Endemic species are plants and animals that exist only in one geographic region. Species can be endemic to large or small areas of the earth: some are endemic to a particular continent, some to part of a continent, and others to a single island. Usually an area that contains endemic species is isolated in some way, so that species have difficulty spreading to other areas, or it has unusual environmental characteristics to which endemic species are uniquely adapted. Endemism, or the occurrence of endemic animals and plants, is more common in some regions than in others. In isolated environments such as the Hawaiian Islands , Australia , and the southern tip of Africa, as many of 90% of naturally occurring species are endemic. In less isolated regions, including Europe and much of North America, the%age of endemic species can be very small.
Biologists who study endemism do not only consider species, the narrowest classification of living things; they also look at higher level classifications of genus, family, and order. These hierarchical classifications are nested so that, in most cases, an order of plants or animals contains a number of families, each of these families includes several genera (plural of "genus"), and each genus has a number of species. These levels of classification are known as "taxonomic" levels.
Species is the narrowest taxonomic classification, with each species closely adapted to its particular environment . Therefore species are often endemic to small areas and local environmental conditions. Genera, a broader class, are usually endemic to larger regions. Families and orders more often spread across continents. As an example, the order Rodentia, or rodents, occurs throughout the world. Within this order, the family Heteromyidae occurs only in western North America and the northern edge of South America. One member of this family, the genus Dipodomys, or kangaroo rats, is restricted to several western states and part of Mexico. Finally, the species Dipodomys ingens, occurs only in a small portion of the California coast. Most often endemism is considered on the lowest taxonomic levels of genus and species.
Animals and plants can become endemic in two general ways. Some evolve in a particular place, adapting to the local environment and continuing to live within the confines of that environment. This type of endemism is known as "autochthonous," or native to the place where it is found. An "allochthonous" endemic species, by contrast, originated somewhere else but has lost most of its earlier geographic range. A familiar autochthonous endemic species is the Australian koala, which evolved in its current environment and continues to occur nowhere else. A well-known example of allochthonous endemism is the California coast redwood (Sequoia sempervirens ), which millions of years ago ranged across North America and Eurasia, but today exists only in isolated patches near the coast of northern California. Another simpler term for allochthonous endemics is "relict," meaning something that is left behind.
In addition to geographic relicts, plants or animals that have greatly restricted ranges today, there are what is known as "taxonomic relicts." These are species or genera that are sole survivors of once-diverse families or orders. Elephants are taxonomic relicts: millions of years ago the family Elephantidae had 25 different species (including woolly mammoths) in five genera. Today only two species remain, one living in Africa (Loxodonta africana ) and the other in Asia (Elephas maximus ). Horses are another familiar species whose family once had many more branches. Ten million years ago North America alone had at least 10 genera of horses. Today only a few Eurasian and African species remain, including the zebra and the ass. Common horses, all members of the species Equus caballus, returned to the New World only with the arrival of Spanish conquistadors.
Taxonomic relicts are often simultaneously geographic relicts. The ginkgo tree, for example was one of many related species that ranged across Asia 100 million years ago. Today the family Ginkgoales contains only one genus, Ginkgo, with a single species, Ginkgo biloba, that occurs naturally in only a small portion of eastern China. Similarly the coelacanth, a rare fish found only in deep waters of the Indian Ocean near Madagascar , is the sole remnant of a large and widespread group that flourished hundreds of millions of years ago.
Where living things become relict endemics, some sort of environmental change is usually involved. The redwood, the elephant, the ginkgo, and the coelacanth all originated in the Mesozoic era, 245–65 million years ago, when the earth was much warmer and wetter than it is today. All of these species managed to survive catastrophic environmental change that occurred at the end of the Cretaceous period, changes that eliminated dinosaurs and many other terrestrial and aquatic animals and plants. The end of the Cretaceous was only one of many periods of dramatic change; more recently two million years of cold ice ages and warmer interglacial periods in the Pleistocene substantially altered the distribution of the world's plants and animals. Species that survive such events to become relicts do so by adapting to new conditions or by retreating to isolated refuges where habitable environmental conditions remain.
When endemics evolve in place, isolation is a contributing factor. A species or genus that finds itself on a remote island can evolve to take advantage of local food sources or environmental conditions, or its characteristics may simply drift away from those of related species because of a lack of contact and interbreeding. Darwin's Galapagos finches, for instance, are isolated on small islands, and on each island a unique species of finch has evolved. Each finch is now endemic to the island on which it evolved. Expanses of water isolated these evolving finch species, but other sharp environmental gradients can contribute to endemism, as well. The humid southern tip of Africa, an area known as the Cape region, has one of the richest plant communities in the world. A full 90% of the Cape's 18,500 plant species occur nowhere else. Separated from similar habitat for millions of years by an expanse of dry grasslands and desert , local families and genera have divided and specialized to exploit unique local niches. Endemic speciation, or the evolution of locally unique species, has also been important in Australia, where 32% of genera and 75% of species are endemic. Because of its long isolation, Australia even has family-level endemism, with 40 families and sub-families found only on Australia and a few nearby islands.
Especially high rates of endemism are found on long-isolated islands, such as St. Helena, New Caledonia, and the Hawaiian chain. St. Helena, a volcanic island near the middle of the Atlantic, has only 60 native plant species, but 50 of these exist nowhere else. Because of the island's distance from any other landmass, few plants have managed to reach or colonize St. Helena. Speciation among those that have reached the remote island has since increased the number of local species. Similarly Hawaii and its neighboring volcanic islands, colonized millions of years ago by a relatively small number of plants and animals, now has a wealth of locally-evolved species, genera, and sub-families. Today's 1,200–1,300 native Hawaiian plants derive from about 270 successful colonists; 300–400 arthropods that survived the journey to these remote islands have produced over 6,000 descendent species today. Ninety-five percent of the archi pelago's native species are endemic, including all ground birds. New Caledonia, an island midway between Australia and Fiji, consists partly of continental rock, suggesting that at one time the island was attached to a larger landmass and its resident species had contact with those of the mainland. Nevertheless, because of long isolation 95% of native animals and plants are endemic to New Caledonia.
Ancient, deep lakes are like islands because they can retain a stable and isolated habitat for millions of years. Siberia's Lake Baikal and East Africa's Lake Tanganyika are two notable examples. Lake Tanganyika occupies a portion of the African Rift Valley, 0.9 mi (1.5 km) deep and perhaps 6 million years old. Fifty percent% of the lake's snail species are endemic, and most of its fish are only distantly related to the fish of nearby Lake Nyasa. Siberia's Lake Baikal, another rift valley lake, is 25 million years old and 1 mi (1.6 km) deep. Eighty-four percent of the lake's 2,700 plants and animals are endemic, including the nerpa, the world's only freshwater seal.
Because endemic animals and plants by definition have limited geographic ranges, they can be especially vulnerable to human invasion and habitat destruction. Island species are especially vulnerable because islands commonly lack large predators, and many island endemics evolved without defenses against predation. Cats, dogs, and other carnivores introduced by sailors have decimated many island endemics. The flora and fauna of Hawaii, exceptionally rich before Polynesians arrived with pigs, rats, and agriculture, were severely depleted because their range was limited and they had nowhere to retreat as human settlement advanced. Tropical rain forests, with extraordinary species diversity and high rates of endemism, are also vulnerable to human invasion. Many of the species eliminated daily in Amazonian rain forests are locally endemic, so that their entire range can be eliminated in a short time.
[Mary Ann Cunningham Ph.D. ]
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