August 9, 2012 report
Researchers find evidence that island biodiversity really is different from mainland
In the study, turnover was defined as the number of animals of the same species that live in different areas. They chose Anolis lizards and Terrarana frogs, because both have common ancestors that have radiated to virtually all of the Caribbean islands as well as to the mainland and because both are part of a large species group that has evolved different traits in different areas; 400 for Anolis and 850 for Terrarana.
To analyze the lizards and frogs and their turnover rates, the team randomly divided mainland areas into virtual islands of territory to allow for comparison with islands. They then turned to the historical record to determine how many of which kinds of the lizards and frogs live in which areas, and used that information to come up with turnover rates for each.
In posting their results, they showed three kinds of turnover comparison rates, mainland to mainland (M-M), mainland to island (M-I), and island to island (I-I). They found that the turnover rates for M-I were greater than for M-M, but that I-I were the greatest of all. This shows, albeit in a limited fashion, that turnover rates of island animals does appear to be something that is truly special as has often been observed by visitors and offers at least some degree of proof that the distinctive animals that are endemic to such places as Madagascar, the Caribbean islands and Hawaii, truly are unique and different due to their isolation over long periods of time.
Many oceanic islands are notable for their high endemism, suggesting that islands may promote unique assembly processes. However, mainland assemblages sometimes harbour comparable levels of endemism, suggesting that island biotas may not be as unique as is often assumed. Here, we test the uniqueness of island biotic assembly by comparing the rate of species turnover among islands and the mainland, after accounting for distance decay and environmental gradients. We modelled species turnover as a function of geographical and environmental distance for mainland (MM) communities of Anolis lizards and Terrarana frogs, two clades that have diversified extensively on Caribbean islands and the mainland Neotropics. We compared mainlandisland (MI) and islandisland (II) species turnover with predictions of the MM model. If island assembly is not unique, then the MM model should successfully predict MI and II turnover, given geographical and environmental distance. We found that MI turnover and, to a lesser extent, II turnover were significantly higher than predicted for both clades. Thus, in the first quantitative comparison of mainlandisland species turnover, we confirm the long-held but untested assumption that island assemblages accumulate biodiversity differently than their mainland counterparts.
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