In biology, a
taxon
In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...
with a disjunct distribution is one that has two or more groups that are related but considerably separated from each other geographically. The causes are varied and might demonstrate either the expansion or contraction of a species' range.
Range fragmentation
Also called range fragmentation, disjunct distributions may be caused by changes in the environment, such as
mountain building
Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust (tectonic plates). Folding, faulting, volcanic activity, igneous intr ...
and
continental drift or rising
sea level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardis ...
s; it may also be due to an
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...
expanding its range into new areas, by such means as
rafting, or other animals transporting an organism to a new location (plant seeds consumed by birds and animals can be moved to new locations during bird or animal migrations, and those seeds can be deposited in new locations in fecal matter). Other conditions that can produce disjunct distributions include: flooding, or changes in wind, stream, and current flows, plus others such as anthropogenic introduction of alien
introduced species
An introduced species, alien species, exotic species, adventive species, immigrant species, foreign species, non-indigenous species, or non-native species is a species living outside its native distributional range, but which has arrived there ...
either accidentally or deliberately (agriculture and horticulture).
Habitat fragmentation
Disjunct distributions can occur when suitable
habitat
In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
is
fragmented, which produces fragmented populations, and when that fragmentation becomes so divergent that species movement between one suitable habitat to the next is disrupted, isolated population can be produced. Extinctions can cause disjunct distribution, especially in areas where only scattered areas are habitable by a species;
for instance, island chains or specific elevations along a mountain range or areas along a coast or between bodies of water like streams, lakes and ponds.
Examples
There are many patterns of disjunct distributions at many scales:
Irano-Turanian disjunction, Europe - East Asia, Europe-South Africa (e.g. genus ''
Erica''),
Mediterranean-Hoggart disjunction (genus ''
Olea''), etc.
Lusitanian distribution
This kind of disjunct distribution of a species, such that it occurs in
Iberia
The Iberian Peninsula (),
**
* Aragonese language, Aragonese and Occitan language, Occitan: ''Peninsula Iberica''
**
**
* french: Péninsule Ibérique
* mwl, Península Eibérica
* eu, Iberiar penintsula also known as Iberia, is a pe ...
and in
Ireland
Ireland ( ; ga, Éire ; Ulster-Scots: ) is an island in the North Atlantic Ocean, in north-western Europe. It is separated from Great Britain to its east by the North Channel, the Irish Sea, and St George's Channel. Ireland is the s ...
, without any intermediate localities, is usually called "Lusitanian" (named after the Roman Province
Lusitania
Lusitania (; ) was an ancient Iberian Roman province located where modern Portugal (south of the Douro river) and
a portion of western Spain (the present Extremadura and the province of Salamanca) lie. It was named after the Lusitani or Lu ...
, corresponding roughly to modern-day
Portugal
Portugal, officially the Portuguese Republic ( pt, República Portuguesa, links=yes ), is a country whose mainland is located on the Iberian Peninsula of Southwestern Europe, and whose territory also includes the Atlantic archipelagos of th ...
).
Examples of animal species with a Lusitanian distribution are: the Kerry slug ''
Geomalacus maculosus
The Kerry slug or Kerry spotted slug (''Geomalacus maculosus'') is a species of terrestrial, pulmonate, gastropod mollusc. It is a medium-to-large sized, air-breathing land slug in the family of roundback slugs, Arionidae.
Adult Kerry slugs g ...
'' and the
Pyrenean glass snail ''
Semilimax pyrenaicus''. Plant species with this kind of distribution include several
heather species (''Calluna'' spp.) and the
strawberry tree (''Arbutus unedo'').
The theory behind the name "Lusitanian" is now discredited; it posited that there was an ice-free land mass that served as a
refugium off of the south-west of Ireland during the
Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million year ...
(last)
glaciation
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate bet ...
. In this refugium, relic fauna and flora from a previous ice-free period survived until the present warmer
interstadial
Stadials and interstadials are phases dividing the Quaternary period, or the last 2.6 million years. Stadials are periods of colder climate while interstadials are periods of warmer climate.
Each Quaternary climate phase is associated with a Ma ...
period. Although the theory is no longer accepted, the term Lusitanian is still used as a descriptive term for faunal elements such as the Kerry slug.
Recently a better explanation of the occurrence of the Kerry slug and similar faunal elements in southwestern Ireland has been developed. This new theory is supported by two recent discoveries: the genetic similarity of much of
Ireland's fauna to that of northern Spain, and the genetic similarity of much of Ireland's human population to that of northern Spain.
Mascheretti et al. (2003) examined the genotypes of
Eurasian pygmy shrew
The Eurasian pygmy shrew (''Sorex minutus''), often known simply as the pygmy shrew, is a widespread shrew of the northern Palearctic.
Description
It measures about in length, not including the tail, and has an average weight of .The hair on t ...
, a small mammal, across its range in Europe. The Irish population showed close genetic affinity to a population from
Andorra
, image_flag = Flag of Andorra.svg
, image_coat = Coat of arms of Andorra.svg
, symbol_type = Coat of arms
, national_motto = la, Virtus Unita Fortior, label=none (Latin)"United virtue is stro ...
but not to that of
Britain or other places in Europe. The genetic structure of the population further showed that the entire Irish population of the Eurasian pygmy shrew had originated from a single
founder event
In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, usi ...
. The authors concluded that it had been introduced in the early (
Palaeolithic
The Paleolithic or Palaeolithic (), also called the Old Stone Age (from Greek: παλαιός '' palaios'', "old" and λίθος ''lithos'', "stone"), is a period in human prehistory that is distinguished by the original development of stone too ...
) or middle (
Mesolithic
The Mesolithic ( Greek: μέσος, ''mesos'' 'middle' + λίθος, ''lithos'' 'stone') or Middle Stone Age is the Old World archaeological period between the Upper Paleolithic and the Neolithic. The term Epipaleolithic is often used synonymo ...
)
Stone Age
The Stone Age was a broad prehistoric period during which stone was widely used to make tools with an edge, a point, or a percussion surface. The period lasted for roughly 3.4 million years, and ended between 4,000 BC and 2,000 BC, with ...
, by boat, probably from south-west Europe. This coincides with work on human populations, which found
[McEvoy B., Richards M., Forster P. & Bradley D. G. 2004. ''The longue durée of genetic ancestry: multiple genetic marker systems and Celtic origins on the Atlantic facade of Europe''. Am. J. Hum. Genet. 75: 693-702.] a strong
genetic similarity in make-up between populations in western Ireland and in northern Spain. This would be explained by a human migration from Spain to Ireland in the late
Paleolithic
The Paleolithic or Palaeolithic (), also called the Old Stone Age (from Greek: παλαιός '' palaios'', "old" and λίθος ''lithos'', "stone"), is a period in human prehistory that is distinguished by the original development of stone too ...
or early
Mesolithic
The Mesolithic ( Greek: μέσος, ''mesos'' 'middle' + λίθος, ''lithos'' 'stone') or Middle Stone Age is the Old World archaeological period between the Upper Paleolithic and the Neolithic. The term Epipaleolithic is often used synonymo ...
.
It seems increasingly likely that much of Ireland's Lusitanian fauna is in reality an artefact of this era of human expansion in the early part of the Postglacial era. In other words, it seems likely that these species were introduced accidentally with trade items or goods brought by boat from Iberia.
See also
*
Geographical isolation
*
Habitat
In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
*
Habitat fragmentation
Habitat fragmentation describes the emergence of discontinuities (fragmentation) in an organism's preferred environment (habitat), causing population fragmentation and ecosystem decay. Causes of habitat fragmentation include geological process ...
References
{{Reflist
Biogeography
Biology terminology
Habitat
Ecology terminology