The geographical limits to the

distribution Distribution may refer to: Mathematics *Distribution (mathematics), generalized functions used to formulate solutions of partial differential equations *Probability distribution, the probability of a particular value or value range of a varia ...

of a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

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. The inability of a species to expand its range beyond a certain geographic area is because of some

limiting factor A limiting factor is a variable of a system that causes a noticeable change in output or another measure of a type of system. The limiting factor is in a pyramid shape of organisms going up from the producers to consumers and so on. A factor not l ...

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. In other cases the specific reasons why species do not pass these boundaries are unknown, however,

ecology Ecology () is the natural science of the relationships among living organisms and their Natural environment, environment. Ecology considers organisms at the individual, population, community (ecology), community, ecosystem, and biosphere lev ...

is the main determinant of the distribution of a species. 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. 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.

Terminology

The science of understanding the distributions of organisms is known as

chorology Chorology (from Greek , , "place, space"; and , ''-logia'') can mean * the study of the causal relations between geographical phenomena occurring within a particular region * the study of the spatial distribution of organisms (biogeography). Use ...

, a branch of

biogeography Biogeography is the study of the species distribution, distribution of species and ecosystems in geography, geographic space and through evolutionary history of life, geological time. Organisms and biological community (ecology), communities o ...

. 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 Niche may refer to: Science *Developmental niche, a concept for understanding the cultural context of child development and growth *Ecological niche, a term describing the relational position of an organism's species *Niche differentiation, in ec ...

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. 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 In ecology, habitat refers to the array of resources, 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 manifestation of its ...

is most ideal at the centre of a distribution and ecological conditions decline towards the margin. Because the

population size In population genetics and population ecology, population size (usually denoted ''N'') is a countable quantity representing the number of individual organisms in a population. Population size is directly associated with amount of genetic drift, a ...

at the margin is likely to be smaller,

genetic drift Genetic drift, also known as random genetic drift, allelic drift or the Wright effect, is the change in the Allele frequency, frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene va ...

can have a larger effect and reduce the

genetic variation Genetic variation is the difference in DNA among individuals or the differences between populations among the same species. The multiple sources of genetic variation include mutation and genetic recombination. Mutations are the ultimate sources ...

of marginal populations. Reduced

gene flow In population genetics, gene flow (also known as migration and allele flow) is the transfer of genetic variation, genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent ...

between central and peripheral populations also limits the genetic diversity at the margins. High

selection pressure Evolutionary pressure, selective pressure or selection pressure is exerted by factors that reduce or increase reproductive success in a portion of a population, driving natural selection. It is a quantitative description of the amount of change oc ...

, 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. The

spatial distribution A spatial distribution in statistics is the arrangement of a phenomenon across the Earth's surface and a graphical display of such an arrangement is an important tool in geographical and environmental statistics. A graphical display of a spatial ...

often differs, with the population being more dense in the centre as opposed to the margins, this can often have a simple

probability distribution In probability theory and statistics, a probability distribution is a Function (mathematics), function that gives the probabilities of occurrence of possible events for an Experiment (probability theory), experiment. It is a mathematical descri ...

pattern. The gene flow between central and peripheral populations may prevent range expansion when it does not allow the

gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survi ...

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. The asymmetrical gene flow hypothesis posits that there is more

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 The Iberian Peninsula ( ), also known as Iberia, is a peninsula in south-western Europe. Mostly separated from the rest of the European landmass by the Pyrenees, it includes the territories of peninsular Spain and Continental Portugal, comprisin ...

Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...

and the

Balkans The Balkans ( , ), corresponding partially with the Balkan Peninsula, is a geographical area in southeastern Europe with various geographical and historical definitions. The region takes its name from the Balkan Mountains that stretch throug ...

served as refugia for warmth-adapted species during the

Ice Ages An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages, and Gre ...

Abiotic factors

Gradients in any

abiotic In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them und ...

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 Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves. Reproduct ...

and cause reduced fitness. Changes in temperature resulting from

global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...

, for example, may cause a species to change its geographical distribution northward.

Precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

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. Moisture in the soil can also put limits on the distribution of an organism. There are many other abiotic factors that can determine a species range, including

dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It can ...

, conductivity,

alkalinity Alkalinity (from ) is the capacity of water to resist Freshwater acidification, acidification. It should not be confused with base (chemistry), basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength of a buffer s ...

and pH.

Biotic factors

Interactions between organisms can cause limitations to the distribution of a species. One interaction that may limit a distribution is

predation Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

, 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.

Interspecific competition Interspecific competition, in ecology, is a form of competition in which individuals of ''different'' species compete for the same resources in an ecosystem (e.g. food or living space). This can be contrasted with mutualism, a type of symbiosis. ...

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. 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 Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The en ...

mutualism Mutualism may refer to: * Mutualism (biology), positive interactions between species * Mutualism (economic theory), associated with Pierre-Joseph Proudhon * Mutualism (movement), social movement promoting mutual organizations * Mutualism model o ...

, where survival is not possible without the

hosts A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County * Host Island, in the Wilhelm Archipelago, Antarctica People * ...

. Parasitism can also play another role in determining the distribution of a species: marginal populations with suboptimal habitats can carry a higher

parasite load Parasite load is a measure of the number and virulence of the parasites that a host organism harbours. Quantitative parasitology deals with measures to quantify parasite loads in samples of hosts and to make statistical comparisons of parasitism ac ...

. This may be because less favourable conditions at the margins of a distribution lead to lower resistance to infection.

Anthropogenic factors

Humans can cause changes to the environment and alter distributions.

Deforestation Deforestation or forest clearance is the removal and destruction of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. Ab ...

can increase the habitat of certain species and allow them to expand their distribution, or change distributions in response to a decrease in habitat. Recent changes in average temperatures, which may be caused by humans, are causing changes in the distribution of some species, such as northward expansion. 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 In biogeography, a native species is indigenous to a given region or ecosystem if its presence in that region is the result of only local natural evolution (though often popularised as "with no human intervention") during history. The term is equi ...

that cannot tolerate the novel competition.

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 A seed bank (also seed banks, seeds bank or seed vault) stores seeds to preserve genetic diversity; hence it is a type of gene bank. There are many reasons to store seeds. One is to preserve the genes that plant breeders need to increase yield, ...

to germinate, and also use dispersal of their seeds mediated by ants.

References