Marginal distribution (biology)

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The geographical limits to the distribution of a species are determined by biotic or abiotic factors. Core populations are those occurring within the centre of the range, and marginal populations (also called peripheral populations) are found at the boundary of the range.

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The inability of a species to expand its range beyond a certain geographic area is because of some limiting factor or factors to which the species cannot successfully adapt. In some cases, geographical range limits are entirely predictable, such as the physical barrier of an ocean for a terrestrial species. [1] In other cases the specific reasons why species do not pass these boundaries are unknown, however, ecology is the main determinant of the distribution of a species. [2] The fitness of a species falls at the edges of its distributional range, with population growth and fitness falling to zero beyond where a species can survive. [3]

For many species of invertebrate animals, the exact geographic range limits have never been precisely ascertained, because not enough scientific field work has been carried out in many parts of the world to map distribution more precisely, therefore finding a range extension for species, especially marine species, is not an uncommon occurrence.

Marginal distributions can have conservation implications. [4]

Terminology

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. [5] [6] [7] 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. [5]

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. [7] [8] [9] 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. [10] 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. [11] 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. [7]

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. [7] [12]

Abiotic factors

The pine moth, Thaumetopoea pityocampa, expanded its range northward in France. Moth September 2008-3.jpg
The pine moth, Thaumetopoea pityocampa , 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. [1] [13] 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. [14] Moisture in the soil can also put limits on the distribution of an organism. [15] There are many other abiotic factors that can determine a species range, including dissolved oxygen, conductivity, alkalinity and pH. [16]

Biotic factors

Individuals of the freshwater gastropod Lymnaea stagnalis were found to have higher occurrences of trematode parasites at the edge of their distribution as compared with those of the core. Spitzschlammschnecke.jpg
Individuals of the freshwater gastropod 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. [17] 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. [17] [18] [19] 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. [20] 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. [21]

Anthropogenic factors

The bronzed cowbird, Molothrus aeneus, continues to expand its range in New Mexico and Texas due to deforestation. Molothrus aeneus -Tucson, Arizona, USA-8 (3).jpg
The bronzed cowbird, 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, [22] or change distributions in response to a decrease in habitat. [23] Recent changes in average temperatures, which may be caused by humans, are causing changes in the distribution of some species, such as northward expansion. [24] 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. [25]

Combined influences

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. [26]

See also

Related Research Articles

<span class="mw-page-title-main">Ecological niche</span> Fit of a species living under specific environmental conditions

In ecology, a niche is the match of a species to a specific environmental condition. It describes how an organism or population responds to the distribution of resources and competitors and how it in turn alters those same factors. "The type and number of variables comprising the dimensions of an environmental niche vary from one species to another [and] the relative importance of particular environmental variables for a species may vary according to the geographic and biotic contexts".

<span class="mw-page-title-main">Edge effects</span> Ecological concept

In ecology, edge effects are changes in population or community structures that occur at the boundary of two or more habitats. Areas with small habitat fragments exhibit especially pronounced edge effects that may extend throughout the range. As the edge effects increase, the boundary habitat allows for greater biodiversity.

Phylogeography is the study of the historical processes that may be responsible for the past to present geographic distributions of genealogical lineages. This is accomplished by considering the geographic distribution of individuals in light of genetics, particularly population genetics.

Allopatric speciation – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from each other to an extent that prevents or interferes with gene flow.

<span class="mw-page-title-main">Environmental gradient</span>

An environmental gradient, or climate gradient, is a change in abiotic (non-living) factors through space. Environmental gradients can be related to factors such as altitude, depth, temperature, soil humidity and precipitation. Often times, a multitude of biotic (living) factors are closely related to these gradients; as a result of a change in an environmental gradient, factors such as species abundance, population density, morphology, primary productivity, predation, and local adaptation may be impacted.

<span class="mw-page-title-main">Peripatric speciation</span> Speciation in which a new species is formed from an isolated smaller peripheral population

Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population. Since peripatric speciation resembles allopatric speciation, in that populations are isolated and prevented from exchanging genes, it can often be difficult to distinguish between them. Nevertheless, the primary characteristic of peripatric speciation proposes that one of the populations is much smaller than the other. The terms peripatric and peripatry are often used in biogeography, referring to organisms whose ranges are closely adjacent but do not overlap, being separated where these organisms do not occur—for example on an oceanic island compared to the mainland. Such organisms are usually closely related ; their distribution being the result of peripatric speciation.

<span class="mw-page-title-main">Biological dispersal</span> Movement of individuals from their birth site to a breeding site

Biological dispersal refers to both the movement of individuals from their birth site to their breeding site, as well as the movement from one breeding site to another . Dispersal is also used to describe the movement of propagules such as seeds and spores. Technically, dispersal is defined as any movement that has the potential to lead to gene flow. The act of dispersal involves three phases: departure, transfer, settlement and there are different fitness costs and benefits associated with each of these phases. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population genetics, and species distribution. Understanding dispersal and the consequences both for evolutionary strategies at a species level, and for processes at an ecosystem level, requires understanding on the type of dispersal, the dispersal range of a given species, and the dispersal mechanisms involved. Biological dispersal can be correlated to population density. The range of variations of a species' location determines expansion range.

<span class="mw-page-title-main">Habitat fragmentation</span> Discontinuities in an organisms environment causing population fragmentation.

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<span class="mw-page-title-main">Chalkhill blue</span> Species of butterfly

The chalkhill blue is a butterfly in the family Lycaenidae. It is a small butterfly that can be found throughout the Palearctic realm, where it occurs primarily in grasslands rich in chalk. Males have a pale blue colour, while females are brown. Both have chequered fringes around their wings.

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<span class="mw-page-title-main">Latitudinal gradients in species diversity</span> Global increase in species richness from polar regions to tropics

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<span class="mw-page-title-main">Species distribution</span> Geographical area in which a species can be found

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<span class="mw-page-title-main">Dainty damselfly</span> Species of damselfly

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<span class="mw-page-title-main">Forest migration</span>

Forest migration is the movement of large seed plant dominated communities in geographical space over time.

<span class="mw-page-title-main">Ecological fitting</span> Biological process

Ecological fitting is "the process whereby organisms colonize and persist in novel environments, use novel resources or form novel associations with other species as a result of the suites of traits that they carry at the time they encounter the novel condition". It can be understood as a situation in which a species' interactions with its biotic and abiotic environment seem to indicate a history of coevolution, when in actuality the relevant traits evolved in response to a different set of biotic and abiotic conditions.

<span class="mw-page-title-main">Ecological speciation</span>

Ecological speciation is a form of speciation arising from reproductive isolation that occurs due to an ecological factor that reduces or eliminates gene flow between two populations of a species. Ecological factors can include changes in the environmental conditions in which a species experiences, such as behavioral changes involving predation, predator avoidance, pollinator attraction, and foraging; as well as changes in mate choice due to sexual selection or communication systems. Ecologically-driven reproductive isolation under divergent natural selection leads to the formation of new species. This has been documented in many cases in nature and has been a major focus of research on speciation for the past few decades.

<span class="mw-page-title-main">Climate change and invasive species</span> Increase of invasive organisms caused by climate change

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In biology, parallel speciation is a type of speciation where there is repeated evolution of reproductively isolating traits via the same mechanisms occurring between separate yet closely related species inhabiting different environments. This leads to a circumstance where independently evolved lineages have developed reproductive isolation from their ancestral lineage, but not from other independent lineages that inhabit similar environments. In order for parallel speciation to be confirmed, there is a set of three requirements that has been established that must be met: there must be phylogenetic independence between the separate populations inhabiting similar environments to ensure that the traits responsible for reproductive isolation evolved separately, there must be reproductive isolation not only between the ancestral population and the descendent population, but also between descendent populations that inhabit dissimilar environments, and descendent populations that inhabit similar environments must not be reproductively isolated from one another. To determine if natural selection specifically is the cause of parallel speciation, a fourth requirement has been established that includes identifying and testing an adaptive mechanism, which eliminates the possibility of a genetic factor such as polyploidy being the responsible agent.

References

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