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Ecsystem Diversity and Stability
Cnservatin bilgists have lng been cncerned that species extinctin culd have significant cnsequences fr the stability f entire ecsystems—grups f interacting rganisms and the physical envirnment that they inhabit. An ecsystem culd survive the lss f sme species, but if enugh species were lst, the ecsystem wuld be severely degraded. In fact, it is pssible that the lss f a single imprtant species culd start a cascade f extinctins that might dramatically change an entire ecsystem. A gd illustratin f this ccurred after sea tters were eliminated frm sme Pacific kelp (seaweed) bed ecsystems: the kelp beds were practically bliterated t because in the absence f sea tter predatin, sea urchin ppulatins explded and cnsumed mst f the kelp and ther macralgae.
It is usually claimed that species-rich ecsystems tend t be mre stable than species-pr ecsystems. Three mechanisms by which higher diversity increases ecsystem stability have been prpsed. First, if there are mre species in an ecsystem, then its fd web will be mre cmplex, with greater redundancy amng species in terms f their nutritinal rles. In ther wrds, in a rich system if a species is lst, there is a gd chance that ther species will take ver its functin as prey, predatr, prducer, decmpser, r whatever rle it played. Secnd, diverse ecsystems may be less likely t be invaded by new species, ntably extics (freign species living utside their native range), that wuld disrupt the ecsystem’s structure and functin. Third, in a species-rich ecsystem, diseases may spread mre slwly because mst species will be relatively less abundant, thus increasing the average distance between individuals f the same species and hampering disease transmissin amng individuals.
Scientific evidence t illuminate these ideas has been slw in cming, and many shadws remain. ne f the first studies t prvide data supprting a relatinship between diversity and stability examined hw grassland plants respnded t a drught. Researchers D. Tilman and J A. Dwning used the rati f abve-grund bimass in 1988 (after tw years f drught) t that in 1986 (predrught) in 207 plts in a grassland field in the Cedar Creek Natural Histry Area in Minnesta as an index f ecsystem respnse t disruptin by drught. In an experiment that began in 1982, they cmpared these values with the number f plant species in each plt and discvered that the plts with a greater number f plant species experienced a less dramatic reductin in bimass. Plts with mre than ten species had abut half as much bimass in 1988 as in 1986, whereas thse with fewer than five species nly prduced rughly ne-eighth as much bimass after the tw-year drught. Apparently, species-rich plts were likely t cntain sme drught-resistant plant species that grew better in drught years, cmpensating fr the pr grwth f less-tlerant species.
T put this result in mre general terms, a species-rich ecsystem may be mre stable because it is mre likely t have species with a wide array f respnses t variable cnditins such as drughts. Furthermre, a species-rich ecsystem is mre likely t have species with similar eclgical functins, s that if a species is lst frm an ecsystem, anther species, prbably a cmpetitr, is likely t flurish and ccupy its functinal rle. Bth f these, variability in respnses and functinal redundancy, culd be thught f as insurance against disturbances.
The Minnesta grassland research has been widely accepted as strng evidence fr the diversity- stability thery; hwever, its findings have been questined, and similar studies n ther ecsystems have nt always fund a psitive relatinship between diversity and stability. Clearly, this is a cmplex issue that requires further field research with a brad spectrum f ecsystems and species: grassland plants and cmputer mdels will nly take us s far. In the end, despite insightful attempts t detect sme general patterns, we may find it very difficult t reduce this tpic t a simple, universal truth.
1. The wrd "significant" in the passage is clsest in meaning t
direct
imprtant
lng-term
surprising
2. Accrding t paragraph 1, why has the extinctin f species been a cncern fr cnservatin bilgists?
When ecsystems lse just ne species, they underg permanent change.
The extinctin f a particular predatr species culd cause an verppulatin f certain prey species.
The lss f ne r mre species culd cause the decline f a whle ecsystem.
The extinctin f a single species is evidence that plant-fd surces are in danger f disappearing.
3. Accrding t paragraph 1, what was the result f the remval f Pacific sea tters?
The kelp and sea urchins were destryed by new predatrs.
The uncntrlled ppulatin f sea urchins ate mst f the kelp plants.
Withut sea tters, the kelp beds sn became vergrwn.
Macralgae remained as the primary ppulatin in the ecsystem.
4. The wrd “redundancy” in the passage is clsest in meaning t
duplicatin
variety
requirements
flexibility
5. What is the functin f paragraph 2 in the passage?
T present a hypthesis abut ecsystem diversity and sme reasns why it might be true
T give examples f types f ecsystems that have the greatest diversity
T cntradict a previus belief abut the stability f species-rich ecsystems
T cntrast species-rich and species-pr ecsystems
6. Accrding t paragraph 2, which f the fllwing increases the stability f an ecsystem?
Species in which prducers utnumber predatrs
New r extic species that increase ecsystem diversity
Heavily ppulated species that are free f disease
Species that are diverse but have similar nutritinal rles
7. Which f the sentences belw best expresses the essential infrmatin in the highlighted sentence in the passage? Incrrect chices change the meaning in imprtant ways r leave ut essential infrmatin.
In any ecsystem, as the number f individuals in the same species increases, the rate f disease transmissin slws dwn.
Ecsystems that have a small number f different species tend t be disease-free, because the species’ habitats are at a safe distance frm each ther.
In ecsystems with many species, diseases spread mre slwly because there are fewer individuals in a species and, as a result, the individuals are mre widely scattered.
The average distance between individuals in a species-rich ecsystem increases, s diseases are prevented frm being cmmunicated between species.
8. The phrase ”cmpensating fr" in the passage Is clsest in meaning t
wrking against
leaving ut
making up fr
spreading ver
9. What Is the main imprtance f the study discussed in paragraph 3?
It examines the respnse f certain grassland plants t a drught.
It cntains an index f plants that survived well in times f drught.
It prvides scientific evidence that diversity helps t make ecsystems stable.
It shws that ecsystems cntain bth resistant species and less tlerant nes.
10. Select the TW answer chices that, accrding t paragraph 4, are cnclusins that can be drawn frm the study by Tilman and Dwning. T receive credit yu must select TW answer chices.
A diverse ecsystem will have species that respnd differently t a variety f cnditins.
Species within a species-rich ecsystem are mre likely t have cmpetitrs.
An ecsystem is mre likely t develp diverse and stable species when it is expsed t extreme cnditins.
Species with similar eclgical functins will perfrm the functin f a lst species.
11. The wrd “detect” in the passage is clsest in meaning t
repeat
alter
find
emphasize
12. Accrding t paragraph 5, which f the fllwing is true abut Tilman and Dwning’s findings?
General patterns f diversity and stability have been established as a result f the findings.
Questins abut the findings have been refuted by cmputer mdels.
The findings have been tested in a brad spectrum f ecsystems with similar results.
The findings are nt sufficient t prve a definite link between diversity and stability in ecsystems.
13. Lk at the fur squares [] that indicate where the fllwing sentence culd be added t the passage.
It seems clear that there is rm fr a great deal mre research, althugh sme wrk has been dne.
Where wuld the sentence best fit? Click n a square [] t add the sentence t the passage.
14. Directins: An intrductry sentence fr a brief summary f the passage is prvided belw. Cmplete the summary by selecting the THREE answer chices that express the mst imprtant ideas in the passage. Sme sentences d nt belng in the summary because they express ideas that are nt presented in the passage r are minr ideas in the passage. This questin is wrth 2 pints.
Drag yur answer chices t the spaces where they belng T remve an answer chice, click n it
T review the passage, click VIEW TEXT
Bilgists have lng been wrried abut the pssible effect f the extinctin f species n whle ecsystems.
Answer Chices
Cnservatin bilgy studies Indicate that the lss f a single imprtant species may bring temprary change t an ecsystem but it seldm results in lasting damage.
Ecsystems having species with similar functins but different respnses t adverse cnditins can survive envirnmental disturbances.
The Minnesta grassland study by Tilman and Dwning presented evidence that the greater the diversity f species in an ecsystem, the mre stable the ecsystem.
The absence f sea tter predatin in a Pacific kelp bed ecsystem dramatically changed the entire ecsystem by stabilizing the ttal kelp ppulatin.
The findings f the Minnesta grassland study by Tilman and Dwning indicated an equal number f drught-resistant and drught-tlerant plant species in species-rich plts.
Mre research is needed n the relatinship between species diversity and ecsystem stability, thugh a simple explanatin is unlikely.