Biogeography

Endemism

April 19, 2011, 9:29 pm
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Black lemur, Nosy Komba, a Madagascar endemic species. @ C.Michael Hogan

Endemism is the association of a biological taxon with a unique and well-defined geographic area. While an island is the most sharply delineated geographic zone that may comprise endemic status, any effective geographic barriers or realms of ecological continuity may define boundaries of endemism; examples of such boundaries are: mountain range, river, transition to desert or other new biome. Factors promoting endemism are uniqueness of meteorology, soils and presence of other species that may be predators, prey or symbionts, or merely provide nesting or cover areas.

Even though the concept of endemism is often used in association with geopolitical boundaries (e.g..endemic to California), the underlying association is almost always rather an area delimited by one or more natural boundaries; for example, the statement that Welwitschia mirabilis is endemic to the Namib Desert is a more meaningful designation than the plant being endemic to Namibia and Angola.

Terminology

There are several terms which offer nuances and sometimes can be confused with endemism. The chief ones are:

  • Indigenous or native species are found in a given area, but may also occur elsewhere.
  • Paleoendemic is the term for a species that was widespread in earlier times, but is now restricted to a much more delimited area
  • Neoendemic refers to a species which has recently appeared in a locale, due to speciation or geographic isolation from historic ancestral populations
  • Cosmopolitan is the antithesis of endemic, and refers to a taxon which is extremely widespread in many world regions.
  • Alien species refers to a taxon which does not occur naturally in a given area, but has been introduced, usually by human intervention. An alternative name is exotic species; moreover, if the alien species disrupts the new ecosystem, it may be termed an invasive species.

Natural boundaries for taxon distribution

caption Massive Andean ice sheets form an effective
biogeographical barrier. Source: C.Michael Hogan
Generalizing from the notion of island biogeography, one can imagine any set of natural barriers as effective delimiters of a species distribution; in fact, the barriers need not be mechanical in nature such as a mountain range or lava flow. Ecological barriers can be effective even if more subtle, such as transition of soil pH, microclimate or range of a predator.

The range of a species is typically time variant, with fluctuations in food and water availabiltiy being chief determinants. In addition, the concept of distribution is linked to home range size in the case of animals, since the faunal range is influenced by the need not only to secure adequate territory , but also to enable an overall species density to enable ready finding of a mate.

When physical or environmental separation isolates taxa for a long time period, a species can develop phenotypic differentiation from other species members; this process usually occurs upon one or more mutation events that produces DNA material which enables the new phenotype to adapt better to the environment than its ancestors; the entire process can be considered as speciation and natural selection. When the species distribution is drastically reduced from a predecessor larger range, the population is often termed a relict population and the shrunken range is termed a refugium.

Biodiversity hotspots

A biodiversity hotspot is defined as a region that has 1500 or more endemic vascular plant species (or 0.5 % of the world total); to qualify as a biodiversity hotspot, the region must have lost at least 70 % of its native vegetation. This concept is used to identify important regions deserving of exceptional conservation practices. Biodiversity hotspots are sometimes tantmount to high endemism ecoregions.The following ecoregions have high endemism based on vegetative species:

Relation to soils

Soil characteristics have a strong influence in determining species endemism, particularly for plant species, which derive most of their nutrients from the soil. Abiotic factors such as pH, nitrogen and phosphorus content as well as soil granularity are examples of soil characteristics that define appropriateness of habitat and may be determinants of species range.

An example of soil endemism is Vine Hill manzanita (Arctostaphylos densiflora), which thrives only in a small area of western Sonoma County, California, where the soils have an unusual composition of high sand content and low pH. Another plant example is the Cedars buckwheat (Eriogonum cedrorum) which occurs a highly restricted area of Sonoma County, where serpentine soils and talus environments are present.

Relation to climate

Meteorological factors such as temperature, humidity, wind velocity and sunlight are important determinants of plant or animal survival. When these conditions are not optimal for a given species metabolism, the organism will not thrive and reproduce in a manner that is competitive. Thus these climate parameters form natural boundaries in which each species can be expected to persist. Often these factors along with soil and topographic variables will serve jointly to define the endemic range for a given species.

References

  • Robert H. MacArthur, Edward O. Wilson. 2001. The theory of island biogeography. Princeton University Press. 203 pages
  • Pierre Dansereau. 1957. Biogeography: An Ecological Perspective. Ronald Press, New York, USA. ISBN 0826023304.
  • C.Barry Cox and Peter D. Moore. 2010. Biogeography: An Ecological and Evolutionary Approach. John Wiley and Sons. 506 pages
  • J.F.Lamoreux et al. 2006. Global tests of biodiversity concordance and the importance of endemism. Nature 440:212–214
  • Russell A. Mittermeier, Norman Myers and Cristina Goettsch Mittermeier. 2000. Hotspots: Earth's Biologically Richest and Most Endangered Terrestrial Ecoregions. Conservation International ISBN 978-9686397581
  • Robert Ornduff. 1974. An introduction to California plant life. University of California Press. 152 pages
  • G.C.Stevens. 1989. The latitudinal gradients in geographical range: how so many species co-exist in the tropics. American Naturalist 133, 240-256.
Glossary

Citation

Hogan, C. (2011). Endemism. Retrieved from http://www.eoearth.org/view/article/161345

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