Effect of Distance

While the species-area effect is documented for nearly all quantitative studies of islands, a similar effect of distance is more rarely demonstrated. This is probably true for two reasons: first, the lack of variation in distance from an outside source for islands within single archipelagos, and second, even where a variety of distances are available, the necessity of taking into account the usually very strong effect of area before a distance effect can be detected. An example is given for lizards of the Lovely Bay archipelago in the Bahamas (Figure 7): lizards are expected to be especially sensitive to variation in distance because they can only disperse by rafting (or swimming) over water, a rather tenuous process at best. Distance is not always a hindrance to species, however, and may even be an advantage. Thus migrating birds in the Bahamas tend to increase their species numbers with increasing island distance; this is the opposite of resident birds in the same region, perhaps because the latter's relative scarcity on distant islands results in more resources for migrants there.

The impact of distance is also involved in the rescue effect, whereby near islands have their populations boosted more frequently by immigration, conferring dual advantages - large population size and genetic variability - both would tend to forestall extinction. Somewhat surprisingly, few examples from water-surrounded islands are known,

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Figure 7 Number of species of diurnal lizards on near (N) and far (F) islands in Lovely Bay, Bahamas, as a function of island area. Reproduced from Schoener TW and Schoener A (1983) Distribution of vertebrates on some very small islands. II: Patterns in species number. Journal of Animal Ecology 52: 237-262, with permission from Blackwell Publishing Ltd.

although more examples exist from islands sensu latu, so-called habitat islands (see above), including the founding study on arthropods inhabiting thistle heads.

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