Effect of Habitat

We have so far reviewed two kinds of explanations for the species-area effect: (1) relation to a species-abundance distribution (as in sampling) and (2) via the MacArthurWilson mechanism, extinction rate being assumed a function of population size. A third sort of explanation was advanced by Williams, who argued that area is just a proxy for habitat diversity, and it is the latter that directly drives the number of species on islands. By this interpretation, each species has its associated habitat type, and as area increases, the amount of each habitat type also increases, exceeding the thresholds for each species'

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50 100 150 200 Habitat diversity

50 100 150 200 Habitat diversity

Figure 8 Breeding passerine birds on Aegean islands: Number of species vs habitat diversity. Reproduced from Watson GE (1964) Ecology and evolution of passerine birds in the islands of the Aegean Sea. PhD Dissertation, Yale University, with permission.

occurrence one-by-one. A classic example is the avifauna of the Aegean Islands. Number of species shows a very precise relation to habitat diversity (Figure 8), equaling or surpassing many of the highly regular species-area relations exhibited by other groups. Multiple regression showed habitat diversity a more important prediction than area, just as was shown for elevation with respect to Galapagos plants (see above). Similar results have been obtained for species counts in pieces of larger, mainland areas, for example, forest birds of eastern North America.

In addition to the species-area effect, a correlation with habitat diversity has also been postulated as accounting for the distance effect: Lack argued that far islands had fewer species than near because their habitats were less varied. A definitive test of the Lack vs. MacArthurWilson explanation of the distance effect was performed for birds and lizards of the Bahamas. Measuring the occurrences of various habitat types directly, investigators found using partial correlation that both isolation and habitat poverty contributed to a tendency for fewer species to occur on more distant islands.

Although most research concerning the effect of area, habitat, etc. is directed toward understanding the number of species on islands, recent work is exploring effects of such biogeographic variables on population and life-history properties of island species. We measured survivorship of the lizard Anolis sagrei on Bahamian islands and found a very strong inverse relation to height of vegetation on the island. The result may perhaps be surprising in

view of the tendency of these lizards to be arboreal and the apparent lack of resources on the scrubbiest islands (where annual survival was up to 80%!). The likely explanation is that itinerant bird predators prefer islands with higher vegetation, increasing the risk ofpredation on those islands. Studies involving life-history traits are becoming increasingly common, and we now turn to an area where this is of conservation importance, the understanding of extinction.

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