Zoogeographic Regionalization

the division of the earth and its individual land areas and waters into zoo-geographical zones, based on zoogeographic data.

The most widely accepted units of zoogeographic regionalization are the realm, the region, the subregion, the province, the district and the sector. Sharp boundaries between these divisions occur only in instances when the boundaries are based on certain physicogeographic characteristics, for example, the boundary between water and dry land, the barriers formed by mountain ranges, and sharp boundaries between natural regions.

Transitional zones of different widths usually lie between zoogeographic regions, and here the individual faunal elements, such as species and groups of species, mix and overlap. The conditions of existence and the composition of the fauna in the sea and on land are so different that independent systems of zoogeographic regionalization have been worked out for marine and terrestrial faunas. The distribution of freshwater fauna is generally similar to that of terrestrial fauna. Hence it is subdivided according to the system used for terrestrial fauna.

The large and small faunal zones identified by zoogeo-graphic regionalization differ in rank and in degree of endemism, as well as in the historical development of their faunas. The largest subdivisions, realms, are characterized by endemism of orders and by a large proportion of endemic forms, the regions by endemism of families, the subregions by endemism of genera, and the provinces by the presence of endemic species.

In the division of dry land into zones, the highest zoogeo-graphic categories, the vertebrates—chiefly mammals—form the main groups. Three realms are usually distinguished, according to the presence or absence of representatives of certain subclasses of mammals: the Notogea, comprising the Australian region, with oviparous mammals, many marsupials, and few placental mammals; the Neogea, consisting of the neotropical region, with no oviparous mammals and few marsupials; and the Arctogea, comprising all other regions, with only placental mammals.

Many different systems of zoogeographic subdivision of land into regions have been proposed from time to time. The division of land into six zoogeographic regions is more or less generally accepted: the Australian region (Australia and the Pacific islands), the neotropical region (South and Central America), the Ethiopian region (Africa below the Sahara and Madagascar), the Indo-Malay, or Oriental, region (Hindustan, Indochina, Malay Archipelago), the Holarctic region (North America, Asia, excluding the area occupied by the Indo-Malay region, Europe, and North Africa, including the Sahara), and the antarctic region (Antarctica and adjacent islands). The Holarctic region is sometimes divided into two regions, the Nearctic (North America) and the Palaearctic (the remaining part).

The above zoogeographic regionalization of land is appli-cable to all groups of the animal kingdom with comparatively few deviations in the boundaries of regions and their main subdivisions. This is explained by the fact that the fauna of each region had a common geological history and now dwells under similar conditions. Zoogeographic division is based on the present distribution of animals, but insofar as all groups of organisms and all faunas are products of historical development, each zoogeographic subdivision is also determined historically and bears features of the epoch in which the fauna developed. For example, the fauna of the Australian region, on the whole the most ancient, preserves features of the Cretaceous, while Holarctic fauna, the youngest, preserves features of the Pleistocene.

In the zoogeographic regionalization of the ocean, separate systems have been adopted, on the one hand, for the water strata and the sea bottom and, on the other hand, for each vertical zone. This may be explained, first, by the sharp differentiation between the water strata population (pelagic fauna) and bottom-living population (benthic fauna) and, second, by the pronounced vertical zonation in the distribution of marine organisms. Each vertical zone has its particular habitat conditions and its distinct species of fauna and historical development.

In the pelagic portion of the sea the zoogeographic regions generally coincide with climatic latitudinal zones, and the differentiation between regions is less sharp than on the sea bottom, where two wholly different faunas may be distinguished—the comparatively shallow-water fauna of the continental shelf (at a depth of 200-300 m) and the deep-water fauna inhabiting the ocean depths. Three realms are usually distinguished on the continental shelf: the realm of the cold and temperate seas of the northern hemisphere, the tropical realm, and the realm of the cold and temperate seas of the southern hemisphere.

The realm of the cold and temperate seas of the northern hemisphere may be divided into two regions, the arctic and the boreal, or into three regions, the arctic and two boreal regions, called the North Atlantic and the North Pacific. Each region has its endemic families, pointing to the existence of ancient centers where contemporary North Atlantic and North Pacific faunas originated. The fauna of the North Pacific region, very rich and diverse, has preserved many ancient Tertiary forms because of comparatively few climatic changes in the geological past. The fauna of the North Atlantic region is impoverished because it became largely extinct in the glacial period, although representatives of several endemic North Atlantic families have survived. The tropical realm is characterized by the development of coral reefs and mangrove thickets, by an abundance of endemic orders and families, by the presence of endemic classes, and by a richness and diversity of marine vertebrates and invertebrates. Three regions may be distinguished here: the Indo-West Pacific region, the West African, or Guinean, region, and the Central American region, which includes both the Atlantic and Pacific coasts. Some zoologists distinguish a Mediterranean-Lusitanian region adjoining the tropical realm. The fauna of this region is linked with the warm-water fauna of the ancient Tethys marine basin, from which was derived the fauna of the entire tropical realm. The region contains typical tropical families and genera and possesses a large number of endemic species.

Three regions may be distinguished in the realm of the cold and temperate seas of the southern hemisphere: the Kerguelen region (Kerguelen Islands, Prince Edward Islands, Crozet Islands, Macquarie Island), the Patagonian region (the shelf off South America, Tierra del Fuego, Strait of Magellan, Falkland Islands), and the antarctic region (the continental shelf off Antarctica, South Georgia and nearby islands). Also, faunas with endemic elements are characteristic of the shelf off New Zealand, Southwest Africa, and the southern coasts of Australia. They have been little studied and their zoogeo-graphic rank has not been established. Each region is sub-divided into subregions, provinces, and sometimes districts.

The bottom-living, or abyssal, fauna is less differentiated zoogeographically. The ocean is divided into three regions: the Pacific-North Indian, the Atlantic, and the antarctic, with each subdivided into subregions and provinces.