Marginal distribution (biology)

The science of understanding the distributions of organisms is known as chorology, a branch of biogeography. The core population of a species are those individuals occurring within the centre of the range. Although one cannot ever truly know the ideal niche of a particular species, it can be approximated from the core of the distribution, this is known as the "realized ecological niche". Marginal or peripheral populations are those found at the boundary of the range.{{cite report |title=Policy brief on Marginal and peripheral forests |url=http://www.fao.org/forestry/44385-0d0f329cc9c7563f42f3417c43caf23b3.pdf |publisher=COST European Cooperation in Science and Technology |pages=1–6 |docket=FPS COST Acti on FP1202 |access-date=5 October 2020}}{{cite journal |last=Guo |first=Qinfeng |date=11 June 2014 |title=Central-marginal population dynamics in species invasions |journal=Frontiers in Ecology and Evolution |volume=2 |issue=23 |doi=10.3389/fevo.2014.00023 |doi-access=free }}{{cite journal |last1=Yang |first1=Aihong |last2=Dick |first2=Christopher W. |last3=Yao |first3=Xiaohong |last4=Huang |first4=Hongwen |date=10 May 2016 |title=Impacts of biogeographic history and marginal population genetics on species range limits: a case study of Liriodendron chinense |url= |journal=Scientific Reports |volume=6 |issue=25632 |page=25632 |doi=10.1038/srep25632 |pmid=27162176 |pmc=4861920 |bibcode=2016NatSR...625632Y }} When the distribution of a species is changing, the leading edge populations are at the expanding geographic edge of the distribution range whilst rear edge populations are undergoing retreat.

The central‐marginal hypothesis, also sometimes called the "central-peripheral population hypothesis", posits that there is less genetic diversity and greater inter‐population genetic differentiation at the range margins, as compared to the range cores. This is based on the assumption that the habitat is most ideal at the centre of a distribution and ecological conditions decline towards the margin. Because the population size at the margin is likely to be smaller, genetic drift can have a larger effect and reduce the genetic variation of marginal populations. Reduced gene flow between central and peripheral populations also limits the genetic diversity at the margins. High selection pressure, due to a less than ideal habitat at the margin, furthermore reduces genetic diversity. Although exceptions to this hypothesis are common, in general this rule appears to hold empirically true.{{cite journal |last1=Kennedy |first1=John Paul |last2=Preziosi |first2=Richard F. |last3=Rowntree |first3=Jennifer K. |last4=Feller |first4=Ilka C. |date=February 2020 |title=Is the central‐marginal hypothesis a general rule? Evidence from three distributions of an expanding mangrove species, Avicennia germinans (L.) L. |journal=Molecular Ecology |volume=29 |issue=4 |pages=704–719 |doi=10.1111/mec.15365 |pmid=31990426 |pmc=7065085 |doi-access=free }}{{cite journal|last=Dai|first=Q|author2=Fu JZ|title=When central populations exhibit more genetic diversity than peripheral populations: a simulation study|journal=Chinese Science Bulletin|year=2011|volume=56|issue=24|pages=2531–2540|doi=10.1007/s11434-011-4605-x|bibcode=2011ChSBu..56.2531D|doi-access=free}} The spatial distribution often differs, with the population being more dense in the centre as opposed to the margins, this can often have a simple probability distribution pattern.{{cite journal|last=Brown|first=JH|title=On the relationship between abundance and distribution of species|journal=The American Naturalist|year=1984|volume=124|issue=2|pages=255–279|doi=10.1086/284267|s2cid=84276000}} The gene flow between central and peripheral populations may prevent range expansion when it does not allow the gene pool at margin to differentiate. Conditions at the centre of the range differ from those at the periphery, therefore adapted alleles at the centre may not benefit marginal populations experiencing different conditions.{{cite journal|last=Kirkpatrick |first=M |author2=Barton NH |title=Evolution of a species' range |journal=American Naturalist |year=1997 |volume=150 |issue=1 |pages=1–23 |doi=10.1086/286054 |pmid=18811273|s2cid=28389132 }} The asymmetrical gene flow hypothesis posits that there is more gene flow from central to peripheral populations. Empirical data supporting this theory is less robust.

When circumstances, usually climatic, restrict the distribution to a small area, this is known as a refugium. In Europe, for example, the geographical spokes sticking out of the continent in the south - the Iberian Peninsula, Italy and the Balkans served as refugia for warmth-adapted species during the Ice Ages.{{cite journal |last1=Harter |first1=D. E. V. |last2=Jentsch |first2=A. |last3=Durka |first3=W. |date=May 2015 |title=Holocene re‐colonisation, central–marginal distribution and habitat specialisation shape population genetic patterns within an Atlantic European grass species |journal=Plant Biology |volume=17 |issue=3 |pages=684–693 |doi=10.1111/plb.12269 |pmid=25266560 }}

File:Moth September 2008-3.jpg, expanded its range northward in France.]]

Gradients in any abiotic factor, such as climate, create physiological barriers to dispersal. All species have limits of tolerance to abiotic factors. Too much or too little of anything can lower their survival and reproductive success and cause reduced fitness. Changes in temperature resulting from global warming, for example, may cause a species to change its geographical distribution northward.{{cite journal|last=Battisti|first=A|author2=Stastny M |author3=Netherer S |author4=Robinet C |author5=Schopf A |author6=Roques A |author7=Larsson S |s2cid=85727778|title=Expansion of geographic range in the pine processionary moth caused by increased winter temperatures|journal=Ecological Applications|year=2005|volume=15|issue=6|pages=2084–2096 |doi=10.1890/04-1903|hdl=11577/1421038|hdl-access=free}} Precipitation can also be a key determinant in limiting the geographic range edges of species. This is often seen in organisms with high water demands, whose survival and reproduction is limited by dry conditions.{{cite journal|last=Bateman|first=BL|author2=Abell-Davis SE |author3=Johnson CN |s2cid=18311626|title=Climate-driven variation in food availability between the core and range edge of the endangered northern betting (Bettongia tropica)|journal=Australian Journal of Zoology|year=2011|volume=59|issue=3|pages=177–185|doi=10.1071/zo11079}} Moisture in the soil can also put limits on the distribution of an organism.{{cite journal|last=Duckworth|first=G|author2=Altwegg R |author3=Guo D |title=Soil moisture limits foraging: a possible mechanism for the range dynamics of the hadeda ibis in southern Africa|journal=Diversity and Distributions|year=2010|volume=16|issue=5|pages=765–772|doi=10.1111/j.1472-4642.2010.00683.x|s2cid=83593117|doi-access=free}} There are many other abiotic factors that can determine a species range, including dissolved oxygen, conductivity, alkalinity and pH.{{cite journal|last=Neff|first=MR|author2=Jackson DA|title=Effects of broad-scale geological changes on patterns in macroinvertebrate assemblages|journal= Journal of the North American Benthological Society|year=2011|volume=30|issue=2|pages=459–473|doi=10.1899/10-052.1|s2cid=53642682}}

File:Spitzschlammschnecke.jpg Lymnaea stagnalis were found to have higher occurrences of trematode parasites at the edge of their distribution as compared with those of the core.]]

Interactions between organisms can cause limitations to the distribution of a species. One interaction that may limit a distribution is predation, where prey species are limited from a particular area by very efficient predators, or where these predators may permit certain prey to have larger ranges.{{cite journal|last=Holt |first=RD |author2=Barfield M |title=Trophic interactions and range limits: the diverse roles of predation |journal=Proc. R. Soc. B |year=2009 |volume=276 |issue=1661 |pages=1435–1442 |doi=10.1098/rspb.2008.1536 |pmid=19324814 |pmc=2677217}} Interspecific competition is another common determinant of the distribution of individual species. Where two similar species share an overlapping range, competition often causes the distributions to shift to exclude one of the two.{{cite journal|last=Hersteinsson|first=P|author2=Macdonald W|title=Interspecific competition and the geographical distribution of red and arctic foxes Vulpes vulpes and Alopex lagopus|journal=Oikos|year=1992|volume=64|issue=3|pages=505–515|doi=10.2307/3545168|jstor=3545168}}{{cite journal|last=Armitage|first=DW|author2=Jones SE|title=Coexistence barriers confine the poleward range of a globally distributed plant|journal=Ecology Letters|year=2020|volume=23|issue=12|pages=1838–1848|doi=10.1111/ele.13612|pmid=33022085|s2cid=222183571 |url=https://www.biorxiv.org/content/biorxiv/early/2020/02/25/2020.02.24.946574.full.pdf}} The geographic range of one species may be linked to another, where the range of one species cannot extend independent of the other. This is seen in parasitism or mutualism, where survival is not possible without the hosts.{{cite journal|last=Stanton-Geddes|first=J|author2=Anderson C|title=Does a facultative mutualism limit species expansion?|journal=Oecologia|year=2011|volume=167|issue=1|pages=149–155|doi=10.1007/s00442-011-1958-4|pmid=21380848|s2cid=25646719}} Parasitism can also play another role in determining the distribution of a species: marginal populations with suboptimal habitats can carry a higher parasite load. This may be because less favourable conditions at the margins of a distribution lead to lower resistance to infection.{{cite journal|last=Briers|first=R|title=Range limits and parasite prevalence in a freshwater snail|journal=Proceedings of the Royal Society of London|year=2003|volume=270|issue=Suppl 2|pages=178–180|doi=10.1098/rsbl.2003.0046|pmid=14667375|pmc=1809944}}

File:Molothrus aeneus -Tucson, Arizona, USA-8 (3).jpg, Molothrus aeneus, continues to expand its range in New Mexico and Texas due to deforestation.]]

Humans can cause changes to the environment and alter distributions. Deforestation can increase the habitat of certain species and allow them to expand their distribution,{{cite journal|last=Kostecke|first=RM|author2=Ellison K |author3=Summers SG |title=Continued range expansion by bronzed cowbirds in the southwestern united states|journal=The Southwestern Naturalist|year=2004|volume=49|issue=4|pages=487–492|doi=10.1894/0038-4909(2004)049<0487:crebbc>2.0.co;2|s2cid=85732049 }} or change distributions in response to a decrease in habitat.{{cite journal|last=Larsen|first=TH|title=Upslope range shifts of Andean dung beetles in response to deforestation: compounding and confounding effects of microclimactic change |journal=Biotropica |year=2012 |volume=44|issue=1|pages=82–89|doi=10.1111/j.1744-7429.2011.00768.x|s2cid=83292640 }} Recent changes in average temperatures, which may be caused by humans, are causing changes in the distribution of some species, such as northward expansion.{{cite journal|last=Crozier|first=L|title=Warmer winters drive butterfly range expansion by increasing survivorship|journal=Ecology|year=2004|volume=85|issue=1|pages=231–241|doi=10.1890/02-0607}} Humans have also initiated many range expansions by introducing species to new locations both intentionally and accidentally. These species may survive and reproduce in these new locations and thus expand their distribution. These species may also cause changes in the distributions of native species that cannot tolerate the novel competition.{{cite journal|last=McNeil|first=JN|author2=Duchesne R|title=Transport of hay and its importance in the passive dispersal of the European skipper, Thymelicus lineola (Lepidoptera:Hesperiidae)|journal=Can. Entomol.|year=1977|volume=109|issue=9|pages=1253–1256|doi=10.4039/ent1091253-9|s2cid=86013170 }}

In most cases combinations of factors are responsible for limiting the geographic range edge of species. Abiotic and biotic factors may work together in determining the range of a species. An example might be some obligate seeder plants where the distribution is limited by the presence of wildfires, which are needed to allow their seed bank to germinate, and also use dispersal of their seeds mediated by ants.{{cite journal|last=Arnan|first=X|author2=Rodrigo A |author3=Molowny-Horas R |author4=Retana J |title=Ant-mediated expansion of an obligate seeder species during the first years after fire|journal=Plant Biology|year=2010|volume=12|issue=6|pages=842–852|doi=10.1111/j.1438-8677.2009.00294.x |pmid=21040299}}

{{clear|right}}

• Edge effects
• Species distribution
• Niche construction
• Mutualism Parasitism Continuum

{{reflist|32em}}

Category:Population ecology