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ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overl ...
, the competitive exclusion principle, sometimes referred to as Gause's law, is a proposition that two
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
which
compete Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indivi ...
for the same limited
resource Resource refers to all the materials available in our environment which are technologically accessible, economically feasible and culturally sustainable and help us to satisfy our needs and wants. Resources can broadly be classified upon thei ...
cannot coexist at constant population values. When one species has even the slightest advantage over another, the one with the advantage will dominate in the long term. This leads either to the extinction of the weaker competitor or to an
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
ary or behavioral shift toward a different
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...
. The principle has been paraphrased in the maxim "complete competitors can not coexist".


History

The competitive exclusion principle is classically attributed to
Georgy Gause Georgy Frantsevich Gause (russian: Гео́ргий Фра́нцевич Га́узе; December 27, 1910 – May 2, 1986), was a Soviet and Russian biologist and evolutionist, who proposed the competitive exclusion principle, fundamental to the sc ...
, although he actually never formulated it. The principle is already present in Darwin's theory of natural selection. Throughout its history, the status of the principle has oscillated between ''a priori'' ('two species coexisting ''must'' have different niches') and experimental truth ('we find that species coexisting do have different niches').


Experimental basis

Based on field observations,
Joseph Grinnell Joseph Grinnell (February 27, 1877 – May 29, 1939) was an American field biologist and zoologist. He made extensive studies of the fauna of California, and is credited with introducing a method of recording precise field observations known as ...
formulated the principle of competitive exclusion in 1904: "Two species of approximately the same food habits are not likely to remain long evenly balanced in numbers in the same region. One will crowd out the other".
Georgy Gause Georgy Frantsevich Gause (russian: Гео́ргий Фра́нцевич Га́узе; December 27, 1910 – May 2, 1986), was a Soviet and Russian biologist and evolutionist, who proposed the competitive exclusion principle, fundamental to the sc ...
formulated the law of competitive exclusion based on laboratory competition experiments using two species of ''
Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...
'', ''P. aurelia'' and ''P. caudatum''. The conditions were to add fresh water every day and input a constant flow of food. Although ''P. caudatum'' initially dominated, ''P. aurelia'' recovered and subsequently drove ''P. caudatum'' extinct via exploitative
resource Resource refers to all the materials available in our environment which are technologically accessible, economically feasible and culturally sustainable and help us to satisfy our needs and wants. Resources can broadly be classified upon thei ...
competition. However, Gause was able to let the ''P. caudatum'' survive by differing the environmental parameters (food, water). Thus, Gause's law is valid only if the ecological factors are constant. Gause also studied competition between two species of yeast, finding that ''
Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have b ...
'' consistently outcompeted '' Schizosaccharomyces kefir'' by producing a higher concentration of
ethyl alcohol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a hyd ...
.


Prediction

Competitive exclusion is predicted by mathematical and theoretical models such as the Lotka–Volterra models of competition. However, for poorly understood reasons, competitive exclusion is rarely observed in natural
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
s, and many biological communities appear to violate Gause's law. The best-known example is the so-called "
paradox of the plankton In aquatic biology, the paradox of the plankton describes the situation in which a limited range of resources supports an unexpectedly wide range of plankton species, apparently flouting the competitive exclusion principle which holds that when ...
". All
plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
species live on a very limited number of resources, primarily
solar energy Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture. It is an essen ...
and minerals dissolved in the water. According to the competitive exclusion principle, only a small number of plankton species should be able to coexist on these resources. Nevertheless, large numbers of plankton species coexist within small regions of open sea. Some communities that appear to uphold the competitive exclusion principle are MacArthur's
warbler Various Passeriformes (perching birds) are commonly referred to as warblers. They are not necessarily closely related to one another, but share some characteristics, such as being fairly small, vocal, and insectivorous. Sylvioid warblers The ...
s and
Darwin's finches Darwin's finches (also known as the Galápagos finches) are a group of about 18 species of passerine birds. They are well known for their remarkable diversity in beak form and function. They are often classified as the subfamily Geospizinae or ...
, though the latter still overlap ecologically very strongly, being only affected negatively by competition under extreme conditions.


Paradoxical traits

A partial solution to the paradox lies in raising the dimensionality of the system. Spatial
heterogeneity Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
, trophic interactions, multiple
resource Resource refers to all the materials available in our environment which are technologically accessible, economically feasible and culturally sustainable and help us to satisfy our needs and wants. Resources can broadly be classified upon thei ...
competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, ind ...
, competition-colonization trade-offs, and lag may prevent exclusion (ignoring stochastic extinction over longer time-frames). However, such systems tend to be analytically intractable. In addition, many can, in theory, support an unlimited number of species. A new paradox is created: Most well-known models that allow for stable coexistence allow for unlimited number of species to coexist, yet, in nature, any community contains just a handful of species.


Redefinition

Recent studies addressing some of the assumptions made for the models predicting competitive exclusion have shown these assumptions need to be reconsidered. For example, a slight modification of the assumption of how growth and body size are related leads to a different conclusion, namely that, for a given ecosystem, a certain range of species may coexist while others become outcompeted. One of the primary ways niche-sharing species can coexist is the competition-colonization trade-off. In other words, species that are better competitors will be specialists, whereas species that are better colonizers are more likely to be generalists. Host-parasite models are effective ways of examining this relationship, using host transfer events. There seem to be two places where the ability to colonize differs in ecologically closely related species. In feather lice, Bush and Clayton provided some verification of this by showing two closely related genera of lice are nearly equal in their ability to colonize new host pigeons once transferred. Harbison continued this line of thought by investigating whether the two genera differed in their ability to transfer. This research focused primarily on determining how colonization occurs and why wing lice are better colonizers than body lice. Vertical transfer is the most common occurrence, between parent and offspring, and is much-studied and well understood. Horizontal transfer is difficult to measure, but in lice seems to occur via phoresis or the "hitchhiking" of one species on another. Harbison found that body lice are less adept at phoresis and excel competitively, whereas wing lice excel in colonization.


Phylogenetic context

An
ecological Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
community is the assembly of species which is maintained by ecological (Hutchinson, 1959; Leibold, 1988) and evolutionary process (Weiher and Keddy, 1995; Chase ''et al''., 2003). These two processes play an important role in shaping the existing community and will continue in the future (Tofts ''et al''., 2000; Ackerly, 2003; Reich ''et al''., 2003). In a local community, the potential members are filtered first by environmental factors such as temperature or availability of required resources and then secondly by its ability to co-exist with other resident species. In an approach of understanding how two species fit together in a community or how the whole community fits together, ''The Origin of Species'' ( Darwin, 1859) proposed that under homogeneous environmental condition struggle for existence is greater between closely related species than distantly related species. He also hypothesized that the functional traits may be conserved across phylogenies. Such strong phylogenetic similarities among closely related species are known as phylogenetic effects (Derrickson ''et al''., 1988.) With field study and mathematical models, ecologist have pieced together a connection between functional traits similarity between species and its effect on species co-existence. According to competitive-relatedness hypothesis (Cahil ''et al''., 2008) or phylogenetic limiting similarity hypothesis (Violle ''et al''., 2011) interspecific competition is high among the species which have similar functional traits, and which compete for similar resources and habitats. Hence, it causes reduction in the number of closely related species and even distribution of it, known as phylogenetic overdispersion (Webb ''et al''., 2002). The reverse of phylogenetic overdispersion is phylogenetic clustering in which case species with conserved functional traits are expected to co-occur due to environmental filtering (Weiher ''et al''., 1995; Webb, 2000). In the study performed by Webb ''et al''., 2000, they showed that a small-plots of Borneo forest contained closely related trees together. This suggests that closely related species share features that are favored by the specific environmental factors that differ among plots causing phylogenetic clustering. For both phylogenetic patterns (phylogenetic overdispersion and phylogenetic clustering), the baseline assumption is that phylogenetically related species are also ecologically similar (H. Burns et al., 2011). There are no significant number of experiments answering to what degree the closely related species are also similar in niche. Due to that, both phylogenetic patterns are not easy to interpret. It’s been shown that phylogenetic overdispersion may also result from convergence of distantly related species (Cavender-Bares ''et al.'' 2004; Kraft ''et al.'' 2007). In their study, they have shown that traits are convergent rather than conserved. While, in another study, it’s been shown that phylogenetic clustering may also be due to historical or bio-geographical factors which prevents species from leaving their ancestral ranges. So, more phylogenetic experiments are required for understanding the strength of species interaction in community assembly.


Application to humans

Evidence showing that the competitive exclusion principle operates in human groups has been reviewed and integrated into regality theory to explain warlike and peaceful societies. For example, hunter-gatherer groups surrounded by other hunter-gatherer groups in the same
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...
will fight, at least occasionally, while hunter-gatherer groups surrounded by groups with a different means of subsistence can coexist peacefully.


See also

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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 ...
*
Limiting similarity Limiting similarity (informally "limsim") is a concept in theoretical ecology and community ecology that proposes the existence of a maximum level of niche overlap between two given species that will allow continued coexistence. This concept is a ...
*
Paradox of the plankton In aquatic biology, the paradox of the plankton describes the situation in which a limited range of resources supports an unexpectedly wide range of plankton species, apparently flouting the competitive exclusion principle which holds that when ...


References

{{DEFAULTSORT:Competitive Exclusion Principle Biological rules Biological interactions it:Competizione interspecifica#La mutua esclusione