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An aggregated distribution, commonly found among
predator 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 ...
s,
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
s, and
plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s is a highly uneven (
skewed In probability theory and statistics, skewness is a measure of the asymmetry of the probability distribution of a real-valued random variable about its mean. The skewness value can be positive, zero, negative, or undefined. For a unimodal ...
)
statistical distribution In statistics, an empirical distribution function ( an empirical cumulative distribution function, eCDF) is the distribution function associated with the empirical measure of a sample. This cumulative distribution function is a step functio ...
pattern in which they collect or aggregate in regions. These regions may be widely separated, particularly within animal distributions that are influenced by prey or host densities. This distribution makes sampling difficult and invalidates commonly-used
parametric statistics Parametric statistics is a branch of statistics which leverages models based on a fixed (finite) set of parameters. Conversely nonparametric statistics does not assume explicit (finite-parametric) mathematical forms for distributions when modeli ...
. A similar pattern is found among predators that search for their prey.


In predators

When predators need to search for their prey, they could search at random, as has been assumed in models such as those made by Lotka in 1925 and Volterra in 1928. This would imply that they scatter themselves evenly across the environment. However, prey may be concentrated at high densities in some areas and scarce elsewhere. The zoologists M. P. Hassell and R. M. May noted that predators and parasites, too, might aggregate themselves where prey was abundant, choosing some response curve: they observed for example that redshanks (predatory birds) adopted a sigmoid (s-shaped) response to the density of '' Corophium'' ( amphipod) prey per square metre of
mudflat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal ...
s. They noted, too, that several different behaviours of predators or parasites could cause them to aggregate selectively in areas where prey are at high density: they could be attracted by a volatile substance liberated by prey or the plants they are feeding on; they could choose to spend more time in areas where they have caught prey, as many predators appear to do; or, predators could follow an individual predator which had located prey, as is seen in feeding
terns Terns are seabirds in the family (biology), family Laridae, subfamily Sterninae, that have a worldwide distribution and are normally found near the sea, rivers, or wetlands. Terns are treated in eleven genus, genera in a subgroup of the fam ...
. Aggregated distributions of predators where they tend to spend time in areas where prey are concentrated have the effect of stabilising prey populations; when the time to travel between such concentrated areas is high; and when the prey populations too are more highly clumped.


In parasites

Parasite aggregation with respect to hosts is, according to Robert Poulin "a defining feature of metazoan parasite populations." The main reason for this is probably either that some hosts are more exposed to parasites, or that they are more susceptible to them. Aggregation is seen in directly transmitted parasites (those not using a
vector Vector most often refers to: * Euclidean vector, a quantity with a magnitude and a direction * Disease vector, an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematics a ...
) from many groups: ectoparasites like
lice Louse (: lice) is the common name for any member of the infraorder Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera was previously recognized as an order, until a 2021 genetic study determined th ...
and mites, marine parasites like
copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...
s and cyamid amphipods, and many kinds of nematode, fungi, protozoa, bacteria, and viruses. It is widespread, too, in trophically transmitted parasites of animals. The observed pattern is that most individual hosts are free or almost free of parasites, while a minority carry a large number of parasites.


Quantitative ecology

Aggregated distribution of parasites across their hosts poses considerable problems for students of parasite ecology, as it renders
parametric statistics Parametric statistics is a branch of statistics which leverages models based on a fixed (finite) set of parameters. Conversely nonparametric statistics does not assume explicit (finite-parametric) mathematical forms for distributions when modeli ...
as commonly used by biologists invalid. Log-transformation of data before the application of parametric test, or the use of
non-parametric statistics Nonparametric statistics is a type of statistical analysis that makes minimal assumptions about the underlying distribution of the data being studied. Often these models are infinite-dimensional, rather than finite dimensional, as in parametric s ...
is recommended by several authors, but this can give rise to further problems, so quantitative parasitology is based on more advanced biostatistical methods.


In plants

Intraspecific aggregation within a plant
community A community is a social unit (a group of people) with a shared socially-significant characteristic, such as place, set of norms, culture, religion, values, customs, or identity. Communities may share a sense of place situated in a given g ...
increases competition and crowding, altering competitive interactions among and between species. Aggregated distributions tend to decrease performance of dominant plant species, while benefiting the performance of competitively inferior species. Aggregation can exacerbate the effect of density-dependent mortality, further benefiting otherwise inferior species by providing space and resources for the species growth. The presence of density-dependent mortality has been shown to shift distributions away from an aggregated pattern to a random or repulsion distribution pattern


References

{{reflist Biological models Predation Parasitism Plant ecology