Species distribution is the manner in which a biological taxon is spatially
arranged. Species distribution is not to be confused with dispersal, which is the
movement of individuals away from their area of origin or from centers of high
population density. A similar concept is the species range. A species range is
often represented with a species range map. Biogeographers try to understand
the factors determining a species' distribution. The pattern of
distribution is not permanent for each species. Distribution patterns can
change seasonally, in response to the availability of resources, and also
depending on the scale at which they are viewed. Dispersion usually takes place
at the time of reproduction. Populations within a species are translocated
through many methods, including dispersal by people, wind, water and
animals. Humans are one of the largest distributors due to the current trends
in globalization and the expanse of the transportation industry. For example,
large tankers often fill their ballasts with water at one port and empty them in
another, causing a wider distribution of aquatic species.
Biogeography is the study of the distribution of biodiversity over space
and time. It is very useful in understanding species distribution
through factors such as speciation, extinction, continental drift,
glaciation, variation of sea levels, river capture and available resources.
This branch of study not only gives a description of the species distribution,
but also a geographical explanation for the distribution of particular species.
The traditional biogeographic regions were first modeled by Alfred Wallace in
The Geographical Distribution of Animals. These were based on the work of
Sclater's terrestrial biogeographic regions. Wallace's system was based on
both birds and vertebrates, including non-flying mammals, which better reflect
the natural divisions of the Earth due to their limited dispersal abilities.
Clumped distribution Clumped distribution is the most common
type of dispersion found in nature. In clumped distribution, the distance
between neighboring individuals is minimized. This type of distribution is
found in environments that are characterized by patchy resources.
Animals need certain resources to survive, and when these resources become
rare during certain parts of the year animals tend to “clump” together around
these crucial resources. Individuals might be clustered together in an area
due to social factors such as selfish herds and family groups. Organisms that
usually serve as prey form clumped distributions in areas where they can
hide and detect predators easily. Other causes of clumped distributions
are the inability of offspring to independently move from their habitat.
This is seen in juvenile animals that are immobile and strongly dependent upon
parental care. For example, the bald eagle's nest of eaglets exhibits a
clumped species distribution because all the offspring are in a small subset of a
survey area before they learn to fly. Clumped distribution can be beneficial
to the individuals in that group. However, in some herbivore cases, such
as cows and wildebeests, the vegetation around them can suffer, especially if
animals target one plant in particular. Clumped distribution in species acts as
a mechanism against predation as well as an efficient mechanism to trap or corner
prey. African wild dogs, Lycaon pictus, use the technique of communal hunting to
increase their success rate at catching prey. Studies have shown that larger
packs of African wild dogs tend to have a greater number of successful kills. A
prime example of clumped distribution due to patchy resources is the wildlife
in Africa during the dry season; lions, hyenas, giraffes, elephants, gazelles,
and many more animals are clumped by small water sources that are present in
the severe dry season. It has also been observed that extinct and threatened
species are more likely to be clumped in their distribution on a phylogeny. The
reasoning behind this is that they share traits that increase vulnerability to
extinction because related taxa are often located within the same broad
geographical or habitat types where human-induced threats are concentrated.
Using recently developed complete phylogenies for mammalian carnivores and
primates it has been shown that the majority of instances threatened species
are far from randomly distributed among taxa and phylogenetic clades and display
clumped distribution. Regular or uniform distribution
Less common than clumped distribution, uniform distribution, also known as even
distribution, is evenly spaced. Uniform distributions are found in populations
in which the distance between neighboring individuals is maximized.
The need to maximize the space between individuals generally arises from
competition for a resource such as moisture or nutrients, or as a result of
direct social interactions between individuals within the population, such
as territoriality. For example, penguins often exhibit uniform spacing by
aggressively defending their territory among their neighbors. Plants also
exhibit uniform distributions, like the creosote bushes in the southwestern
region of the United States. Salvia leucophylla is a species in California
that naturally grows in uniform spacing. This flower releases chemicals called
terpenes which inhibit the growth of other plants around it and results in
uniform distribution. This is an example of allelopathy, which is the release of
chemicals from plant parts by leaching, root exudation, volatilization, residue
decomposition and other processes. Allelopathy can have beneficial,
harmful, or neutral effects on surrounding organisms. Some
allelochemicals even have selective effects on surrounding organisms; for
example, the tree species Leucaena leucocephala exudes a chemical that
inhibits the growth of other plants but not those of its own species, and thus
can affect the distribution of specific rival species. Allelopathy usually
results in uniform distributions, and its potential to suppress weeds is being
researched. Farming and agricultural practices often create uniform
distribution in areas where it would not previously exist, for example, orange
trees growing in rows on a plantation. Random distribution
Random distribution, also known as unpredictable spacing, is the least
common form of distribution in nature and occurs when the members of a given
species are found in homogeneous environments in which the position of
each individual is independent of the other individuals: they neither attract
nor repel one another. Random distribution is rare in nature as biotic
factors, such as the interactions with neighboring individuals, and abiotic
factors, such as climate or soil conditions, generally cause organisms to
be either clustered or spread apart. Random distribution usually occurs in
habitats where environmental conditions and resources are consistent. This
pattern of dispersion is characterized by the lack of any strong social
interactions between species. For example; When dandelion seeds are
dispersed by wind, random distribution will often occur as the seedlings land
in random places determined by uncontrollable factors. Oyster larvae
can also travel hundreds of kilometers powered by sea currents, which can
result in their random distribution. Species distribution models
Species distribution can now be potentially predicted based on the
pattern of biodiversity at spatial scales. A general hierarchical model can
integrate disturbance, dispersal and population dynamics. Based on factors of
dispersal, disturbance, resources limiting climate, and other species
distribution, predictions of species distribution can create a bio-climate
range, or bio-climate envelope. The envelope can range from a local to a
global scale or from a density independence to dependence. The
hierarchical model takes into consideration the requirements, impacts
or resources as well as local extinctions in disturbance factors.
Models can integrate the dispersal/migration model, the
disturbance model, and abundance model. Species distribution models can be used
to assess climate change impacts and conservation management issues. Species
distribution models include: presence/absence models, the
dispersal/migration models, disturbance models, and abundance models. A
prevalent way of creating predicted distribution maps for different species
is to reclassify a land cover layer depending on whether or not the species
in question would be predicted to habit each cover type. This simple SDM is
often modified through the use of range data or ancillary information- such as
elevation or water distance. Recent studies have indicated that the
grid size used can have an effect on the output of these species distribution
models. The standard 50x50 km grid size can select up to 2.89 times more area
than when modeled with a 1x1 km grid for the same specie. This has several
effects on the species conservation planning under climate change
predictions which could lead to over-prediction of future ranges in
species distribution modeling. This can result in the misidentification of
protected areas intended for a species future habitat.
Abiotic and biotic factors The distribution of species into
clumped, uniform, or random depends on different abiotic and biotic factors.
Any non-living chemical or physical factor in the environment is considered
an abiotic factor. There are three main types of abiotic factors: climatic
factors consist of sunlight, atmosphere, humidity, temperature, and salinity;
edaphic factors are abiotic factors regarding soil, such as the coarseness
of soil, local geology, soil pH, and aeration; and social factors include
land use and water availability. An example of the effects of abiotic
factors on species distribution can be seen in drier areas, where most
individuals of a species will gather around water sources, forming a clumped
distribution. Biotic factors, such as predation,
disease, and competition for resources such as food, water, and mates, can also
affect how a species is distributed. A biotic factor is any behavior of an
organism that affects another organism, such as a predator consuming its prey.
For example, biotic factors in a quail’s environment would include their prey,
competition from other quail, and their predators, such as the coyote. An
advantage of a herd, community, or other clumped distribution allows a population
to detect predators earlier, at a greater distance, and potentially mount
an effective defense. Due to limited resources, populations may be evenly
distributed to minimize competition, as is found in forests, where competition
for sunlight produces an even distribution of trees.
Species Distribution Grids Project The Species Distribution Grids Project
is an effort led out of the University of Columbia to create maps and databases
of the whereabouts of various animal species. This work is centered on
preventing deforestation and prioritizing areas based on species
richness. As of April 2009, data are available for global amphibian
distributions, as well as birds and mammals in the Americas. The map gallery
[1] contains sample maps for the Species Grids data set. These maps are not
inclusive but rather contain a representative sample of the types of
data available for download: Statistical determination of
distribution patterns There are various ways to determine the
distribution pattern of species. The Clark-Evans nearest neighbor method can
be used to determine if a distribution is clumped, uniform or random. To
utilize the Clark-Evans nearest neighbor method, researchers examine a population
of a single species. The distance of an individual to its nearest neighbor is
recorded for each individual in the sample. For two individuals that are
each other's nearest neighbor, the distance is recorded twice, once for
each individual. To receive accurate results, it is suggested that the number
of distance measurements is at least 50. The average distance between nearest
neighbors is compared to the expected distance in the case of random
distribution to give the ratio: If this ratio is equal to 1, then the
population is randomly dispersed. If R is significantly greater than 1, the
population is evenly dispersed. Lastly, if R is significantly less than 1, the
population is clumped. Statistical tests can then be used to determine whether R
is significantly different from 1. The Variance/Mean ratio method focuses
mainly on determining whether a species fits a randomly spaced distribution, but
can also be used as evidence for either an even or clumped distribution. To
utilize the Variance/Mean ratio method, data is collected from several random
samples of a given population. In this analysis, it is imperative that data
from at least 50 sample plots is considered. The number of individuals
present in each sample is compared to the expected counts in the case of
random distribution. The expected distribution can be found using Poisson
distribution. If the variance/mean ratio is equal to 1, the population is found
to be randomly distributed. If it is significantly greater than 1, the
population is found to be clumped distribution. Finally, if the ratio is
significantly less than 1, the population is found to be evenly
distributed. Typical statistical tests used to find the significance of the
variance/mean ratio include Student's t-test and chi squared.
However, many researchers believe that species distribution models based on
statistical analysis, without including ecological models and theories, are too
incomplete for prediction. Instead of conclusions based on presence-absence
data, probabilities that convey the likelihood a species will occupy a given
area are more preferred because these models include an estimate of confidence
in the likelihood of the species being present/absent. Additionally, they are
also more valuable than data collected based on simple presence or absence
because models based on probability allow the formation of spatial maps that
indicates how likely a species is to be found in a particular area. Similar
areas can then be compared to see how likely it is that a species will occur
there also; this leads to a relationship between habitat suitability and species
occurrence. Global warming effects
Researchers from the Arctic Ocean Diversity project have documented rising
numbers of warm-water crustaceans in the seas around Norway's Svalbard Islands.
Arcod is part of the Census of Marine Life, a huge 10-year project involving
researchers in more than 80 nations that aims to chart the diversity,
distribution and abundance of life in the oceans. Marine Life has become
largely affected by increasing effects of global warming. This study shows that
as the ocean temperatures rise species are beginning to travel into the cold
and harsh Arctic waters. Even the Snow Crab has extended its range 500 km
north. See also
Range Notes
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