Endemism
Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsewhere. For example, the Cape sugarbird is found exclusively in southwestern South Africa and is therefore said to be endemic to that particular part of the world. An endemic species can also be referred to as an endemism or, in scientific literature, as an endemite.
Endemism is an important concept in conservation biology for measuring biodiversity in a particular place and evaluating the risk of extinction for species. Endemism is also of interest in evolutionary biology, because it provides clues about how changes in the environment cause species to undergo range shifts, go extinct, or diversify into more species.
The extreme opposite of an endemic species is one with a cosmopolitan distribution, having a global or widespread range.
A rare alternative term for a species that is endemic is precinctive, which applies to species that are restricted to a defined geographical area. Other terms that sometimes are used interchangeably, but less often, include autochthonal, autochthonic, and indigenous; however, these terms do not reflect the status of a species that specifically belongs only to a determined place.
Etymology
History of the concept
The word endemic is from Neo-Latin endēmicus, from Greek ἔνδημος, éndēmos, "native". Endēmos is formed of en, meaning "in", and dēmos, meaning "the people". The word entered the English language as a loan word from the French endémique, and originally seems to have been used in the sense of diseases that occur at a constant amount in a country, as opposed to epidemic diseases, which are exploding in cases. The word was used in biology in 1872 to mean a species restricted to a specific location by Charles Darwin.The less common term 'precinctive' has been used by some entomologists as the equivalent of 'endemic'. Precinctive was coined in 1900 by David Sharp when describing the Hawaiian insects, as he was uncomfortable with the fact that the word 'endemic' is often associated with diseases. 'Precinctive' was first used in botany by Vaughan MacCaughey in Hawaii in 1917.
Overview
A species is considered to be endemic to the area where it is found naturally, to the exclusion of other areas; presence in captivity or botanical gardens does not disqualify a species from being endemic. In theory, the term "endemic" could be applied on any scale; for example, the cougar is endemic to the Americas, and all known life is endemic to Earth. However, endemism is normally used only when a species has a relatively small or restricted range. This usage of "endemic" contrasts with "cosmopolitan". Endemics are not necessarily rare; some might be common where they occur. Likewise, not all rare species are endemics; some may have a large range but be rare throughout this range.Origins
The evolutionary history of a species can lead to endemism in multiple ways. Allopatric speciation, or geographic speciation, is when two populations of a species become geographically separated from each other and, as a result, develop into different species. In isolated areas where there is little possibility for organisms to disperse to new places or to receive new gene flow from outside, the rate of endemism is particularly high. For example, many endemic species are found on remote islands, such as Hawaii, the Galápagos Islands and Socotra. Populations on an island are isolated, with little opportunity to interbreed with outside populations, which eventually causes reproductive isolation and separation into different species. Darwin's finches in the Galápagos archipelago are examples of species endemic to islands. Similarly, isolated mountainous regions like the Ethiopian Highlands, or large bodies of water far from other lakes, like Lake Baikal, can also have high rates of endemism.Endemism can also be created in areas that act as refuges for species during times of climate change like ice ages. These changes may have caused species to become repeatedly restricted to regions with unusually stable climate conditions, leading to high concentrations of endemic species in areas resistant to climate fluctuations. Endemic species that used to exist in a much larger area but died out in most of their range are called paleoendemic, in contrast to neoendemic species, which are new species that have not dispersed beyond their range. The ginkgo tree, Ginkgo biloba, is one example of a paleoendemic species.
In many cases, biological factors, such as low rates of dispersal or returning to the spawning area, can cause a particular group of organisms to have high speciation rates and thus many endemic species. For example, cichlids in the East African Rift Lakes have diversified into many more endemic species than the other fish families in the same lakes, possibly due to such factors. Plants that become endemic on isolated islands are often those that have a high rate of dispersal and are able to reach such islands by being dispersed by birds. While birds are less likely to be endemic to a region based on their ability to disperse via flight, there are over 2,500 species that are considered endemic, meaning that the species is restricted to an area less than.
Microorganisms were traditionally not believed to form endemics. The hypothesis 'everything is everywhere,' first stated in Dutch by Lourens G.M. Baas Becking in 1934, describes the theory that the distribution of organisms smaller than 2 mm is cosmopolitan where habitats occur that support their growth.
Subtypes and definitions
Endemism can reflect a wide variety of evolutionary histories, so researchers often use more specialized terms that categorize endemic species based upon how they came to be endemic to an area. Different categorizations of endemism also capture the uniqueness and irreplaceability of biodiversity hotspots differently and impact how those hotspots are defined, affecting how resources for conservation are allocated.The first subcategories were first introduced by Claude P. E. Favager and Juliette Contandriopoulis in 1961: schizoendemics, apoendemics and patroendemics. Using this work, Ledyard Stebbins and Jack Major then introduced the concepts of neoendemics and paleoendemics in 1965 to describe the endemics of California. Endemic taxa can also be classified into autochthonous, allochthonous, taxonomic relicts and biogeographic relicts.
Paleoendemism refers to species that were formerly widespread but are now restricted to a smaller area. Neoendemism refers to species that have recently arisen, such as through divergence and reproductive isolation or through hybridization and polyploidy in plants, and have not dispersed beyond a limited range.
Paleoendemism is more or less synonymous with the concept of a 'relict species': a population or taxon of organisms that were more widespread or more diverse in the past. A 'relictual population' is a population that currently occurs in a restricted area but whose original range was far wider during a previous geologic epoch. Similarly, a 'relictual taxon' is a taxon that is the sole surviving representative of a formerly diverse group.
The concept of phylogenetic endemism has also been used to measure the relative uniqueness of the species endemic to an area. Measurements that incorporate phylogenetic endemism weight the branches of the evolutionary tree according to their narrow distribution. This captures not only the total number of taxa endemic to the area but also how distant those species are from their living relatives.
Types of neoendemics include schizoendemics, apoendemics, and patroendemics. Schizoendemics arise from a wider distributed taxon that has become reproductively isolated without becoming genetically isolated—a schizoendemic has the same chromosome count as the parent taxon it evolved from. An apoendemic is a polyploid of the parent taxon, whereas a patroendemic has a lower, diploid chromosome count than the related, more widely distributed polyploid taxon. Mikio Ono coined the term 'aneuendemics' in 1991 for species that have more or fewer chromosomes than their relatives due to aneuploidy.
Pseudoendemics are taxa that have possibly recently evolved from a mutation. Holoendemics is a concept introduced by Richardson in 1978 to describe taxa that have remained endemic to a restricted distribution for a very long time.
In a 2000 paper, Myers and de Grave further attempted to redefine the concept. In their view, everything is endemic; even cosmopolitan species are endemic to Earth, and earlier definitions restricting endemics to specific locations are wrong. So, the categories of neoendemics and paleoendemics do not really help in studying where species are found, because they assume that an endemic species only exists in one specific area. Instead, they propose four different categories: holoendemics, euryendemics, stenoendemics and rhoendemics. In their scheme, cryptoendemics and euendemics are further subdivisions of rhoendemics. In their view, a holoendemic is a cosmopolitan species. Stenoendemics, also known as local endemics, have a reduced distribution and are synonymous with the word 'endemics' in the traditional sense, whereas euryendemics have a larger distribution—both these have distributions that are more or less continuous. A rhoendemic has a disjunct distribution. In cases where the disjunct distribution happens because of vicariance, a euendemic's separation is due to geological events like tectonic plate movements, while a cryptoendemic's separation occurs because the populations in between have gone extinct. There is yet another possible situation that can cause a disjunct distribution, where a species is able to colonize new territories by crossing over areas of unsuitable habitat, such as plants colonizing an island—which they dismiss as extremely rare and for which they do not devise a name. Traditionally, none of Myers and de Grave's categories would be considered endemics except stenoendemics.