Tests of Species Equilibrium

A major claim of the MacArthur-Wilson theory was that a substantial portion of the world's islands are in fact at equilibrium, whereby the number of species colonizing balances the number becoming extinct. The evidence for a species equilibrium ranges from highly supportive to contradictory; we review the major cases in decreasing order of support.

1. Birds of Krakatau. In 1883 a tremendous volcanic eruption destroyed two-thirds of the Indonesian island of Krakatau and left its remnants and two neighboring islands under 30-60 m of ash, with no observable plants or animals surviving. As major confirmation of their theory, MacArthur and Wilson argued that equilibrium for land birds was re-attained only 25-36 years after the eruption. However, subsequent studies showed that the conclusion was premature: equilibrium has perhaps not yet been quite reached even after 100 years after the eruption. Much smaller gross extinction rates are now known to occur as compared to those predicted by MacArthur and Wilson (0.25-0.42% vs. 1-6% species per year).

2. Birds of the Channel Islands. The first systematic survey of birds on nine California Channel islands was accomplished in 1917. Since that time several surveys showed that the number of species over 50+ years has not changed much (Figure 2a). The degree to which the islands showed turnover was controversial and will be discussed in the next section.

3. Arthropods ofRed Mangrove Islets . The red mangrove Rhizophora mangle grows as an emergent shrub or tree from small floating dispersal structures that root on shallow marine banks; islands so created can range from moderately large down to extremely small. Investigators took advantage of the plethora of such islands to study experimentally the recolonization by arthropods that would occur after a devastating extinction event. A pestextermination company was hired to cover each mangrove islet with plastic sheeting and spray insecticide within; this eliminated most of the arthropods, so that when the sheeting was removed, artificially 'defaunated' islands resulted. Numbers of species rapidly increased, slightly overshooting the old equilibrium values before settling near it 1-2 years after inception of recolonization (Figure 2b).

4. Marine epifaunal invertebrates on rocks. To simulate colonization of rocks in the intertidal, artificial panels were set out in the Massachusetts (USA) subtidal. This experiment produced an oscillating equilibrium (Figure 2c), not unexpected in this highly seasonal environment. Thus the species equilibrium may manifest itself as a rather predictable oscillation, rather than necessarily having a constant value.

5. Plants on Krakatau. Plants showed a much slower rate of recovery after volcanic eruption than did birds ('1' above). In MacArthur and Wilson's original analysis, there was no real tendency for the colonization curve for plants even to begin leveling off. The most recent censuses (c. 100 years since the eruption) show that equilibrium has been nearly attained for seed plants (Figure 2d) and ferns. However, like birds, the gross extinction and immigration curves are not monotonically related to number of species present as assumed by MacArthur and Wilson (Figure 1); rather apparently ecological succession causes several changes in direction, especially for immigration. For example, water- and wind-dispersal plants were the first to colonize, whereas animal-dispersed species did not colonize until a certain amount of successional change had taken place.

6. Birds on islands off Australia and New Zealand. Records for 15 islands taken over periods ranging from 50 to 124 years showed that the number of passerine bird species increased on 14 islands, up to 900% of the original values. While humans have had effects on these islands, including habitat diversification in some cases (which would allow more species), those effects did not seem to be sufficient to account for such huge increases; perhaps climatic warming for these rather high-latitude islands was involved. The investigators characterized this variation as 'nonequilibrial'; certainly it stands in contrast to Channel Island birds ('2' above), as change was quite unidirectional.

7. Birds on Skokholm Island. Data on numbers of bird species for this rather northerly island off the British mainland (taken 1928-39 and then 1946-67) showed that number of species fluctuated between 5 and 13, with substantial temporal autocorrelation. These are large percent changes (over 200% by one measure), so is this evidence against equilibrium? Certainly it is evidence against species constancy; however, MacArthur and Wilson's algebraic theory (as opposed to the graphical one presented above) was a stochastic model with per-unit-time probabilities of immigration and extinction rather than fixed rates. Indeed, if we calculate for the Skokholm data the temporal variance and the mean number of species, the ratio is about 2/3, well within the possible range from the MacArthur-Wilson stochastic model, although higher than expected for equal (absolute) slopes of the gross extinction and immigration curves; an especially high extinction rate is consistent with the high ratio, unsurprising for such a small island.

8. Spiders and lizards on islands decimated by hurricanes. In 1997, certain Bahamas islands were completely inundated by the nearly 5 m storm surge of a major hurricane. As for Krakatau, no spider nor lizard individual survived; reco-lonization data a year later found spider species counts about where they were before the hurricane, whereas few islands to this day have been recolonized by lizards. In this case, the rapidly dispersing arthropods would be expected to reestablish equilibrium more quickly than large, terrestrial vertebrates such as lizards. Were hurricanes sufficiently frequent, certain taxa such as lizards might never reach a species equilibrium before the next disaster wiped them out again.

9. Arthropods in soybean fields. Soybean fields are an example of a highly temporary habitat that is frequently 'defaunated', here by scheduled human activities combined with climatic seasonality. The arthropods inhabiting such communities do not have time to reach equilibrium before being 'zeroed' again; hence they must constitute permanently nonequilibrium communities.

In summary, the value of the equilibrium concept varies from very useful for undisturbed islands to not so great for those that are frequently disturbed. The theory, however, does contain nonequilibrium dynamics, so that it is descriptively useful even for pre-equilibrium stages.

Year

Days

Species present
JJASONDJFMAMJJASONDJ

Time (months)

Figure 2 (Continued)

Figure 2 (a) Birds on the Channel Islands, California (USA). Number of breeding species S for each island plotted against survey year. The number written over the line connecting each pair of points is the percent turnover between those surveys. (b) Colonization curves of four mangrove islets, Florida (USA). E2 is the nearest island and E1 is the farthest island. (c) Oscillating equilibrium in marine epifaunal invertebrates on rocks. Gross immigration and extinction rate curves (top) and colonization curves (bottom). (d) Species number vs. time, that is, colonization curves (left), and gross immigration or extinction curves vs. average species number in intersurvey interval for seed plants of Krakatau (Rakata) (right). (a) Reproduced from Jones HL and Diamond JM (1976) Short-time-base studies of turnover in breeding bird populations on the California Channel Islands. Condor 78: 526-549, with permission. (b) Reproduced from Simberloff D and Wilson EO (1970) Experimental zoogeography of islands. A two-year record of colonization. Ecology 51: 934-937, with permission. (c) Reproduced from Osman RW (1977) The influence of seasonality and stability on the species equilibrium. Ecology 59: 383-399, with permission. (d) Reproduced from Thornton JWB, etal. (1993) Colonization of Rakata (Krakatau Islands) by nonmigrant land birds from 1983-1992 and implications for the value of island biogeography theory. Journal of Biogeography 20: 441-452, with permission from Blackwell Publishing Ltd.

Figure 2 (a) Birds on the Channel Islands, California (USA). Number of breeding species S for each island plotted against survey year. The number written over the line connecting each pair of points is the percent turnover between those surveys. (b) Colonization curves of four mangrove islets, Florida (USA). E2 is the nearest island and E1 is the farthest island. (c) Oscillating equilibrium in marine epifaunal invertebrates on rocks. Gross immigration and extinction rate curves (top) and colonization curves (bottom). (d) Species number vs. time, that is, colonization curves (left), and gross immigration or extinction curves vs. average species number in intersurvey interval for seed plants of Krakatau (Rakata) (right). (a) Reproduced from Jones HL and Diamond JM (1976) Short-time-base studies of turnover in breeding bird populations on the California Channel Islands. Condor 78: 526-549, with permission. (b) Reproduced from Simberloff D and Wilson EO (1970) Experimental zoogeography of islands. A two-year record of colonization. Ecology 51: 934-937, with permission. (c) Reproduced from Osman RW (1977) The influence of seasonality and stability on the species equilibrium. Ecology 59: 383-399, with permission. (d) Reproduced from Thornton JWB, etal. (1993) Colonization of Rakata (Krakatau Islands) by nonmigrant land birds from 1983-1992 and implications for the value of island biogeography theory. Journal of Biogeography 20: 441-452, with permission from Blackwell Publishing Ltd.

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