gain experience as juveniles before expending the highcosts of reproduction. In long-lived animals, the relativeadvantage of juvenile experience outweighs the energy in-vestment in survival and growth. In shorter-lived animals,on the other hand, quick reproduction is more critical thanjuvenile training, and reproduction tends to occur earlier.

Life history adaptations involve many trade-offsbetween reproductive cost and investment in survival.

Different kinds of animals and plants employ quitedifferent approaches.


0 2 4 6 8 10 12 14

17.5

18.0

18.5

19.0

19.5

Clutch size

Nestlingsize(weightingrams)

FIGURE 24.13The relationship between clutch size and offspring size. Ingreat tits, the size of nestlings is inversely related to the number of

eggs laid. The more mouths they have to feed, the less the parentscan provide to any one nestling.

Vole

0.81.2

Mouse

Warthog

0.8

1.2

Pig Lynx

Impala

Beaver

Hippo

Sheep

Otter

Cottontail rabbit

Red squirrel

African elephant

Pika

Chipmunk

Kob

0.4

0.0

0.4

0.8

0.4

Relative life expectancy

Relativeage

atfirstre

production

0.0 0.4 0.8

FIGURE 24.14

Age at first reproduction. Among mammals, compensating for theeffects of size, age at first reproduction increases with lifeexpectancy at birth. Each dot represents a species. Values arerelative to the species symbolized . (After Begon et al., 1996.)


14/22

Biotic Potential

Populations often remain at a relatively constant size, re-

gardless of how many offspring they produce. As you sawin chapter 1, Darwin based his theory of natural selectionpartly on this seeming contradiction. Natural selection oc-curs because of checks on reproduction, with some individ-uals reproducing less often than others. To understandpopulations, we must consider how they grow and whatfactors in nature limit population growth.

The Exponential Growth Model

The actual rate of population increase, r, is defined as thedifference between the birth rate and the death rate cor-rected for any movement of individuals in or out of thepopulation, whether net emigration (movement out ofthe area) or net immigration (movement into the area).Thus,

r= (b d) + (i e)

Movements of individuals can have a major impact on

population growth rates. For example, the increase inhuman population in the United States during the closingdecades of the twentieth century is mostly due to immi-grants. Less than half of the increase came from the repro-duction of the people already living there.

The simplest model of population growth assumes apopulation growing without limits at its maximal rate. Thisrate, called the biotic potential, is the rate at which a pop-ulation of a given species will increase when no limits areplaced on its rate of growth. In mathematical terms, this isdefined by the following formula:

dN= riNdt

where N is the number of individuals in the population,dN/dtis the rate of change in its numbers over time, and riis the intrinsic rate of natural increase for that populationits innate capacity for growth.

The innate capacity for growth of any population is ex-

ponential (red line in figure 24.15). Even when the rate ofincrease remains constant, the actual increase in the num-ber of individuals accelerates rapidly as the size of thepopulation grows. The result of unchecked exponentialgrowth is a population explosion. A single pair of house-flies, laying 120 eggs per generation, could produce morethan 5 trillion descendants in a year. In 10 years, their de-scendants would form a swarm more than 2 meters thickover the entire surface of the earth! In practice, such pat-terns of unrestrained growth prevail only for short peri-

ods, usually when an organism reaches a new habitat withabundant resources (figure 24.16). Natural examples in-clude dandelions reaching the fields, lawns, and meadows



1250

1000

750

= 1.0 NdN

dt

500

250

0

0 5 10

Number of generations (t)

Populationsize(N)

15

= 1.0 N

Carrying

capacity

dN

dt

1000 N1000

FIGURE 24.15Two models of population growth.The red line illustrates theexponential growth model for a population with an rof 1.0. The

blue line illustrates the logistic growth model in a population withr= 1.0 andK= 1000 individuals. At first, logistic growthaccelerates exponentially, then, as resources become limiting, thedeath rate increases and growth slows. Growth ceases when thedeath rate equals the birthrate. The carrying capacity (K)ultimately depends on the resources available in the environment.

FIGURE 24.16An example of a rapidly increasing population. Europeanpurple loosestrife,Lythrum salicaria, became naturalized overthousands of square miles of marshes and other wetlands in NorthAmerica. It was introduced sometime before 1860 and has had anegative impact on many native plants and animals.


15/22

of North America from Europe for the first time; algaecolonizing a newly formed pond; or the first terrestrialimmigrants arriving on an island recently thrust up fromthe sea.

Carrying CapacityNo matter how rapidly populations grow, they eventuallyreach a limit imposed by shortages of important environ-mental factors, such as space, light, water, or nutrients. Apopulation ultimately may stabilize at a certain size, calledthe carrying capacityof the particular place where it lives.The carrying capacity, symbolized by K, is the maximumnumber of individuals that a population can support.

The Logistic Growth Model

As a population approaches its carrying capacity, its rate ofgrowth slows greatly, because fewer resources remain foreach new individual to use. The growth curve of such apopulation, which is always limited by one or more factorsin the environment, can be approximated by the followinglogistic growth equation:

dN= rN(K - N)dt K

In this logistic model of population growth, the growthrate of the population (dN/dt) equals its rate of increase (rmultiplied by N, the number of individuals present at anyone time), adjusted for the amount of resources available.The adjustment is made by multiplying rNby the fractionof K still unused (K minus N, divided by K). As N in-creases (the population grows in size), the fraction bywhich r is multiplied (the remaining resources) becomes

smaller and smaller, and the rate of increase of the popu-lation declines.In mathematical terms, as Napproaches K, the rate of

population growth (dN/dt) begins to slow, reaching 0 whenN=K(blue line in figure 24.18). Graphically, if you plot Nversus t (time) you obtain an S-shaped sigmoid growthcurve characteristic of many biological populations. Thecurve is called sigmoid because its shape has a doublecurve like the letter S. As the size of a population stabilizesat the carrying capacity, its rate of growth slows down,

eventually coming to a halt (figure 24.17a).In many cases, real populations display trends corre-

sponding to a logistic growth curve. This is true not only inthe laboratory, but also in natural populations (figure24.17b). In some cases, however, the fit is not perfect (fig-ure 24.17c) and, as we shall see shortly, many populationsexhibit other patterns.

The size at which a population stabilizes in a particular

place is defined as the carrying capacity of that place forthat species. Populations often grow to the carryingcapacity of their environment.


0 5 10 15 20 25

Time (days)

500

400

300

200

100

0Numberofparam

ecia(percm3)

(a)

(c)

10

8

6

4

2

0

Time (years)

Numb

erofbreedingmale

fur

seals(thousands)

1915 1925 1935 1945

(b)

500

400

300

200

100

0

200 10 30 5040 60

Time (days)

Numberofcladocerans

(per200ml)

FIGURE 24.17Most natural populations exhibit logistic growth. (a)Paramecium grown in a laboratory environment. (b) A fur seal(Callorhinus ursinus) population on St. Paul Island, Alaska. (c)Laboratory populations of two populations of the cladoceran

Bosmina longirsotris. Note that the populations first exceeded thecarrying capacity, before decreasing to a size which was thenmaintained.


16/22

The Influence of PopulationDensity

The reason that population growth rates are affected bypopulation size is that many important processes are

density-dependent. When populations approach theircarrying capacity, competition for resources can be severe,leading both to a decreased birthrate and an increased riskof mortality (figure 24.18). In addition, predators oftenfocus their attention on particularly common prey, whichalso results in increasing rates of mortality as populationsincrease. High population densities can also lead to an ac-cumulation of toxic wastes, a situation to which humansare becoming increasingly accustomed.

Behavioral changes may also affect population growth

rates. Some species of rodents, for example, become antiso-cial, fighting more, breeding less, and generally actingstressed-out. These behavioral changes result from hor-monal actions, but their ultimate cause is not yet clear;most likely, they have evolved as adaptive responses to situ-ations in which resources are scarce. In addition, incrowded populations, the population growth rate may de-crease because of an increased rate of emigration of indi-viduals attempting to find better conditions elsewhere (fig-

ure 24.19).However, not all density-dependent factors are nega-tively related to population size. In some cases, growthrates increase with population size. This phenomenon is re-ferred to as theAllee effect (after Warder Allee, who firstdescribed it). The Allee effect can take several forms. Mostobviously, in populations that are too sparsely distributed,individuals may have difficulty finding mates. Moreover,some species may rely on large groups to deter predators orto provide the necessary stimulation for breeding activities.


0.4

0.5

0.7

0.6

0.8

0.9

40 80

Number of adults

Juvenilemortality

12010060200 140 160

2.0

1.0

3.0

4.0

5.0

20 40

Number of breeding females

Numberofsurviving

youngperfemale

605030100 70 80

(a)

(b)

FIGURE 24.18Density dependence in the song sparrow (Melospiza melodia)on Mandarte Island. Reproductive success decreases (a) andmortality rates increase (b) as population size increases.

FIGURE 24.19Density-dependent effects.Migratorylocusts,Locusta migratoria, are a legendaryplague of large areas of Africa and Eurasia.At high population densities, the locusts havedifferent hormonal and physicalcharacteristics and take off as a swarm. Themost serious infestation of locusts in 30 yearsoccurred in North Africa in 1988.


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Density-Independent Effects

Growth rates in populations sometimesdo not correspond to the logisticgrowth equation. In many cases, suchpatterns result because growth is under

the control of density-independenteffects. In other words, the rate ofgrowth of a population at any instant islimited by something other than thesize of the population.

A variety of factors may affect popula-tions in a density-independent manner.Most of these are aspects of the externalenvironment. Extremely cold winters,droughts, storms, volcanic eruptionsindividuals often will be affected by theseactivities regardless of the size of thepopulation. Populations that occur inareas in which such events occur rela-tively frequently will display erratic pop-ulation growth patterns in which thepopulations increase rapidly when condi-tions are benign, but suffer extreme reductions wheneverthe environment turns hostile.

Population Cycles

Some populations exhibit another type of pattern incon-sistent with simple logistic equations: they exhibit cyclicpatterns of increase and decrease. Ecologists have studiedcycles in hare populations since the 1920s. They havefound that the North American snowshoe hare (Lepusamericanus) follows a 10-year cycle (in reality, it varies

from 8 to 11 years). Its numbers fall tenfold to 30-fold ina typical cycle, and 100-fold changes can occur. Two fac-tors appear to be generating the cycle: food plants andpredators.

Food plants. The preferred foods of snowshoe haresare willow and birch twigs. As hare density increases, thequantity of these twigs decreases, forcing the hares tofeed on high-fiber (low-quality) food. Lower birthrates,low juvenile survivorship, and low growth rates follow.

The hares also spend more time searching for food, ex-posing them more to predation. The result is a precipi-tous decline in willow and birch twig abundance, and acorresponding fall in hare abundance. It takes two tothree years for the quantity of mature twigs to recover.Predators. A key predator of the snowshoe hare is theCanada lynx (Lynx canadensis). The Canada lynx shows a10-year cycle of abundance that seems remarkably en-trained to the hare abundance cycle (figure 24.20). Ashare numbers increase, lynx numbers do, too, rising in

response to the increased availability of lynx food.When hare numbers fall, so do lynx numbers, their foodsupply depleted.

Which factor is responsible for the predator-prey oscil-lations? Do increasing numbers of hares lead to overhar-

vesting of plants (a hare-plant cycle) or do increasing num-bers of lynx lead to overharvesting of hares (a hare-lynxcycle)? Field experiments carried out by C. Krebs andcoworkers in 1992 provide an answer. Krebs set up experi-mental plots in Canadas Yukon-containing hare popula-tions. If food is added (no food effect) and predators ex-cluded (no predator effect) from an experimental area, harenumbers increase tenfold and stay therethe cycle is lost.However, the cycle is retained if either of the factors is al-

lowed to operate alone: exclude predators but dont addfood (food effect alone), or add food in presence of preda-tors (predator effect alone). Thus, both factors can affectthe cycle, which, in practice, seems to be generated by theinteraction between the two factors.

Population cycles traditionally have been considered tooccur rarely. However, a recent review of nearly 700 long-term (25 years or more) studies of trends within popula-tions found that cycles were not uncommon; nearly 30% ofthe studiesincluding birds, mammals, fish, and crus-

taceansprovided evidence of some cyclic pattern in popu-lation size through time, although most of these cycles arenowhere near as dramatic in amplitude as the snowshoehare and lynx cycles.

Density-dependent effects are caused by factors thatcome into play particularly when the population size islarger; density-independent effects are controlled byfactors that operate regardless of population size.


1845 1855 1865 1875 1885 1895 1905 1915 1925 1935

40

0

80

120

160

Year

Numberofpelts(in

thousands)

Snowshoe hare

Lynx

FIGURE 24.20Linked population cycles of the snowshoe hare and the northern lynx.These dataare based on records of fur returns from trappers in the Hudson Bay region of Canada.The lynx populations carefully track the snowshoe hares, but lag behind them slightly.


18/22

Population Growth Rates and LifeHistory Models

As we have seen, some species usually have stable popula-tion sizes maintained near the carrying capacity, whereas

the populations of other species fluctuate markedly and areoften far below carrying capacity. As we saw in our discus-sion of life histories, the selective factors affecting suchspecies will differ markedly. Populations near their carryingcapacity may face stiff competition for limited resources.By contrast, resources are abundant in populations farbelow carrying capacity.

We have already seen the consequences of such differ-ences. When resources are limited, the cost of reproduc-tion often will be very high. Consequently, selection will

favor individuals that can compete effectively and utilize re-sources efficiently. Such adaptations often come at the costof lowered reproductive rates. Such populations are termedK-selected because they are adapted to thrive when thepopulation is near its carrying capacity (K). Table 24.2 listssome of the typical features of K-selected populations. Ex-amples of K-selected species include coconut palms,whooping cranes, whales, and humans.

By contrast, in populations far below the carrying ca-

pacity, resources may be abundant. Costs of reproductionwill be low, and selection will favor those individuals thatcan produce the maximum number of offspring. Selec-tion here favors individuals with the highest reproductiverates; such populations are termed r-selected. Examplesof organisms displaying r-selected life history adaptationsinclude dandelions, aphids, mice, and cockroaches(figure 24.21).

Most natural populations show life history adaptationsthat exist along a continuum ranging from completely r-

selected traits to completely K-selected traits. Althoughthese tendencies hold true as generalities, few populationsare purely r- or K-selected and show all of the traits listedin table 24.2. These attributes should be treated as general-ities, with the recognition that many exceptions do exist.

Some life history adaptations favor near-exponentialgrowth, others the more competitive logistic growth.

Most natural populations exhibit a combination of thetwo.


Table 24.2 r-Selected andK-Selected LifeHistory Adaptations

r-Selected K-SelectedAdaptation Populations Populations

Age at first Early Late

reproductionLife span Short LongMaturation time Short LongMortality rate Often high Usually lowNumber of offspring Many Few produced perreproductive episodeNumber of Usually one Often severalreproductions per

lifetimeParental care None Often extensiveSize of offspring Small Largeor eggs

Source: Data from E. R. Pianka,Evolutionary Ecology, 4th edition, 1987,Harper & Row, New York.

FIGURE 24.21The consequences of exponential growth.All organisms havethe potential to produce populations larger than those thatactually occur in nature. The German cockroach (Blatellagermanica), a major household pest, produces 80 young every sixmonths. If every cockroach that hatched survived for threegenerations, kitchens might look like this theoretical culinarynightmare concocted by the Smithsonian Museum of NaturalHistory.


19/22

The Advent of Exponential Growth

Humans exhibit many K-selected life history traits, in-

cluding small brood size, late reproduction, and a highdegree of parental care. These life history traits evolvedduring the early history of hominids, when the limitedresources available from the environment controlled pop-ulation size. Throughout most of human history, ourpopulations have been regulated by food availability, dis-ease, and predators. Although unusual disturbances, in-cluding floods, plagues, and droughts no doubt affectedthe pattern of human population growth, the overall sizeof the human population grew only slowly during ourearly history. Two thousand years ago, perhaps 130 mil-lion people populated the earth. It took a thousand yearsfor that number to double, and it was 1650 before it haddoubled again, to about 500 million. For over 16 cen-turies, the human population was characterized by veryslow growth. In this respect, human populations resem-bled many other species with predominantly K-selectedlife history adaptations.

Starting in the early 1700s, changes in technology have

given humans more control over their food supply, en-abled them to develop superior weapons to ward offpredators, and led to the development of cures for manydiseases. At the same time, improvements in shelter andstorage capabilities have made humans less vulnerable toclimatic uncertainties. These changes allowed humans toexpand the carrying capacity of the habitats in which theylived, and thus to escape the confines of logistic growthand reenter the exponential phase of the sigmoidal growth

curve.Responding to the lack of environmental constraints, thehuman population has grown explosively over the last 300years. Whi le the bir thrate has remained unchanged atabout 30 per 1000 per year over this period, the death ratehas fallen dramatically, from 20 per 1000 per year to itspresent level of 13 per 1000 per year. The difference be-tween birth and death rates meant that the population grewas much as 2% per year, although the rate has now declinedto 1.4% per year.

A 1.4% annual growth rate may not seem large, but ithas produced a current human population of 6 billion peo-ple (figure 24.22)! At this growth rate, 77 million peopleare added to the world population annually, and the humanpopulation will double in 39 years. As we will discuss inchapter 30, both the current human population level andthe projected growth rate have potential consequences forour future that are extremely grave.


24.5 The human population has grown explosively in the last three centuries.

4000 B.C.

2

1

3

4

5

6

3000 B.C.2000 B.C.1000 B.C.

Year

IndustrialRevolution

Significant advancesin medicine throughscience and technology

Bubonic plague"Black Death"

Billionsofpeople

0 1000 2000

FIGURE 24.22History of human population size.Temporary increases indeath rate, even severe ones like the Black Death of the 1400s,have little lasting impact. Explosive growth began with theIndustrial Revolution in the 1700s, which produced a significantlong-term lowering of the death rate. The current population is 6billion, and at the current rate will double in 39 years.


20/22

Population Pyramids

While the human population as a whole continues to growrapidly at the close of the twentieth century, this growth isnot occurring uniformly over the planet. Some countries,like Mexico, are growing rapidly, their birthrate greatly ex-

ceeding their death rate (figure 24.23). Other countries aregrowing much more slowly. The rate at which a populationcan be expected to grow in the future can be assessedgraphically by means of a population pyramida bargraph displaying the numbers of people in each age cate-gory. Males are conventionally shown to the left of the ver-tical age axis, females to the right. A human populationpyramid thus displays the age composition of a populationby sex. In most human population pyramids, the number ofolder females is disproportionately large compared to thenumber of older males, because females in most regionshave a longer life expectancy than males.

Viewing such a pyramid, one can predict demographictrends in births and deaths. In general, rectangularpyramids are characteristic of countries whose popula-tions are stable, their numbers neither growing norshrinking. A triangular pyramid is characteristic of acountry that will exhibit rapid future growth, as most ofits population has not yet entered the child-bearing

years. Inverted triangles are characteristic of populationsthat are shrinking.Examples of population pyramids for the United States

and Kenya in 1990 are shown in figure 24.24. In the nearlyrectangular population pyramid for the United States, thecohort (group of individuals) 55 to 59 years old representspeople born during the Depression and is smaller in size

than the cohorts in the preceding and following years. Thecohorts 25 to 44 years old represent the baby boom. Therectangular shape of the population pyramid indicates thatthe population of the United States is not expandingrapidly. The very triangular pyramid of Kenya, by contrast,predicts explosive future growth. The population of Kenyais predicted to double in less than 20 years.


18951899

19201924

19451949

Time

Death rateMex

ico

Numberper10

00population

0

10

20

30

40

50

Birthrate

19851990

19701975

FIGURE 24.23Why the population of Mexico is growing.The death rate (redline) in Mexico fell steadily throughout the last century, while thebirthrate (blue line) remained fairly steady until 1970. Thedifference between birth and death rates has fueled a high growthrate. Efforts begun in 1970 to reduce the birthrate have beenquite successful, although the growth rate remains rapid.

75+

707465696064555950544549404435393034252920

24

151910145904

Percent of population

Age Kenya

024 2 46810 6 8 10

United States

024 2 4

Male

Female

FIGURE 24.24

Population pyramids from 1990. Population pyramids are graphed according to a populations age distribution. Kenyas pyramid has abroad base because of the great number of individuals below child-bearing age. When all of the young people begin to bear children, thepopulation will experience rapid growth. The U.S. pyramid demonstrates a larger number of individuals in the baby boom cohortthepyramid bulges because of an increase in births between 1945 and 1964.


21/22

An Uncertain Future

The earths rapidly growing human population constitutesperhaps the greatest challenge to the future of the bio-sphere, the worlds interacting community of living things.Humanity is adding 77 million people a year to the earth'spopulationa million every five days, 150 every minute! Inmore rapidly growing countries, the resulting populationincrease is staggering (table 24.3). India, for example, had apopulation of 853 million in 1996; by 2020 its populationwill exceed 1.4 billion!

A key element in the worlds population growth is itsuneven distribution among countries. Of the billion people

added to the worlds population in the 1990s, 90% live indeveloping countries (figure 24.25). This is leading to amajor reduction in the fraction of the worlds populationthat lives in industrialized countries. In 1950, fully one-third of the worlds population lived in industrialized coun-tries; by 1996 that proportion had fallen to one-quarter; in2020 the proportion will have fallen to one-sixth. Thus theworlds population growth will be centered in the parts ofthe world least equipped to deal with the pressures of rapidgrowth.

Rapid population growth in developing countries hasthe harsh consequence of increasing the gap between richand poor. Today 23% of the worlds population lives inthe industrialized world with a per capita income of$17,900, while 77% of the worlds population lives in de-veloping countries with a per capita income of only $810.The disproportionate wealth of the industrialized quarterof the worlds population is evidenced by the fact that85% of the worlds capital wealth is in the industrial

world, only 15% in developing countries. Eighty percentof all the energy used today is consumed by the industrialworld, only 20% by developing countries. Perhaps mostworrisome for the future, fully 94% of all scientists andengineers reside in the industrialized world, only 6% indeveloping countries. Thus the problems created by thefutures explosive population growth will be faced bycountries with little of the worlds scientific or technolog-ical expertise.

No one knows whether the world can sustain todayspopulation of 6 billion people, much less the far greaterpopulations expected in the future. As chapter 30 out-lines, the world ecosystem is already under considerable

stress. We cannot reasonably expect to continue to ex-

pand its carrying capacity indefinitely, and indeed we al-ready seem to be stretching the limits. It seems unavoid-able that to restrain the worlds future populationgrowth, birth and death rates must be equalized. If we areto avoid catastrophic increases in the death rate, thebirthrates must fall dramatically. Faced with this grim di-chotomy, significant efforts are underway worldwide tolower birthrates.

The human population has been growing rapidly for300 years, since technological innovations dramaticallyreduced the death rate.


Table 24.3 A Comparison of 1996 Population Data in Developed and Developing Countries

United States Brazil Ethiopia(highly developed) (moderately developed) (poorly developed)

Fertility rate 2.0 2.8 6.8Doubling time at current rate (yr) 114 41 23Infant mortality rate (per 1000 births) 7.5 58 120Life expectancy at birth (yrs) 76 66 50Per capita GNP (U.S. $; 1994) $25,860 $3,370 $130

1

2

3

Time

Worldpopulationinbillions

19501900 1990 2000

Developing countries

World total

2050 2100

4

5

6

7

8

9

10

11

0

Developed countries

FIGURE 24.25Most of the worldwide increase in population since 1950 hasoccurred in developing countries.The age structures ofdeveloping countries indicate that this trend will increase in thenear future. The stabilizing of the worlds population at about 10billion (shown here) is an optimistic World Bank/United Nationsprediction that assumes significant worldwide reductions ingrowth rate. If the worlds population continues to increase at its1996 rate, there will be over 30 billion humans by 2100!


22/22


Chapter 24Summary Questions Media Resources

24.1 Populations are individuals of the same species that live together.

Populations are the same species in one place;communities are populations of different species thatlive together in a particular place. A community andthe nonliving components of its environmentcombine to form an ecosystem.

Populations may be dispersed in a clumped, uniform,or random manner.

1.What are the three types ofdispersion in a population?Which type is most frequentlyseen in nature? Why?2.What are some causes ofclumped distributions?

The growth rate of a population depends on its agestructure, and to a lesser degree, sex ratio.

Survivorship curves describe the characteristics ofmortality in different kinds of populations.

3.What is survivorship?Describe the three types ofsurvivorship curves and giveexamples of each.4.What is demography? Howdoes a life table work?

24.2 Population dynamics depend critically upon age distribution.

Organisms balance investment in currentreproduction with investment in growth and futurereproduction.

5.Why do some birds lay fewerthan the optimal number ofeggs as predicted by DavidLack?

24.3 Life histories often reflect trade-offs between reproduction and survival.

Population size will change if birth and death rates

differ, or if there is net migration into or out of thepopulation. The intrinsic rate of increase of apopulation is defined as its biotic potential.

Many populations exhibit a sigmoid growth curve,with a relatively slow start in growth, a rapid increase,and then a leveling off when the carrying capacity ofthe environment is reached.

Large broods and rapid rates of population growthcharacterize r-strategists.K-strategists are limited inpopulation size by the carrying capacity of theirenvironments; they tend to have fewer offspring andslower rates of population growth.

Density-independent factors have the same impact ona population no matter what its density.

6. Define the biotic potential ofa population. What is thedefinition for the actual rate ofpopulation increase? What othertwo factors affect it?7.What is an exponentialcapacity for growth? When doesthis type of growth naturallyoccur? Give an example.8.What is carrying capacity? Isthis a static or dynamic measure?Why?9.What is the differencebetween r- andK-selectedpopulations?


Exponential growth of the worlds human populationis placing severe strains on the global environment.10. How do populationpyramids predict whether apopulation is likely to grow orshrink?

24.5 The human population has grown explosively in the last three centuries.

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Introduction toPopulations Population

Characteristics

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Human Population

Stages of PopulationGrowth

Population Growth Size Regulation

Scientists on Science:Coral ReefsThreatened

Student Research:Prairie HabitatFragmentation

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Documents

Chapter 24 Population Ecology