Biogeography

The Scientific Study of the Distribution of Animals and Plants

Animal and plant species are not global in distribution; nor are they distributed at random around the globe. Their ranges are constrained by part events and by past and present environments; so we can speak, in broad terms, of historical and ecological biogeography, though these two subfields are very strongly interdependent. One constantly finds that the species on either side of a barrier, such as a river, a mountain range, a sea channel, or a belt of hostile terrain, are different; the question arises whether the division is due more to the physical barrier itself or to subtle ecological differences on either side.

Zoogeographers (those who study animal distribution) and phytogeographers (who study plant distribution) each divide the world into major regions; these coincide in broad outlines, but differ in detail. The corresponding regions are as follows.

Within each of the floral kingdoms (except the Capensic) are distinct subkingdoms, and within each of the faunal realms are distinct regions; in many cases, these broadly correspond, like the larger divisions themselves. For example, the Palaeotropical in both cases divides easily into African (or Afrotropical), Madagascan, and Indian (or Oriental) subdivisions, but the floral kingdom has an extra subkingdom called “Malesian,” which includes all of Southeast Asia, which in the faunal case is divided between the Oriental region and the Australian realm. The Holarctic is divided into a Palaearctic (Eurasian) and a Nearctic (North American) region in zoogeography, but phytogeographers prefer a more complex division of their Holarctic kingdom.

The reasons for the differences between the zooand phytogeographic classifications are in some cases obscure, in others fairly easy to detect. Any faunal version of the floral Capensic kingdom has presumably been long since swamped by the movements of animals from tropical Africa. Possible elements of the floral Antarctic kingdom may be detected in cold regions of Chile, Tasmania, and New Zealand, but these too have largely been lost by faunal interchange. The real puzzle is the position of New Guinea: Why is its flora as typically Malesian as that of western Indonesia (and there are even Malesian elements in [Page 361]the rain forests of Far North Queensland),whereas its fauna is fundamentally Australian? At the moment, there are no clear-cut answers to this conundrum.

How these biogeographic divisions have come to be has been the subject of some polemic in the past. One model, dispersalism, holds that animals and plants spread out from centers of origin, crossing barriers and speciating (forming new species) as they go. The other major model, vicariance, sees animals and plants spreading widely, after which the barriers arose, fragmenting their formerly continuous ranges; plate tectonics, which in geologic time has split and rejoined whole continents, is just one of many ways in which vicariance has taken place. One can assume that vicariance explains most biogeographic patterns, simply because there is such wholesale concordance of distribution patterns between widely disparate species; dispersal across preexisting barriers seems to require more powers of endurance than most organisms possess. Although the most unlikely animals do occasionally survive long sea voyages, whether by swimming or flying or clinging to floating vegetation, this is surely not the usual way in which they got to their present homes.

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