area of animal and plant distribution having similar or shared characteristics throughout.

It is a matter of general experience that the plants and animals of the land and inland waters differ to a greater or lesser degree from one part of the world to another. Why should this be? Why should the same species not exist wherever suitable environmental conditions for them prevail?

Geographic regions around the world that have similar environmental conditions are capable of harbouring the same type of biota. This situation effectively separates the biosphere into biomes (biome)—ecological communities that have the same climatic conditions and geologic features and that support species with similar life strategies and adaptations. The biome is the fundamental unit of which larger biogeographic regions (floral kingdoms and faunal realms) consist. The tropical forest is one type of terrestrial biome; it is located at various points around the planet where climatic and geologic conditions produce similar environments. The tropical forest biome contains the same general kinds of biological communities (community) wherever it occurs; however, the individual species will not be the same from one tropical forest to another. Instead, each forest will support organisms that are ecologically equivalent—i.e., different species that have a similar life cycle and have adapted analogously to environmental conditions.

Biogeography, the study of animal and plant distributions (and known individually as zoogeography and phytogeography, respectively), was a subject that began to receive much attention in the 19th century. One of the first modern delimitations of biogeographic regions was created in 1858 by the English ornithologist Philip L. Sclater, who based his division of the terrestrial world on the distributions of birds. In the 1870s the biologist Adolf Engler devised a schema based on plant distributions. The phytogeographic work of Sir Joseph Dalton Hooker (Hooker, Sir Joseph Dalton), a plant collector and systematist, and the zoogeographic work of Alfred Russel Wallace (Wallace, Alfred Russel) greatly influenced the work of Charles Darwin. The Darwinian (Darwin, Charles) theory of evolution, accordingly, was firmly rooted in the emerging biogeographic understanding of the era; in On the Origin of Species Darwin included two key chapters (12 and 13) on geographic distribution in which he referred to both Hooker and Wallace. At high altitudes in the tropics Hooker had found plants that were normally restricted to temperate zones, and Darwin interpreted these observations as evidence of past climatic change. Darwin also adopted Wallace's view of faunal (faunal region) distribution among islands: those islands exhibiting similar faunas are separated only by shallow water and were once a contiguous landmass that presented no barrier to animal dispersal, whereas those islands whose faunas are dissimilar are separated by deep seaways that have always existed and barred the migration of species.

Geographic factors have played a significant role at every level of taxonomic division. Populations that become isolated by means of a geographic barrier will tend to diverge from their species. Although these barriers—which include seaways, rivers, mountain ranges, deserts, and other hostile environments—appear minor, they nevertheless can put a wedge between taxa, eventually causing related species, genera, families, and so on (on up the taxonomic hierarchy) to diverge. An example of this mechanism is seen in the Gregory Rift Valley, the eastern branch of the East African Rift System; distinctive subspecies of wildebeest are represented on either side of the rift valley, with the subspecies Connochaetes taurinus albojubatus occurring on the east side and C.t. hecki on the west. Other mammals such as blue, or diadem, monkeys (Cercopithecus mitis) exhibit similar geographic variation. The broad Congo River in central Africa is a barrier between many congeneric species (those that share the same genus) of primates, such as the common chimpanzee (Pan troglodytes) found on the north side of the river and the pygmy chimpanzee (P. paniscus), or bonobo, living to the south of the river. More significant biogeographic divisions occur between genera of the same family that live on different continents, as is the case with African elephants (Loxodonta) and Asian elephants (Elephas). Whole families or suborders may differ from one major biogeographic realm to another, as is seen in the primate divisions of Old World monkeys (catarrhines), which are found in Africa and Asia, and the New World monkeys (platyrrhines) from South America.

Within historical biogeography, two views—the dispersalist and vicariance hypotheses of biotic distribution patterns—have been at odds. According to the dispersalist view, speciation occurs as animals spread out from a centre of origin, crossing preexisting barriers that they would not readily recross and that would cut them off from the original group. The vicariance explanation states that a species that is present over a wide area becomes fragmented (vicariated) as a barrier develops, as occurred through the process of continental drift. These patterns, however, are not mutually exclusive, and both provide insight into the modes of biogeographic distribution. Traditionally biogeographers—and of these mainly zoogeographers such as William Diller Matthew, George Gaylord Simpson, and Philip J. Darlington, Jr.—accepted a number of explanations for the modes of species distribution and differentiation that generally fell into a dispersalist view.

In a series of works from the 1950s and '60s the maverick Venezuelan phytogeographer Leon Croizat strongly objected to this dispersalist explanation of species distribution, which he interpreted as ad hoc events used to explain the geographic distribution of living organisms. He maintained that the regularity in biogeographic relationships was too great to be explained by the chance crossings of barriers. In the 1970s his works sparked the development of the theory of vicarianism.

In spite of the polarization of these views among biogeographers, patterns of distribution can be explained by a combination of dispersalist and vicariance biogeography. Many biogeographers believe that the vicariance process forms the underlying mechanism of distributional diversity, with the dispersalist mode operating more sporadically.

A taxon whose distribution is confined to a given area is said to be endemic to that area. The taxon may be of any rank, although it is usually at a family level or below, and its range of distribution may be wide, spanning an entire continent, or very narrow, covering only a few square metres: a species of squirrel (Sciurus kaibabensis) is endemic to the Kaibab Plateau in Arizona (U.S.), the primate family Lemuridae is endemic to Madagascar, and the mammalian subclass Prototheria (monotremes) is endemic to the Notogaean (Australian) realm (see below The distribution boundaries of flora and fauna: Fauna: Notogaean realm (biogeographic region)). A distinction is often made between neoendemics (taxa of low rank 【e.g., species】 that have not had time to spread beyond their region of origin) and paleoendemics (taxa of high rank 【e.g., class】 that have not yet died out).

The concept of endemism is important because in the past the formulation of biogeographic regions was based on it. The limits of a region are determined by mapping the distributions of taxa; where the outer boundaries of many taxa occur, a line delimiting a biogeographic region is drawn. Major regions (kingdoms and realms) are still determined as those that have the most endemics or, stated another way, those that share the fewest taxa with other regions. As regions are further broken down into subdivisions, they will contain fewer unique taxa.

This method has been criticized because it assumes that species ranges are stable, which they are not. An alternative method of determining biogeographic regions involves calculating degrees of similarity between geographic regions. Similarities of regions can be quantified using Jaccard's coefficient of biotic similarity, which is determined by the equation:

If two areas are being compared, the coefficient of similarity, s, is determined by dividing the number of taxa shared between the areas, c, by the sum of c and the number of taxa peculiar to each area alone, a and b. The larger the coefficient, the more dissimilar are the areas.

Species diversity is determined not only by the number of species within a biological community—i.e., species richness—but also by the relative abundance of individuals in that community. Species abundance is the number of individuals per species, and relative abundance refers to the evenness of distribution of individuals among species in a community. Two communities may be equally rich in species but differ in relative abundance. For example, each community may contain 5 species and 300 individuals, but in one community all species are equally common (e.g., 60 individuals of each species), while in the second community one species significantly outnumbers the other four.

These components of species diversity respond differently to various environmental conditions. A region that does not have a wide variety of habitats usually is species-poor; however, the few species that are able to occupy the region may be abundant because competition with other species for resources will be reduced.

Trends in species richness may reveal a good deal about both past and present conditions of a region. The Antarctic continent has few species because its environment is so inhospitable; however, oceanic islands are species-poor because they are hard to reach, or, as is the case with the Lesser Sunda Islands in south-central Indonesia, because they are of rather recent origin and organisms have not had enough time to establish themselves.

Global gradients also affect species richness. The most obvious gradient is latitudinal: there are more species in the tropics than in the temperate or polar zones. Ecological factors commonly are used to account for this gradation. Higher temperatures, greater climate predictability, and longer growing seasons all conspire to create a more inviting habitat, permitting a greater diversity of species. Tropical rainforests are the richest habitat of all, tropical grasslands exhibit more diversity than temperate grasslands, and deserts in tropical or subtropical regions are populated by a wider range of species than are temperate deserts.

Another factor affecting the species richness of a given area is the distance or barrier that separates the area from potential sources of species. The probability that species will reach remote oceanic islands or isolated valleys is slight. Animal species, especially those that do not fly, are less likely than plant species to do so. The islands of the Lesser Sundas are similar to eastern Java in climate and vegetation, but they have far fewer strictly terrestrial animals. This situation is attributed to the fact that, whereas Java has been connected to a larger landmass in the past, the Lesser Sundas have not. While plants and seeds have been blown across intervening seas, few species of animals that do not have wings have reached these islands.

Neither an environment nor an organism is a static entity. Hence, changes in either will disrupt the relationship that has evolved between the two. Small changes in an organism may actually improve the interaction—a random genetic mutation allowing a plant to utilize a nutrient that has been present but previously unusable by the plant will increase the organism's ability to survive. Changes of an extreme nature, however, are almost always maladaptive. Small environmental variations may present a challenge that organisms can meet by mounting a physiological response or, if they are mobile, by removing themselves to a less stressful area. Catastrophic disruptions, however, may create an environment no longer hospitable to the organisms, and they may die out as a result.

Although the distribution patterns of species are dictated by environmental conditions, the actual range of a species is not identical to its potential range—namely, the area that is ecologically compatible with its needs. For example, the biogeographic regions of the world are related to climatic factors, but they are not coterminous with them. Thus, desert biomes, which are located at latitudes of 30° N and S, and tropical rainforest biomes, which arise around the Equator, can be found in most phytogeographic kingdoms and zoogeographic realms.

The theory of plate tectonics, formulated in the 1960s, is now firmly established. Its explanation of the dynamic nature of continental (continent) landmasses has been important not only within the field of geology but also within the field of biogeography; it has entirely revolutionized the interpretion of the dispersal of flora and fauna (see also plate tectonics: Plate tectonics as an explanation for Earth processes (plate tectonics)). The slow movement of continents has been used to explain both the isolation and intermingling of populations. Prior to the acceptance of this idea, land bridges (land bridge) and sunken continents were invoked as the means by which continents were linked in the geologic past. While land bridges, such as the Bering Strait land bridge that connected western North America to Asia, have existed and contributed to the dispersal of organisms, they no longer are believed to have been as ubiquitous and instrumental in this process as once was thought. Such hypothetical land bridges as Archhelenis, which purportedly connected South America and southwestern Africa, are now regarded by most experts as relics of the fertile imaginations of early biogeographers.

During much of the Mesozoic Era (245 to 66.4 million years ago), the continents formed a single mass that has been named Pangaea (Pangea). In the Early Cretaceous Period (144 to 97.5 million years ago), the Tethys seaway formed and split Pangaea into a northern continent, Laurasia (encompassing Eurasia and North America), and a southern continent, Gondwanaland (Gondwana) (including South America, Antarctica, Africa, India, and Australia). Notwithstanding transient and shifting epicontinental seaways, flora and fauna essentially were able to move freely within the Northern and Southern hemispheres but not between them. During the Late Cretaceous and Tertiary (97.5 to 1.6 million years ago), Gondwanaland split up and its component parts drifted apart, some of them forming connections with Laurasia, which remained more or less a continuous landmass. According to this model, Australia has remained separate from other continents since the Eocene Epoch (57.8 to 36.6 million years ago) and had been in contact only with an already polar Antarctica from the Late Cretaceous onward, which helps to explain its remarkably distinct flora and fauna. The life-forms of South America are only less distinctive than those of Australia. Separated from other continents since the Eocene, South America did not have a permanently established connection with North America until the Pliocene (5.3 to 1.6 million years ago). Only then was some interchange, especially of faunas, permitted. Africa had achieved proximity to Laurasia by the Paleocene (66.4 to 57.8 million years ago) and has remained in tenuous connection to Eurasia ever since, so that its present flora and fauna are much more similar to the rest of the Old World tropics. India had formed a broad connection with Laurasia in the early Tertiary and so has no strongly distinctive (paleoendemic) organisms.

Of what use are biogeographic classifications? In the past, classifying the flora and fauna into regions was primarily a descriptive event. Today, however, biogeographic classification, like biological taxonomy, is not an end in itself but rather a means to understanding the causative factors involved in evolution, whether they be the vicissitudes of geologic events or the dynamics of biological adaptation. In this sense a classification is not right or wrong so much as it is useful or not.

The sorting of animals and plants into major biogeographic regions is a useful, hypothesis-generating activity. When two taxa of organisms show similar variations in distribution, it is theorized that they have been subject to the same kinds of evolutionary processes, such as ecological constraints that favour certain adaptations or random geographic changes. In a survey of many taxa in a biological community, all may have similar distributional patterns; they may have been restrained by the same geographic barriers or been influenced similarly by climatic factors. When comparing the phytogeographic kingdoms with the zoogeographic realms, one is struck by both the broad agreement in outlines and the differences in details.

Curious discrepancies in these patterns do exist. Some organisms have been able to “skip over” climatic zones so that they are found in both northern and southern temperate zones but not in the intervening tropics. Others appear to have exceptional abilities to disperse to remote, isolated regions and survive. For example, members of the bird family Rallidae (rail) have dispersed throughout many islands, including New Caledonia, Lord Howe Island, Guam, and even the aptly named Inaccessible Island, and the giant tortoises (Geochelone) are found on the Galapagos Islands off the west coast of South America as well as on Seychelles off the east coast of Africa.

Discrepancies also exist between animal and plant distributions. For example, a separate kingdom, the South African (Capensic) kingdom, is recognized for plants but not for animals. In New Guinea the flora is classified in the Paleotropical kingdom, but the fauna is not considered to be of the corresponding Paleotropical realm and instead is classified in the Notogaean realm. Some of these discrepancies are more comprehensible than others. The lack of a faunal Capensic division may simply be a function of the greater mobility of animals. Such divisions, if they ever did exist within zoogeography, have been “swallowed up” by the surrounding Neogaean and Afrotropical faunas. Other differences, especially that of the flora and fauna of New Guinea, are less explicable.

Land and freshwater plant groups are older than the groups of animals with which they coexist; thus, the major phytogeographic regions reflect a more ancient phase in Earth history than do the zoogeographic regions. Because plants are less mobile, their associations have survived into the present relatively intact. The division of the major regions into minor subdivisions helps to elucidate more recent events in Earth history as well as the dispersal capabilities, adaptive strategies, and ecological relationships of the biota.

The importance of the climate's influence on biotic dispersal must not be overlooked. Marine organisms tend to be distributed along climatic lines, and many terrestrial groups, such as migratory birds, are so mobile that they have become spread across two or more major biogeographic areas. Although they are widely dispersed, they have specialized within northern and southern temperate zones, which are separated by the unsuitable tropical regions between.

These odd, disjunct distributions serve as reminders that biogeographic regions only sketch the outlines of organismal distributions and that they do not explain every case. What they are useful for is to point toward dispersal mechanisms, past climatic corridors, and other important biological phenomena.

This kingdom is divided into six regions.

The Eurosiberian region extends from Iceland around most of Europe via Siberia to Kamchatka. Conifers of the family Pinaceae—Pinus (pine), Larix (larch), Picea, and Abies (fir)—grow in vast, monospecific stands and give way to temperate deciduous forest to the south, tundra to the north, and moorlands (which contain Ericaceae 【heath family】, Carex 【sedge】, and Sphagnum moss in suitable areas). The western part of the region is much richer in species than the eastern part: there are about 100 genera that are endemic to Europe, with only about 12 endemic to Siberia.

Although different species have different dispersal abilities, even bird and insect distributions can be accounted for by traditional zoogeographic boundaries. In general, the distribution of terrestrial mammals, freshwater fish, and invertebrates seem to correspond well and provide the best evidence of zoogeographic divisions.

The following divisions are based on and modified to a great degree from the work of P.J. Darlington.

Specialists on freshwater fish and invertebrates prefer to divide the Holarctic more finely. Petru Banarescu recognizes the following regions: Euro-Mediterranean; Siberian, Baikal, and Western Mongolian; Eastern, Western, and Arctic North American; and Central Mexican.

Among the families characteristic of this realm are mammals such as Talpidae (moles), Castoridae (beavers), Ochotonidae (pikas); amphibians such as three families of salamanders, Salamandridae, Cryptobranchidae, and Proteidae; and invertebrates such as the freshwater crayfish family Astacidae.

Being in continuous geographic contact, the Paleotropical and the Holarctic realms merge into one another. Nevertheless, each has many distinct elements, in part but not entirely because of their different climates. The mammalian orders Pholidota (pangolins) and Proboscidea (elephants) are endemic to the Paleotropical region. Mammalian families that are confined to and extend across the realm include the Cercopithecidae (Old World monkeys), Lorisidae (lorises, bush babies, angwantibo, and potto), Hystricidae (Old World porcupines), Viverridae (civets and mongooses), Rhinocerotidae (rhinoceroses), and Tragulidae (chevrotains). Endemic avian families include Bucerotidae (hornbills) and Pittidae (pittas); and endemic reptilian families, Chamaeleontidae (Old World chameleons).

In striking contrast to the plant life in the southern tip of Africa, which makes up the South African, or Capensic, kingdom, the fauna of the Cape region cannot be distinguished from that of the surrounding regions. Presumably any unique faunal Capensic element that may have existed at one time has merged with the tropical element. African mainland endemic taxa include the mammalian orders Hyracoidea (hyraxes), Tubulidentata (aardvarks), and Macroscelidea (elephant shrews); the mammalian families Chrysochloridae (golden moles), Pedetidae (springhares), Thryonomyidae (cane rats), and Giraffidae (giraffes and okapi); the bird families Struthionidae (ostriches), Balaenicipitidae (shoebills), and Sagittaridae (secretary birds); the frog subfamily Phrynomerinae; the freshwater fish subclass Palaeopterygii (bichirs), and families Mormyridae (snoutfish) and Malapteruridae (electric catfish); and the snail family Aillyidae.

Seychelles and the Mascarene Islands have distant Madagascan affinities and are generally included in the Madagascan region.

Endemic families in the Oriental, or Sino-Indian, region include, among mammals, the Tupaiidae (tree shrews), Tarsiidae (tarsiers), and Hylobatidae (gibbons); among reptiles, the Lanthanotidae (earless monitor lizards) and Gavialidae (the crocodile-like gharials); and a few bird and invertebrate families.

Mammalian specialists such as G.B. Corbet place the approximate boundary between the Oriental region and the Holarctic in central China; however, Banarescu extends what he calls the Sino-Indian region north to include the Tien Shan mountain system, Tibet, and the Huang Ho, based on evidence of freshwater fish and invertebrates.

In the West Indies, which are an impoverished region within Neogaea, distinctive mammals include two endemic insectivore families, Solenodontidae (solenodon, almiqui) and the recently extinct Nesophontidae. The Galapagos Islands have an impoverished fauna ultimately derived from South America.

The Antarctic, or Archinotic, realm encompasses the Antarctic continent, subantarctic islands, and elements of southwestern New Zealand. The existence of the realm—or rather of its ghost, because nowhere today does it exist in an umixed state—is justified by the common occurrence in New Zealand and South America of such groups as the Eustheniidae (a family of stoneflies), the crustacean order Stygocaridacea, and certain freshwater snails. It is plausible that the marsupial family Microbiotheriidae, which is confined to Chile and is more closely related to the Australian marsupials than to other South American ones, is a relic of an Antarctic connection.