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The Long-Term Stability of Ecosystems

Plant communities assemble themselves flexibly,and their particular structure depends on thespecific history of the area. Ecologists use the term“succession” to refer to the changes that happen in plant communities and ecosystems overtime. The first community in a succession is called a pioneer community, while the long-livedcommunity at the end of succession is called a climax community. Pioneer and successionalplant communities are said to change over periods from 1 to 500 years. These changes—inplant numbers and the mix of species—are cumulative. Climax communities themselveschange but over periods of time greater than about 500 years.

An ecologist who studies a pond today may well find it relatively unchanged in a year’stime. Individual fish may be replaced, but the number of fish will tend to be the same from oneyear to the next. We can say that the properties of an ecosystem are more stable than theindividual organisms that compose the ecosystem.

At one time, ecologists believed that species diversity made ecosystems stable. Theybelieved that the greater the diversity the more stable the ecosystem. Support for this ideacame from the observation that long-lasting climax communities usually have more complexfood webs and more species diversity than pioneer communities. Ecologists concluded thatthe apparent stability of climax ecosystems depended on their complexity. To take anextreme example, farmlands dominated by a single crop are so unstable that one year of badweather or the invasion of a single pest can destroy the entire crop. In contrast, a complexclimax community, such as a temperate forest, will tolerate considerable damage fromweather to pests.

The question of ecosystem stability is complicated, however. The first problem is thatecologists do not all agree what “stability” means. Stability can be defined as simply lack ofchange. In that case, the climax community would be considered the most stable, since, bydefinition, it changes the least over time. Alternatively, stability can be defined as the speedwith which an ecosystem returns to a particular form following a major disturbance, such as afire. This kind of stability is also called resilience. In that case, climax communities would bethe most fragile and the least stable, since they can require hundreds of years to return to theclimax state.

Even the kind of stability defined as simple lack of change is not always associated withmaximum diversity. At least in temperate zones, maximum diversity is often found inmid-successional stages, not in the climax community. Once a redwood forest matures, forexample, the kinds of species and the number of individuals growing on the forest floor arereduced. In general, diversity, by itself, does not ensure stability. Mathematical models ofecosystems likewise suggest that diversity does not guarantee ecosystem stability—just theopposite, in fact. A more complicated system is, in general, more likely than a simple system tobreak down. A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle.

Ecologists are especially interested to know what factors contribute to the resilience ofcommunities because climax communities all over the world are being severely damaged ordestroyed by human activities. The destruction caused by the volcanic explosion of MountSt. Helens, in the northwestern United States, for example, pales in comparison to thedestruction caused by humans. We need to know what aspects of a community are mostimportant to the community’s resistance to destruction, as well as its recovery.

Many ecologists now think that the relative long-term stability of climax communitiescomes not from diversity but from the “patchiness” of the environment, an environmentthat varies from place to place supports more kinds of organisms than an environment that isuniform. A local population that goes extinct is quickly replaced by immigrants from anadjacent community. Even if the new population is of a different species, it canapproximately fill the niche vacated by the extinct population and keep the food web intact.

Paragraph 1: Plant communities assemble themselves flexibly, and their particularstructure depends on the specific history of the area. Ecologists use the term “succession” torefer to the changes that happen in plant communities and ecosystems over time. The firstcommunity in a succession is called a pioneer community, while the long-lived community atthe end of succession is called a climax community. Pioneer and successional plantcommunities are said to change over periods from 1 to 500 years. These changes—in plantnumbers and the mix of species—are cumulative. Climax communities themselves change butover periods of time greater than about 500 years.

1. The word “particular” in the passage is closest in meaning to

○Natural

○Final

○Specific

○Complex

2. According to paragraph 1, which of the following is NOT true of climax communities?

○They occur at the end of a succession.

○They last longer than any other type of community.

○The numbers of plants in them and the mix of species do not change.

○They remain stable for at least 500 years at a time.

Paragraph 2: An ecologist who studies a pond today may well find it relatively unchangedin a year’s time. Individual fish may be replaced, but the number of fish will tend to be the samefrom one year to the next. We can say that the properties of an ecosystem are more stablethan the individual organisms that compose the ecosystem.

3. According to paragraph 2, which of the following principles of ecosystems can be learnedby studying a pond?

○Ecosystem properties change more slowly than individuals in the system.

○The stability of an ecosystem tends to change as individuals are replaced.

○Individual organisms are stable from one year to the next.

○A change in the members of an organism does not affect an ecosystem’s properties

Paragraph 3: At one time, ecologists believed that species diversity made ecosystemsstable. They believed that the greater the diversity the more stable the ecosystem. Supportfor this idea came from the observation that long-lasting climax communities usually havemore complex food webs and more species diversity than pioneer communities. Ecologistsconcluded that the apparent stability of climax ecosystems depended on their complexity.To take an extreme example, farmlands dominated by a single crop are so unstable that oneyear of bad weather or the invasion of a single pest can destroy the entire crop. In contrast, acomplex climax community, such as a temperate forest, will tolerate considerable damagefrom weather of pests.

4. According to paragraph 3, ecologists once believed that which of the following illustratedthe most stable ecosystems?

○Pioneer communities

○Climax communities

○Single-crop farmlands

○Successional plant communities

Paragraph 4: The question of ecosystem stability is complicated, however. The firstproblem is that ecologists do not all agree what “stability” means. Stability can be defined assimply lack of change. In that case, the climax community would be considered the moststable, since, by definition, it changes the least over time. Alternatively, stability can bedefined as the speed with which an ecosystem returns to a particular form following a majordisturbance, such as a fire. This kind of stability is also called resilience. In that case, climaxcommunities would be the most fragile and the least stable, since they can require hundredsof years to return to the climax state.

5. According to paragraph 4, why is the question of ecosystem stability complicated?

○The reasons for ecosystem change are not always clear.

○Ecologists often confuse the word “stability” with the word “resilience.”

○The exact meaning of the word “stability” is debated by ecologists.

○There are many different answers to ecological questions.

6. According to paragraph 4, which of the following is true of climax communities?

○They are more resilient than pioneer communities.

○They can be considered both the most and the least stable communities.

○They are stable because they recover quickly after major disturbances.

○They are the most resilient communities because they change the least over time.

Paragraph 5: Even the kind of stability defined as simple lack of change is not alwaysassociated with maximum diversity. At least in temperate zones, maximum diversity is oftenfound in mid-successional stages, not in the climax community. Once a redwood forestmatures, for example, the kinds of species and the number of individuals growing on the forestfloor are reduced. In general, diversity, by itself, does not ensure stability. Mathematicalmodels of ecosystems likewise suggest that diversity does not guarantee ecosystemstability—just the opposite, in fact. A more complicated system is, in general, more likely thana simple system to break down. (A fifteen-speed racing bicycle is more likely to break down thana child’s tricycle.)

7. Which of the following can be inferred from paragraph 5 about redwood forests?

○They become less stable as they mature.

○They support many species when they reach climax.

○They are found in temperate zones.

○They have reduced diversity during mid-successional stages.

8. The word “guarantee” in the passage is closest in meaning to

○Increase

○Ensure

○Favor

○Complicate

9. In paragraph 5, why does the author provide the information that “(A fifteen-speedracing bicycle is more likely to break down than a child’s tricycle)”?

○To illustrate a general principle about the stability of systems by using an everydayexample

○To demonstrate that an understanding of stability in ecosystems can be applied to helpunderstand stability in other situations

○To make a comparison that supports the claim that, in general, stability increases withdiversity

○To provide an example that contradicts mathematical models of ecosystems

Paragraph 6: Ecologists are especially interested to know what factors contribute to theresilience of communities because climax communities all over the world are being severelydamaged or destroyed by human activities. The destruction caused by the volcanic explosionof Mount St. Helens, in the northwestern United States, for example, pales in comparison tothe destruction caused by humans. We need to know what aspects of a community are mostimportant to the community’s resistance to destruction, as well as its recovery.

10. The word “pales” in the passage is closest in meaning to

○Increases proportionally

○Differs

○Loses significance

○Is common

Paragraph 7：Many ecologists now think that the relative long-term stability of climaxcommunities comes not from diversity but from the “patchiness” of the environment, anenvironment that varies from place to place supports more kinds of organisms than anenvironment that is uniform. A local population that goes extinct is quickly replaced byimmigrants from an adjacent community. Even if the new population is of a differentspecies, it can approximately fill the niche vacated by the extinct population and keep thefood web intact.

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