Distribution and Biogeography

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Distribution and biogeography

Introduction

Mammals are distributed in virtually every part of the globe. The only extensive areas of land from which they are absent are the Antarctic ice caps. Even in the Antarctic, seals occur on the coastal ice and may haul out on shore. A few species live at high Arctic latitudes; polar bears (Ursus maritimus) have been recorded as far as 88°N and ring seals (Phoca hispida) have reached the vicinity of the North Pole. Mammals are found on all the remaining continents, on all except the most remote islands, and in all of Earth's oceans and seas. Marine mammals are known to reach depths of 3,280 ft (1,000m) while land mammals are found from below sea level to elevations above 21,500 ft (6,500 m). They are distributed in all biomes, including tundra, deserts, grasslands, and forests. Species in a wide range of families have adapted to an aquatic lifestyle in swamps, lakes, and rivers. Their distribution extends below the surface of the earth in the case of fossorial or burrowing mammals, and above it through adoption of an arboreal mode of life or by means of flight, in the case of bats. This essay focuses on modern zoogeography (animal distribution) and human effects on zoogeography in modern and historical times.

Mammals are classified into 26 orders, 136 families, and more than 1,150 genera. The number of living and recently extinct species exceeds 4,800. This figure fluctuates as new species are discovered and the status of certain forms is revised. Advances in techniques of molecular genetic analysis have enabled taxonomists to assign taxa with increasing certainty to either specific or subspecific status. During the 1990s intensive fieldwork in forested mountains along the border between Laos and Vietnam revealed the existence of several new large mammals. These consist of a new species and genus of bovid, the saola (Pseudoryx nghetinhensis), and three species of muntjac, a type of small deer. These also included a new genus, Megamuntiacus.

Fewer than 100 mammal species are either known or believed to have become extinct during the last 500 years. Some of these extinct species remain virtually unknown. For example the red gazelle (Gazella rufina) of North Africa is known only from three specimens obtained in Algeria toward the end of the nineteenth century; no living specimen has ever been described, and nothing is known of its ecology. Even more vague is the Jamaican monkey (Xenothrix mcgregori), which is known only from a sub-fossil jaw bone.

The order Tubulidentata contains only one species, the aardvark (Orycteropus afer), which is restricted to Africa. The Monotremata (monotremes) and four orders of marsupials are confined to the Australian region. Two orders of marsupials are found only in a relatively small area of South America. The two largest orders are Rodentia (rodents), with over 2,000 species and Chiroptera (bats), with almost 1,000. Both of these occur naturally on all continents except Antarctica, and they are the only orders to have reached many oceanic islands. Artiodactyla (even-toed ungulates) and Carnivora occur on all continents except Antarctica and Australia, though representatives of both have been introduced to Australia. The Cetacea (whales and dolphins) and Pinnipedia (seals and sea lions) have a worldwide distribution.

Similar wide variations exist at family and species level. No mammal species is truly cosmopolitan, that is, it occurs in every region and every habitat, though a few species have an extensive distribution covering several continents. The gray wolf (Canis lupus) has one of the widest distributions of any terrestrial mammal and is found across North America, Europe, the Middle East, central and northern Asia, and India. The common leopard (Panthera pardus) also has an extensive range through Africa, Arabia, Turkey, the Caucasus and much of Asia, north to the Russian Far East. In the New World, the mountain lion or cougar (Puma concolor) is distributed from Canada to southern Chile. At the other extreme, some species have a distribution that encompasses only a few square miles.

Some mammals show a discontinuous distribution. Thus the mountain hare (Lepus timidus) has its main distribution across the polar and boreal zones of Eurasia, but a disjunct population is still found at high elevations in the Alps, a relict from the last Ice Age when the mountains provided a refuge above the ice sheet. That of the lion (Panthera leo) is a relatively recent phenomenon, and a consequence of human activity. It is now found widely in eastern and southern Africa and in a small area of western India. Well into the historical period the distribution also included North Africa, the Middle East, southeast Europe, and Iran but habitat loss and hunting removed it from the intervening areas. In the late Pleistocene the lion reached even North America, where it

attained a large size, as did other Eurasian immigrants such as the mammoth and moose.

The diversity and richness of the mammal fauna present at any one locality is also subject to great variation. Some places contain representatives of a few orders or groups and have only a few species in total while others have an abundance of species from across a wide range of groups. The reasons for these differences are a complex combination of evolutionary history, degree of isolation, primary productivity, and habitat complexity. A latitudinal gradient is a general pattern, with the number of species increasing from the poles towards the tropics. A similar altitudinal gradient is also commonly observed, with the number of mammal species declining with increasing elevation, though in neither of these cases is the correlation uniform. There is also a generally positive correlation between species diversity and habitat complexity. Smaller islands tend to have impoverished mammal faunas, which is partly a function of the species-area relationship by which smaller areas tend to contain fewer species of all groups.

Ancestral forms of mammals evolved at a time when the continents were still connected in the single land mass of Pangaea. This split into two large continents, Laurasia in the Northern Hemisphere and Gondwana in the Southern Hemisphere. Mammals continued to evolve as these then slowly separated into a series of plates that drifted apart to form the present continents. When plates remained isolated for long periods of geological history as in the case of Madagascar, Australia, and South America, unique mammal faunas evolved that were very different from those elsewhere. In other cases, plates gradually collided or were connected intermittently when lower sea levels exposed land connections, allowing mammals to mingle and disperse. Fluctuations in sea levels alternately exposed and flooded land bridges between regions, thus permitting and preventing dispersal of mammal groups between the two areas at different times. On occasions, a chain of islands formed, rather than a full land bridge, allowing some species to cross, but preventing others from doing so. The result of these movements is that each region has a mammal fauna composed of elements that originated there and others that arrived subsequently and at different times from somewhere else. Dispersing species interact with the existing fauna and may replace or modify the existing species.

In many cases, groups of mammals became extinct in their center of origin but survived elsewhere. Equids first evolved in North America and dispersed into Asia, Europe, and Africa and also later into South America. Wild equids survive in Asia and Africa but disappeared from the Americas around 10,000

years ago until domesticated varieties were brought back by the Spanish conquistadors in the early part of the fifteenth century. Tapirs also evolved in North America and dispersed southwards into Central and South America and northwards into Asia. They subsequently became extinct in North America, leaving the current discontinuous distribution, with three species in Central and South America and one in Southeast Asia.

Offshore islands can be reached by swimming mammals, and a chain of islands allows species to reach the farthest islands on the stepping-stone principle, moving from one island to another. However, the farther away an island is from the nearest mainland, the fewer the number of species that succeed in reaching them. Bats can fly to them and small mammals may arrive by chance, carried there on logs or rafts of floating vegetation.

There are many other barriers to dispersal, such as large rivers, mountain ranges, deserts or other unsuitable habitat, the presence of predators, or competing species. Any factor or combination of factors that cause isolation may lead to further speciation (formation of new species). Variations in climatic conditions during the Pleistocene, especially the more than 20 major glaciations that occurred in the Northern Hemisphere, have also had a significant effect on the distribution of species.

Conditions during the Pleistocene also affected marine environments. For example, cold Arctic and Antarctic waters, rich in dissolved CO₂ and O₂, together with upswellings of mineral-rich deep ocean currents generated tremendous plantation productivity, providing a foundation for the evolution of baleen whale diversity.

Major regions

These differences are conventionally discussed in terms of the division of the world into six major regions as originally proposed in the nineteenth century and followed since with some modifications. These are the Nearctic, Neotropics, Palaearctic, Ethiopian or Afrotropical, Oriental, and Australian.

Nearctic

This region comprises North America up to northern Mexico and Greenland. Ten orders are present, including 37 families, and around 643 species. Two families are endemic, each containing a single species. These are the Antilocapridae (pronghorn) and the Aplodontidae (sewellel, or mountain beaver), endemic to western North America. There are a large number of endemic rodents. These include the woodrats (genus Neotoma), 17 species of ground squirrel (Citellus), three antelope squirrels, 16 chipmunks, 10 squirrels and flying squirrels, 12 pocket gophers, and 37 species of heteromyid rodents (pocket mice, kangaroo mice, and kangaroo rats). Characteristic larger endemic species include bison (Bison bison), mountain goat (Oreamnos americanus), bighorn sheep (Ovis canadensis), and thinhorn sheep (O. dalli).

The Nearctic shares many aspects of its mammal fauna with the Palaearctic and Neotropical regions. More than 80% of Nearctic families and 60% of the genera also occur in the Neotropical region. There are relatively few species of Neotropical origin in the Nearctic fauna. Only the southern opossum (Didelphis marsupialis), the nine-banded armadillo (Dasypus novemcinctus), and some bats have survived of those that entered the region from the south, in contrast to the much larger number of Nearctic species that entered the Neotropical region following formation of the Panamanian land bridge. Nearly half the families are shared with the Palaearctic region, a reflection of the length of time that the two regions have been connected across the Bering Strait either by a land bridge or a chain of islands. Twenty-one genera arrived from the Palaearctic at the end of the Pleistocene. Shared groups include several families of Carnivora (Felidae, Canidae, Mustelidae, Ursidae), deer (Cervidae), shrews (Soricidae), and moles (Talpidae).

A number of species in high latitudes have a circumpolar distribution in the Palaearctic and Nearctic regions, such as caribou (Rangifer tarandus), wolverine (Gulo gulo), gray wolf

(Canis lupus), Arctic fox (Alopex lagopus), and moose (Alces alces). Musk ox (Ovibos moschatus) also once ranged all around the tundra zone but is now extinct in Eurasia apart from some small introduced populations.

Many species in the boreal zones of both regions have a near counterpart in the other, for example pine marten (Martes martes) and stone marten (M. foina) in the Palaearctic and American marten (M. americana) and fisher (M. pennanti) in North America; European otter (Lutra lutra) and river otter (L. canadensis); American mink (Mustela vison) and European mink (M. lutreola).

Neotropical region

The Neotropics includes South America, Central America from southern Mexico southwards, and the West Indies. This region contains a very diverse fauna. Twelve orders are represented, two of them endemic. The Paucituberculata, shrew opossums, consists of three genera and five species, all found in the Andean region. The Microbiotheria has a single species, the monito del monte (Dromiciops australis), distributed in southern South America. The order Xenarthra (sloths, armadillos and anteaters) is mainly restricted to the Neotropical region, with one species occurring in the southern part of the Nearctic. The marsupial order Didelphimorphia has 63 species, all but one of which are restricted to the Neotropics.

Nineteen of the 50 families and about 80% of the almost 1,100 species that occur are also endemic. These include ten endemic families of rodents, including the guinea pigs, chinchillas, and agoutis; several families of bats; and two families of primates. Two genera of wild camelids (Vicugna and Lama) are endemic. The guanaco (L. guanicoe) is evidently the progenitor of two domesticated varieties, the llama and alpaca.

The Neotropical mammal fauna consists of three main strata. The ancestral fauna consisted of some very distinctive extinct orders, the order Xenarthra and early marsupials. These were augmented by intermittent "invaders" from the north, such as primates, rodents, and some carnivores. Then about 2 million years ago during the Pleistocene, formation of the Panamanian land bridge allowed the immigration of many new forms from North America. These included perissodactyls (tapirs and horses), artiodactyls (camelids, deer, and peccaries) and carnivores (felids, canids, and mustelids). This wave of mammalian invasions had a drastic effect on the existing fauna and resulted in many extinctions, including the large herbivore orders Notoungulata and Litopterna, and ground sloths. Ultimately, a unique array of mammals, perhaps as distinctive as those of Australia, disappeared altogether.

This interchange has been mainly, but not exclusively, one-way. Many more species moved south than in the opposite direction. Northern species entering South America have also proved more successful colonizers, penetrating to the southern tip of the continent.

The high degree of mammalian diversity is mainly a result of South America's isolation from other major land masses for long periods of geological history. Climate changes during the Pleistocene and alternating wet and dry periods caused the rainforest to contract and fragment, perhaps becoming in effect a series of forest islands separated by dry savanna or scrub that acted as barriers to dispersal and further leading to the development of new forms and evolution of new species.

There is considerable habitat diversity within the Neotropical region. In addition to the tracts of rainforest there are also extensive areas of dry scrub woodland, tropical savanna, temperate grasslands, desert, and high mountains. The Andes run almost the length of the continent of South America and reach an altitude of 22,830 ft (6,960 m) at their highest point. The rainforest does not cover a continuous extent but is divided into four main blocks by large rivers, mountains, and extensive areas of drier habitat types. The Atlantic rain-forest of southeastern Brazil has been completely isolated from the Amazon rainforest for a very long time and has its own endemic genera and species. During the Pleistocene, alternating wet and dry periods heavily influenced the extent of rainforest. At times this contracted to smaller patches isolated by expanses of arid habitats that acted as barriers to the dispersal of rainforest mammals. Many species subsequently evolved in these isolated forest refugia. Large rivers also act as dispersal barriers and related but separate species occur on opposite banks. For example, the fauna north and south of the Orinoco in northern South America show a number of differences, and the Amazon and Rio Negro also isolate some species.

Much of the characteristic fauna of the West Indies has disappeared; in fact, a disproportionate number of the mammals that have become extinct in recent times were endemic to the West Indies. These include an entire family containing eight species of shrews (Nesophontes), a species of raccoon (Procyon gloveralleni) formerly found on Barbados, 21 species of rodents, and the Caribbean monk seal (Monachus tropicalis). There is still one endemic family of large insectivores—the Solenodontidae—with two extant species, one each on the islands of Cuba and Hispaniola (Haiti and the Dominican Republic). These are thought to be island relicts of a formerly much more widespread group.

Palaearctic region

The Palaearctic region covers Europe, North Africa, most of the Arabian Peninsula, and Asia north of the Himalayas, including Japan and Korea. The Palaearctic is the largest of the major faunal regions in terms of its geographical area and it contains a wide range of habitats. Despite this, it has the lowest rate of endemism of all the major faunal regions. Thirteen orders and 42 families are present, none of them endemic. About 30% of the 262 genera and 60% of the 843 species that occur are endemic. The relatively low level of endemism reflects the long periods of contact at various times with the Nearctic, Oriental, and Ethiopian regions. Around half the Palaearctic families also occur in the Nearctic region, a consequence of the intermittent existence of the Bering land bridge. About 60% of the families also occur in the Oriental region.

The Palaearctic lacks the great diversity of ungulates found in the Ethiopian region, though a few species occur, including an endemic genus of Central Asian gazelles, Procapra. Groups that evolved in southern Eurasia such as deer and caprins are well-represented, with members of the latter group found in virtually all the regions' mountain ranges.

Equids and camelids both arrived from North America and still survive in the Palaearctic, though the distributions of all the surviving species are greatly reduced. Przewalski's horse (Equus caballus) was last seen in the wild in the 1960s, though reintroduction projects aimed at returning it to the wild in Mongolia began during the 1990s. Two species of wild ass still survive. Kulan, or onager (E. hemionus), is widespread in Mongolia, and is still found in fragments of its former range, in western India and in Turkmenistan. Kiang, or Tibetan wild ass (E. kiang), remains numerous on the Tibetan Plateau. Wild bactrian camels are now restricted to three small areas of the Gobi and Takla Makan deserts.

A high-altitude ungulate fauna has evolved on the Qinghai-Tibet Plateau, adapted to elevations above 13,000 ft (4,400 m) and prolonged periods of cold in winter. Component species include Tibetan antelope, or chiru (Pantholops hodgsonii), Tibetan gazelle (Procapra picticaudata), wild yak (Bos grunniens), Kiang (Equus kiang), Tibetan argali (Ovis ammon hodgsoni), and blue sheep (Pseudois nayaur) with white-lipped deer (Cervus albirostris) in the eastern third of the plateau. The diversity of these herbivores does not approach that of the East African savannas but they occurred in large numbers, with several nineteenth century travelers and sportsmen reporting vast herds of thousands of animals. These numbers have since been reduced, but in the eastern steppes of Mongolia, herds of several thousand Mongolian gazelles (Procapra gutturosa) can still be seen.

Of the smaller groups, there are 19 endemic species of pika (Ochotona) distributed through the mountains of Central Asia and the Himalayas. Two species of desmans, endemic insectivores, occur in the Pyrenees and Russia respectively. All but one of 20 species of dormice (Myoxidae) are endemic to the Palaearctic and there is a diverse assemblage of desert rodents in the region: 32 jerboas (Dipodidae), and 41 gerbils and jirds (Muridae).

In the eastern Palaearctic there are several island endemics. Japan has an endemic species of macaque (Macaca fuscata), serow (Capricornis crispus), and hare (Lepus brachyurus). The Ryukyu rabbit (Pentalagus furnessi) is endemic to two of the Ryukyu Islands.

Ethiopian region

This region includes Africa south of the Sahara, Madagascar, and the southwest corner of Arabia. Its mammal fauna exhibits the greatest diversity of all the major regions and 13 out of 26 orders are present. One of these is endemic, Tubulidentata, with a single species, the aardvark. There are 52 families (18 endemic), and over 1,000 species (more than 90% endemic). Endemic families include Giraffidae, Hippopotamidae, Chrysochloridae (golden moles), Tenrecidae (tenrecs), and Macroscelididae (elephant shrews).

Africa was once joined to the Oriental region so there are many elements in common. One order, Pholidota (pangolins or scaly anteaters) contains one genus (Manis) with four Ethiopian species and three in the Oriental region. There are also many families in common, but long periods of isolation have led to the development of unique genera, for example elephants, rhinos, monkeys, apes, hyenas, and cattle.

The region is noted for the impressive array of large herbivores that occur in large numbers on the savannas of central, eastern, and southern Africa. There are also seven endemic genera of Old World monkeys, and two out of the four genera of great apes (chimpanzees and gorillas). There is also a great diversity of rodents and viverrid carnivores (23 out 25 genera are endemic). There are some noteworthy absences from the region, such as deer (Cervidae) and bears (Ursidae).

The unique nature of Madagascar's mammal fauna is well known and results from its long isolation from the mainland of Africa that now lies more than 250 mi (420 km) away. Madagascar finally broke away from Africa during the Middle Tertiary period and only a few groups of mammals appear to have been present at that time, namely lemurs, insectivores, small carnivores, and rodents. In the absence of competitors and later immigration, these ancestral groups were able to radiate into diverse arrays of new species. The lemurs developed into three families (Lemuridae,

Indriidae, Daubentoniidae) containing nine genera and 38 species. The tenrecs provide an excellent example of adaptive radiation, the 27 extant species having a bewildering variety of forms and occupying a great diversity of niches. The early carnivores consisted only of mongooses and civets and both have evolved into distinctive forms. The four extant mongoose species form an endemic subfamily Galidiinae. The civets also show a remarkable development, with seven species belonging to seven separate genera in three subfamilies. There are ten endemic rodents and an endemic bat family (Myzopodidae) containing a single species. The other bats present were presumably able to fly to the island at a later stage. Almost equally striking is the absence of so many widespread African forms such as antelopes, zebras, larger carnivores, lagomorphs, and monkeys. At some point, the bushpig (Potamochoerus porcus) and a small species of hippopotamus (Hippopotamus lemerlei) reached the island; the bushpig survives but the hippo became extinct, probably in prehistoric times.

Oriental region

The Oriental region includes Asia south of the Himalayas, southern China, the Philippines, and Southeast Asia up to Wallace's Line, between the islands of Bali and Lombok. The region has two endemic orders, Scandentia (tree shrews), with 19 species, and Dermoptera (colugos). There are two species of colugos, often also called flying lemurs, a doubly confusing name as they are not lemurs and they glide, rather than fly. There are 50 families in the region, four endemic, and 260 genera, about 35% of them endemic. About two thirds of the more than one thousand species are also endemic. Endemic families include Kitti's hog-nosed bat (Craseonycteridae); tarsiers (Tarsiidae); gibbons (Hylobatidae); and tree shrews (Tupaiidae). The region has strong affinities with the Palaearctic and Ethiopian regions. There is a long land boundary with the Palaearctic along the Himalayas and through China, and almost 75% of the families are shared with the Palaearctic region. These include bears (Ursidae), deer (Cervidae), musk deer (Moschidae), and Felidae, including the tiger (Panthera tigris). Wooded savannas formerly connected the Indian subcontinent with Africa, although these linking areas now consist largely of desert. Groups in common between the Oriental and Ethiopian regions include elephants (one species in each), rhinoceroses, big cats (lion, leopard, and cheetah), viverrids, and great apes. Orangutans (genus Pongo) occur on Borneo and Sumatra and the 14 species of gibbons are distributed from eastern India and southern China through Southeast Asia. There are seven endemic genera of monkeys containing 26 species. Two of these are restricted to Sri Lanka, and two to the Mentawai Islands off the coast of Sumatra. The Oriental region lacks the great diversity of antelopes and other herbivores present in Ethiopian region, though a few species are present. Three are endemic to India, blackbuck (Antilope cervicapra), nilgai (Boselaphus tragocamelus), and four-horned antelope (Tetracerus quadricornis). The herbivore niches are filled in part by several species of deer. Other artiodactyls endemic to the region include five species of wild pigs, and four species of wild cattle including the little-known kouprey (Bos sauveli). The best-known endemic is undoubtedly the giant panda (Ailuropoda melanoleuca), which is indigenous to western China.

Australian region

This region includes Australia, New Zealand, New Guinea, Sulawesi, and some islands in the southwest Pacific. The Australian fauna is the least diverse in terms of the number of orders and species present, but certainly the most distinctive of all the major regions. Excluding Cetacea, only eight orders of mammals are native to Australia, but five of these, Monotremata and four orders of marsupials, are endemic. The other three indigenous orders are Chiroptera, Rodentia, and Carnivora. The latter is represented only by seals, though one terrestrial species, the dingo (Canis familiaris dingo), was brought to Australia by early human inhabitants several thousand years ago. Further introductions have been made by European settlers. There are 17 endemic families, and 60% of the genera and nearly 90% of the species are also endemic.

This distinctive fauna is the result of long isolation after the region broke away from Gondwana around 55 million years ago. When this event took place, only the monotremes

and marsupials were present. The Monotremata are an ancient evolutionary line composed of three species, the duck-billed platypus (Ornithorhynchidae) and two species of echidna or spiny anteater (Tachyglossidae). Marsupials appear to have reached Australia by a filter route from South America via Antarctica. When Australia and Antarctica separated, these original marsupials were isolated. During the 40 million years that followed they radiated to fill most of the niches occupied elsewhere by placental mammals. Larger species of kangaroos and wallabies occupy the large herbivore niche filled by ungulates in most of the rest of the world.

Rodents arrived in two separate migrations. It appears that rodents reached New Guinea from Southeast Asia and moved on into Australia by using islands as stepping stones. Upon reaching Australia, they radiated into many species. There are currently around 13 genera of rats and mice. Some bats are also thought to have reached the continent at a very early stage, with others entering the region later from the north. There are two endemic genera.

There are four orders of marsupials. Dasyuromorphia contains 17 diverse genera including the Tasmanian wolf (Thylacinus cynocephalus), Tasmanian devil, and the numbat or banded anteater. Order Peramelemorphia consists of small species, the bandicoots and bilbies. Order Notoryctemorphia has two species of burrowing marsupial "moles." The Diprodontia contains ten families and about 113 species including the familiar kangaroos, wallabies, and koala, as well as cuscuses and possums. The Australian region contains very few carnivores compared with other regions, only six species in Australia and five in New Guinea.

In more recent times, European settlers introduced several species that have succeeded in colonizing all or large parts of the continent. These include the house mouse (Mus musculus), rabbit (Oryctolagus cuniculus), red fox (Vulpes vulpes), and feral cat (Felis catus). Several domestic herbivores have also established feral populations. Rabbits have degraded vegetation over vast areas and introduced foxes are blamed for the destruction of much of the native fauna. House mice and feral cats are distributed across most of Australia, and the rabbit and fox over about 60%.

The island of Tasmania has many common forms and has acted as a refuge for others. The recently extinct thylacine or Tasmanian wolf (Thylacinus cynocephalus) was present here in historic times and is unconfirmed from the mainland. It is still thought by some to be possibly present. The Tasmanian devil (Sarcophilus harrisii) survives only on the island but was formerly found across Australia. Both may have suffered from competition with the dingo.

Australia and New Guinea have been isolated for most of the last 45 million years by the 100-mi (160-km) wide Torres Strait, and they have been only intermittently connected to each other. New Guinea has many marsupial species including endemic genera and one endemic monotreme species, the long-beaked echidna (Zaglossus bruijni). There are also a number of highly endemic mice and bats. The murid fauna has evolved in isolation from that of Australia. The fauna of the islands at the western end of the region, lying between

the two continental shelves, is a mix of Australian and Oriental forms. The largest island, Sulawesi, contains a number of endemic species. These include the babirusa (Babyrousa babyrussa)—an aberrant type of wild pig, two species of anoa— the smallest of the wild cattle, and four species of macaques.

New Zealand's mammal fauna is a special case. The islands broke away from Gondwana and drifted across what is now the Pacific about 80 million years ago. Only 11 mammal species are indigenous—four bats and seven pinnipeds (seals and sea lions). However, many more have been introduced and its mammal fauna consists of 65 species. The first introductions, of rats and dogs, were made by Polynesian settlers who reached the islands around 1,000–1,200 years ago. European colonists arriving from 1769 onwards brought in many more, mostly for food or game. These include 23 marsupials and 14 ungulates (deer, chamois, and Himalayan tahr). Out of 54 known introductions, 20 came from Europe, especially Great Britain, 14 from Australia, six from Asia (three surviving), and 10 from North or South America (three are established

locally). Introduced species far outnumber native mammals and there are now more large species than small mammals, the opposite of the usual situation.

The smaller Pacific Islands have generally impoverished mammal faunas reflecting the difficulties in colonizing them. The bats are the best represented order, which is unsurprising in view of their ability to fly to small oceanic islands. Sixteen species of endemic bats occur on the Solomon Islands and two more are extinct. There are five endemic species of bats on the Melanesian Islands. One endemic species on Guam (Pteropus tokudae) is extinct, but a new species of bat was discovered on Guadalcanal in 1990. Rodents are the next most frequent group and there are some endemic mice. Ancestors of some of these presumably reached the islands by chance, floating on logs or rafts of vegetation.

Antarctica

While the Arctic is covered within the Nearctic and Palaearctic, the fauna of Antarctica is usually omitted from descriptions of faunal regions. As noted above, no terrestrial mammals occur in the Antarctic but several species of cetaceans and seals use Antarctic waters. Crab-eater seals (Lobodon carcinophagus) occur around the coasts and pack ice of Antarctica and occasionally haul out on the shore. One has even been found on a glacier at an altitude of 3,600 ft (1,100m). The Weddell seal (Leptonycotes weddellii), probably the most southern of the world's mammals, prefers land fast ice to pack ice and is usually found in sight of land. Ross seals (Ommatophoca rossi) also inhabit the Antarctic pack ice. Leopard seals (Hydrurga leptonyx) are distributed throughout Antarctic waters where they prey on smaller species. Orcas and a few other cetaceans are regularly seen in these southern waters. More cetaceans and pinnipeds, including Antarctic fur seals and sea lions, occur farther north in subantarctic waters and around islands such as South Georgia, Macquarie Island, and Kerguelen. Reindeer (Rangifer tarandus) have been introduced to South Georgia.

The marine realm

While seals and possibly sirenians may be included in descriptions of the conventional biogeographic regions, this is not usually the case with cetaceans. It seems worthwhile to consider all the above in a marine realm for the sake of completeness. Marine mammals are distributed from the Arctic Ocean to the Antarctic and occur in the deep ocean, coastal waters, estuaries, and in a few cases in rivers and lakes. Two orders are fully aquatic, Cetacea (whales and dolphins) and Sirenia (manatees and dugongs). The third order, the Pinnipedia, is sometimes classified as part of the Carnivora. It consists of three families, seals, sea lions and fur seals, and walruses. They are mainly aquatic but haul out onto land or ice to rest, mate, and give birth.

The 76 species of whales and dolphins collectively have a worldwide distribution. Several species have extensive individual ranges in all the major oceans. Freshwater species of dolphins live in the Amazon, Yangtze, Indus, and Ganges Rivers. The four living species of sirenians are found in coastal waters, estuaries, and rivers mainly in the tropical and subtropical zones. Steller's sea cow (Hydrodamalis gigas), the only cold-water adapted species, formerly lived in the Bering Sea but was hunted to extinction by 1768.

There are 33 extant species of pinnipeds, mainly distributed in temperate and polar waters but a few species are found in the tropics. The 14 species of fur seals and sea lions are mainly distributed in the Pacific and southern oceans. One species is endemic to the Galápagos Islands, and two more have restricted distributions on Juan Fernandez and nearby islands and islands off California. The walrus (Odobenus rosmarus), the only member of the family Odobenidae is an inhabitant of the Arctic. The 19 species of so-called true seals, Phocidae, are predominantly distributed in northern and southern waters. Six species occur in Arctic and subarctic waters and four in the Antarctic. The Hawaiian monk seal (Monachus schauinslandi) is endemic to those islands. Isolated species occur in the Caspian Sea (Phoca caspica) and in the freshwater of Lake Baikal in Siberia (P. sibirica).

In addition, the polar bear (Ursus maritimus) and the sea otter (Enhydra lutris) of western North America also spend much of their lives in a marine environment. Taken together, marine mammals amount to only about 2.5% of all mammal species, despite the fact that seas and oceans cover a greater proportion of the surface of the earth than land does. This reflects the relative homogeneity of the environment and a lack of natural barriers that allow species to evolve in isolation.

The influence of humans

Humans have had a profound negative impact on the distribution of the world's mammals, through hunting, habitat destruction, and introductions. The consequences of hunting may have begun with early humans, as the Columbian mammoth (Mammuthus columbi) went extinct after only 500 years of contact with Clovis humans. Many of the large mammals of the Mediterranean coasts of North Africa were exterminated by the Romans who exported huge numbers of them to Rome to be killed in public arenas. The invention of modern firearms drastically increased the destructive potential of hunting. An estimated 60 million bison (Bison bison) existed across North America at the beginning of the eighteenth century but 150 years later they had been hunted to the brink of extinction and their former distribution reduced to fragments. During the nineteenth and twentieth centuries Arabian oryx (Oryx leucoryx) and gazelles were severely depleted in Arabia and North Africa, and the tiger (Panthera tigris) disappeared from large swathes of its former distribution and from Bali, Java, and Central Asia altogether. Domestication of sheep, goats, and cattle and their subsequent spread around most of the world have led to them becoming the most widespread of all mammals. The total biomass of domestic animals exceeds that of wild species in many places. The domestic animals compete for grazing and through overgrazing, degrade the habitat so that it can no longer support the original wild populations whose ranges shrink accordingly. Some species such as rats and mice have been introduced accidentally through transport with ships' cargo; others have been introduced deliberately, for sport or amenity. In virtually all cases they have had an adverse effect on the indigenous faunas. The effects of introduced foxes and rabbits on the habitats and wildlife of Australia were mentioned above. In Great Britain, the North American gray squirrel (Sciurus carolinensis) was released in the nineteenth century; since then it has steadily expanded its range in the country at the expense of the native red squirrel (S. vulgaris).

This generally negative trend has been partially reversed by reintroduction programs that restore former distributions. In the European Alps, reintroductions of the Alpine ibex (Capra ibex) in the nineteenth century and of lynx (Lynx lynx) during the 1980s-1990s have proven very successful. In the Arabian Peninsula, the Arabian oryx (Oryx leucoryx), mountain gazelle (Gazella gazella), and Arabian sand gazelle (G. subgutturosa marica) have all been returned to the wild during the 1990s, albeit in limited areas.