Endemism

Q: How and why does a species become endemic? Please answer by giving examples.

Krishna: Endemism is a term used in biology to talk about the distribution of a taxon limited to a small geographic area and which can therefore be found naturally in this place. Therefore, endemic species are those that live in a limited area, such as a mountain range, lake or island, a single continent or country.

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 (2).

A species is considered to be “rare” if it exhibits any one of the following attributes: (1) naturally occurs in a narrow geographical area, (2) occupies only one or a few specialised habitats, (3) forms only small population(s) in its range. An “endemic” species, however, grows naturally in a single geographical area, the size of which could be either narrow or relatively large. Not all endemic species are rare, just as not all rare species must necessarily be endemic. Many rare and/or endemic species exhibit one or more of the following attributes which make them especially prone to extinction: (1) narrow (and single) geographical range, (2) only one or a few populations, (3) small population size and little genetic variability, (4) over-exploitation by people, (5) declining population sizes, (6) low reproductive potential, (7) the need for specialised ecological niches, (8) growth that requires stable and nearly constant environments. When habitats of a rare and/or endemic species are damaged and/or fragmented by various human activities, the distribution ranges and population sizes of the species will be reduced, leaving them vulnerable to extinction at a much higher rate than other comparable species. Species that experience any of the above attributes must be given priority and monitored and managed carefully in an eff ort to promote genetic conservation (1).

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 (2).

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 (2).

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.

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.

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. 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 (2,3).

Invading species sometimes take over and endemic animals or plants gets eliminated in the process.

Footnotes:

1. https://dergipark.org.tr/tr/download/article-file/122363
2. Endemic Species | Encyclopedia.com
3. http://Cox, G. W. Conservation Biology. Dubuque, IA: William C. Bro...