The main lesson of biogeography is that only evolution can explain the diversity of life on continents and islands. But there is another lesson as well: the distribution of life on Earth reflects a blend of chance and lawfulness. Chance, because the dispersal of animals and plants depends on unpredictable vagaries such as winds, currents, and the opportunity to colonize. If the first finches had not arrived in the Galápagos or Hawaii, we might see very different birds there today. If an ancestral lemur-like creature hadn't made it to Madagascar, that island (and likely the Earth) would have no lemurs. Time and chance alone determine who gets marooned; one might call this the "Robinson Crusoe effect." But there is also lawfulness. Evolutionary theory predicts that many animals and plants arriving in new and unoccupied habitats will evolve to thrive there, and will form new species, filling up ecological niches. And we will usually find their relatives on the nearest island or mainland. This is what we see, over and over again. One cannot understand evolution without grasping its unique interaction between chance and lawfulness—an interaction that, as we'll see in the next chapter, is critically important in understanding the idea of natural selection.
But the lessons of biogeography go further, into the realm of biological conservation. Island plants and animals adapt to their environments isolated from species that live elsewhere, their potential competitors, predators, and parasites. Because species on islands don't experience the diversity of life found on continents, they aren't good at coexisting with others. Island ecosystems, then, are fragile things, easily ravaged by foreign invaders who can destroy habitats and species. The worst of these are humans, who not only chop down forests and hunt, but also bring with them an entourage of destructive prickly pears, sheep, goats, rats, and toads. Many of the unique species on oceanic islands are already gone, victims of human activity, and we can confidently (and sadly)
predict that many more will vanish soon. In our lifetime we may see the last of the Hawaiian honeycreepers, the extinction of New Zealand's kakapos and kiwis, the decimation of the lemurs, and the loss of many rare plants that, while perhaps less charismatic, are no less interesting. Each species represents millions of years of evolution, and, once gone, can never be brought back. And each is a book containing unique stories about the past. Losing any of them means losing part of life's history.
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