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 zoo-and 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 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.

One should not think of biogeographic boundaries as static. Paleontological evidence suggests that the Palaearctic/Oriental boundary in China has swung back and forth through the Pleistocene, partly in concert with climatic cycles and partly as a consequence of the gradual uplift of the Tibetan plateau. Again, in the Late Pleistocene, the fauna of the Maghreb (North Africa) was overwhelmingly Afrotropical; the succeeding millennia saw the progressive extinction of Afrotropical elements, and today it is Palaearctic in nature.

Primates follow general biogeographic trends as well as any other group. The Lorisiformes and Catarrhini have an overall Afrotropical and Oriental distribution; the Lemuriformes and Chiromyiformes are wholly Madagascan; the Platyrrhini are Neotropical. There are no Holarctic nonhuman primates, except as marginal “overspills” from the Palaeotropical core (mainly Macaca in China, Japan, and North Africa); and there are no Australian nonhuman primates at all. Within each area, major river barriers (the Congo and its tributaries in Africa, the Amazon and its tributaries in the Neotropics, the Mekong and Brahmaputra in the Oriental region) separate closely related species. Primates also, like other organisms, illustrate the principle of endemism: that isolated areas tend to have unique species. An example would be the Eastern Arc mountain forests of Tanzania, a region that has many endemic species of both plants and animals, only distantly related to their relatives in Central Africa and elsewhere; endemic primates include a species of mangabey (Cercocebus sanjei), a species of colobus (Piliocolobus gordonorum), and at least two species of bushbabies (Galagoides orinus and G. rondoensis).

As far as human evolution goes, the early stages are all Afrotropical; and there is increasing release from vicariance principles through time, first as the descendants of Homo ergaster spread out of Africa, culminating in the invention of boats and crossing of ocean barriers by Homo sapiens. Even at the beginning of the Late Pleistocene, humans were still subject to zoogeographic principles. The nature of the fauna preserved in the caves of the Carmel and the Galilee in Israel fluctuated according to climatic cycles, from predominately Palaearctic to predominately Afrotrop-ical, and the human species changed back and forth alongside the rest of the fauna: Whenever the Israeli cave faunas were Palaearctic, the humans were Neandertal, and when the faunas were Afrotropical, the humans were Homo sapiens. There can be no more eloquent example of the relevance of biogeography to anthropology.