The Structure of Ecosystems13.2

440 MHR • Unit 5 Population Dynamics

As mentioned in section 13.1, ecosystems containboth biotic and abiotic components. Eachecosystem consists of all the organisms in one ormore communities, as well as the physical andchemical factors affecting them. The boundaries ofecosystems are not distinct — one ecosystem mayoverlap or exist inside another. There are terrestrial(land-based) and aquatic (water-based) ecosystems,and ecosystems that contain both land and water.An ecosystem can be small (such as the one shownin Figure 13.14), or as large as the biosphere — aglobal ecosystem.

Figure 13.14 Small insects fall into the pitcher plant’s cup-shaped leaves, drown, and decompose. This plant, whichtypically lives in nitrogen-poor environments, extracts thenutrients it requires from the bodies of insects. The pitcherplant and its surroundings comprise a small ecosystem.

Despite the wide range in sizes and types ofecosystems, the same basic processes take place in all of them. Two processes are particularlyimportant: energy flow and chemical cycling. Theproper functioning of these processes is vital to thesurvival of organisms in the ecosystem and to theintegrity of the ecosystem itself.

Trophic StructureWhen ecologists refer to the trophic structure of anecosystem or community, they are describing thefeeding relationships among its members. Eachspecies is assigned to a specific trophic level in the structure, depending on its main source ofnutrition. Most ecosystems have several trophiclevels through which energy flows and chemicals(matter) cycle.

The first (or lowest) trophic level consists ofautotrophic organisms. Autotrophs are organismsthat can make energy-rich organic molecules (suchas glucose) from the raw materials available in theenvironment. They then break these “homemade”organic molecules down during cellular respirationto provide the energy that fuels the rest of their lifeprocesses. Photosynthetic autotrophs use theenergy of the Sun to drive this manufacturingprocess. Almost all plants, as well as some types of protists (algae) and bacteria (cyanobacteria), are photosynthetic autotrophs.

All organisms need energy to drive cellularprocesses. They must, therefore, have a source oforganic molecules from which they can release thisenergy during cellular respiration. Autotrophs,which are at the first trophic level, produce organicmolecules; this makes the first level the mostimportant. Because the first trophic level supportsall life at the higher levels, autotrophs are referred to as the primary producers in anecosystem. This first trophic level provides all thepotential energy required to drive the other levelsin the ecosystem.

All organisms in the trophic levels above thisone are heterotrophs. Heterotrophs are unable to

EXPECTAT IONS

Describe what is meant by the trophic structure of a community.

Describe the ecosystem roles of producers, consumers, and decomposers.

T H I N K I N G L A B

Shrinking Polar Bears andExpanding Snow Geese BackgroundAn incredible variety of living things inhabits the biosphere.This biotic diversity reflects the abiotic diversity of Earth.Through the process of evolution, populations have evolvedadaptations that enhance their survival and reproductiveability in diverse habitats. The result is the diversity of life.

During the last 100 years, scientists have reportedsignificant changes in the world’s ecosystems. While someof these changes seem to have been beneficial for somespecies, they seem to have had negative effects on others.For example, dramatic changes have been observed inpopulations of polar bears (Ursus maritimus) and lessersnow geese (Chen caerulescens) that breed in Canada’sarctic. Field surveys of polar bears have revealed thatpopulations are declining, fewer cubs are born each year,and individual bears are smaller and weigh less than whathas been considered typical for members of this species. Incontrast, snow goose populations have tripled since 1968.Their numbers are so high that the feeding activity of thesegeese is causing substantial damage to the habitats wherethey and other species breed and overwinter.

Why do you think these changes are happening? Incompleting this assignment, you will draw on yourknowledge of the nature of ecology and evolution. Youmight choose to work on one of these two species, andthen compare your findings with those of students whoworked on the other species.

You Try It1. Describe the ecological niches of the polar bear and the

snow goose. Include diet, preferred habitat, time ofpeak activity during a day, yearly activities such asmigration, and other relevant information.

2. Would you consider the polar bear to be a generalist or a specialist? What about the snow goose? Explainyour answer.

3. To which trophic levels do the polar bear and snowgoose belong?

4. Describe the feeding, sensory, and locomotoryadaptations that improve the ability of each species tosurvive and reproduce in its habitat and niche. Whatfeatures allow each species to cope with environmentalstresses (for example, temperature or moistureextremes) in its habitat?

5. Draw a map illustrating the approximate range of eachspecies.

6. Find out as much as you can about the evolutionaryhistory of these species. What other species are theymost closely related to? When might they haveappeared as a species?

7. How might changes that currently appear to beoccurring in Earth’s climate affect the future evolution of each of these species?

8. Estimate what chance each of these species has forsurviving for the next 100 years. Does either facepossible extinction? Why or why not?

441Chapter 13 Ecological Principles • MHR

make the energy-rich molecules they need to fueltheir life processes. Instead, they must obtain thesemolecules by consuming other organisms, eitherautotrophs or other heterotrophs. Therefore, theyare referred to as consumers.

Herbivores that eat autotrophs are termedprimary consumers, since they are the first eatersin the chain. On land, insects, snails, grazingmammals, and birds and mammals that eat seedsand fruit are the most common herbivores. Inaquatic ecosystems, this role is often taken byheterotrophic protists, various types of smallinvertebrate animals, and some species of fish.

Carnivores that eat mainly herbivores aresecondary consumers. Spiders, frogs, and insect-eating birds are examples of secondary consumers.In most ecosystems, these secondary consumers are themselves eaten by other carnivores, which are known as tertiary consumers (the third set

of eaters). There may also be higher levels ofconsumers above these.

The members of another consumer group, oftenreferred to as decomposers, obtain their energy-richmolecules by eating leftover or waste materialderived from all the trophic levels, including thefeces of living organisms, dead bodies, or bodyparts (for example, fallen leaves). Decomposers arevery important to every ecosystem. Their role is tobreak large molecules (that were once part of livingorganisms) down into smaller ones and return themto the abiotic environment. Thus, decomposersreturn vitally important nutrient elements such ascarbon and nitrogen to the soil and air. Thesematerials can then be used again by autotrophs to make new energy-rich organic molecules.Decomposers are an ecosystem’s recyclers, ensuringthat the biosphere’s limited supply of requirednutrients is not lost.

442 MHR • Unit 5 Population Dynamics

Food Chains and WebsThe trophic structure determines the route taken bythe energy and matter (chemical elements) containedin food as it moves through an ecosystem. Food istransferred from primary producers to primaryconsumers and then to secondary consumers along apathway referred to as a food chain (see Figure 13.15).

In reality, few ecosystems are so simple that theyconsist of only a single, unbranched food chain.More commonly, many species may feed on asingle species below themselves, while at the sametime a single upper-level consumer species may eatmany different species below itself. In addition,organisms may eat individuals from two or morelevels. The hawk shown in Figure 13.15 may eatmice, grasshoppers, and snakes, while the mouse

may eat plant parts (fruits and seeds) andgrasshoppers. In fact, many organisms, includinghumans, are referred to as omnivores because theyeat plants, animals, and other types of organisms.In other words, they are both primary and higher-level consumers. The result is that the feedingrelationships in most ecosystems form complexfood webs, rather than simple food chains. Figure 13.16 illustrates a food web.

In the Thinking Lab on page 441, you exploredhow communities are structured and the specificroles individuals can take within a community. The next section will show you how energy flowsthrough ecosystems.

Figure 13.15 Terrestrial and aquatic communities containdifferent species, but can have the same overall trophicstructure. “Plankton” is a general term referring to smallaquatic animals and protists. What distinguishes the twotypes of plankton?

Quaternaryconsumers

Tertiaryconsumers

Secondaryconsumers

Primaryconsumers

Primaryproducers

Terrestrial food chain Marine food chain

carnivore

plant

herbivore

carnivore

carnivore

carnivore

phytoplankton

zooplankton

carnivore

carnivore

Your Electronic Learning Partner has animation clips that will enhance your understanding of deep-sea ventcommunities.

ELECTRONIC LEARNING PARTNER

Some types of bacteria use energy derived from breakingthe chemical bonds in hydrogen sulfide molecules to formthe organic molecules they use as food. This type ofautotrophic food production is called chemosynthesis. It isquite rare and occurs in some very unusual environments,including ocean depths of 2500 m. At these depths there is no light and very little oxygen. In addition, hot magmafrom Earth’s molten core escapes to superheat thesurrounding water.

Some unusual marine organisms live near underseavents off the coast of British Columbia. These organismsinclude tube worms, limpets, and palm worms.

B I O F A C T

443Chapter 13 Ecological Principles • MHR

S E C T I O N R E V I E W

1. Explain why some ecosystems can supporthighly complex food webs while others can supportonly relatively simple ones.

2. Draw a chart to illustrate a food web that couldbe found in a typical pond ecosystem (show at leastthree distinct trophic levels). Identify the organismsand label each trophic level.

3. Explain how the same species can occupymore than one trophic level within the same foodweb. Explain how this type of ecological interactioncan enhance the stability of a food web.

4. Describe the types of biotic and abiotic factorsthat can lead to the collapse of a food web in anecosystem.

5. Explain why autotrophs rather thandecomposers occupy the lowest level of a food chain.

6. Could photosynthetic producers exist in theabsence of consumers in an ecosystem? Explain your answer.

7. Describe the typical energy sources thatdecomposers rely on in an aquatic ecosystem (such as a pond or lake).

8. Describe the difference between a food chainand a food web. Which is more realistic in itsdepiction of what actually exists in nature?

9. Explain why producer organisms that live deep below the surface of Earth’s oceans rely onchemosynthesis rather than photosynthesis tomanufacture high-energy food molecules.

K/U

K/U

K/U

K/U

K/U

K/U

K/U

C

K/U

Figure 13.16 A simplified food web. There are many other species that feed withor on the ones shown in the diagram. Which species eat organisms on more thanone trophic level?

Tertiary andsecondaryconsumers

Secondaryand primaryconsumers

Primaryconsumers

Primaryproducers

Decomposers

(bacteria, fungi, certain animals)

wastes and dead organisms

(plants, algae, cyanobacteria)