Biogeography

[I]t is...those [species] which range widely over the world, are the most diffused within their own country, and are the most numerous in individuals, which often-est produce well-marked varieties, or as I consider them, incipient species.

—Charles Darwin, 1859

In this passage from the second chapter of The Origin of Species, Charles Darwin was reporting the results of his tabulations of several well-studied regional floras. Darwin never published those data, but his suggestion that species that are widespread tend to be both more abundant and more variable than species with narrower ranges has been supported by recent evidence. Thus, in addition to his contributions to evolutionary theory and ecology, Darwin anticipated modern advances in the field of biogeography.

Widespread species are often abundant locally, but no species is found everywhere. The study of the distribution of organisms over Earth's surface began when eighteenth-century European explorers, settlers, and travelers started to take note of the vast differences among the biota on the different continents and attempted to understand them.

When the first Europeans arrived in Australia, they saw plants and animals that differed in perplexing ways from the ones they knew at home. Among these oddities were flowers pollinated by brush-tongued parrots and mammals that hopped around on their hind legs, carrying their offspring in pouches. In contrast, the first Europeans to visit North America felt at home because the plants (such as oaks, elms, and pines) and animals (such as deer, rabbits, foxes, thrushes, and crows) of North America were similar to those of Europe. Why was North America's biota so similar to Europe's while Australia's was so bizarrely different?

Biogeography is the science that documents and attempts to explain the patterns of distribution of populations, species, and ecological communities across Earth. In this chapter, we will show how biogeographers identify the processes that influence the distributions of species, both those that operated in the remote past and those that are operating today. We will also review Earth's major bio-geographic regions. Finally, we will look at the factors that influence the number of species that live together.

An Australian Endemic The numbat (Myrmecobius fasciatus) is an Australian marsupial mammal that uses its highly specialized tongue to feed on ants and termites. Unrelated and different-appearing animals on other continents also feed on these insects, but the numbat, like many other mammals, is unique to Australia.

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