Competitive exclusion principle

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1: A smaller (yellow) species of bird forages across the whole tree. 2: A larger (red) species competes for resources. 3: Red dominates in the middle for the more abundant resources. Yellow adapts to a new niche restricted to the top and bottom and avoiding competition. Competitive-20Exclusion-20Principle.svg
1: A smaller (yellow) species of bird forages across the whole tree.
2: A larger (red) species competes for resources.
3: Red dominates in the middle for the more abundant resources. Yellow adapts to a new niche restricted to the top and bottom and avoiding competition.

In ecology, the competitive exclusion principle, [1] sometimes referred to as Gause's law, [2] is a proposition that two species which compete for the same limited resource 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 evolutionary or behavioral shift toward a different ecological niche. The principle has been paraphrased in the maxim "complete competitors cannot coexist". [1]

Contents

History

The competitive exclusion principle is classically attributed to Georgy Gause, [3] although he actually never formulated it. [1] The principle is already present in Darwin's theory of natural selection. [2] [4]

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'). [2]

Experimental basis

Paramecium aurelia and Paramecium caudatum grow well individually, but when they compete for the same resources, P. aurelia outcompetes P. caudatum. Graph of competitive exclusion principle.jpg
Paramecium aurelia and Paramecium caudatum grow well individually, but when they compete for the same resources, P. aurelia outcompetes P. caudatum.

Based on field observations, Joseph Grinnell 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". [5] Georgy Gause formulated the law of competitive exclusion based on laboratory competition experiments using two species of Paramecium , 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 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.

Prediction

Cellular automaton model of interspecific competition for a single limited resource Logical deterministic individual-based cellular automata model of interspecific competition for a single limited resource.gif
Cellular automaton model of interspecific competition for a single limited resource

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 ecosystems, and many biological communities appear to violate Gause's law. The best-known example is the so-called "paradox of the plankton". [6] All plankton species live on a very limited number of resources, primarily solar energy 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 warblers [7] and Darwin's finches, [8] though the latter still overlap ecologically very strongly, being only affected negatively by competition under extreme conditions. [9]

Paradoxical traits

A partial solution to the paradox lies in raising the dimensionality of the system. Spatial heterogeneity, trophic interactions, multiple resource competition, 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. [10] [11]

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 [12] 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 [13] 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.

Support for a model of competition-colonization trade-off is also found in small mammals related to fire disturbances. In a project focused on the long-term impacts of the 1988 Yellowstone Fires Allen et al. [14] used stable isotopes and spatial mark-recapture data to show that Southern red-backed voles (Clethrionomys gapperi)), a specialist, are excluding deer mice (Peromyscus maniculatus), a generalist, from food resources in old-growth forests. However, after wildfire disturbance deer mice are more effective colonizers, and able to take advantage of the release from competitive pressure from voles. This dynamic of establishes a pattern of ecological succession in these ecosystems, with competitive exclusion from voles shaping the amount and quality of resources deer mice can access.

Phylogenetic context

An ecological community is the assembly of species which is maintained by ecological (Hutchinson, 1959; [15] Leibold, 1988 [16] ) and evolutionary process (Weiher and Keddy, 1995; [17] 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. [18] )

With field study and mathematical models, ecologists 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 [19] ) or phylogenetic limiting similarity hypothesis (Violle et al., 2011 [20] ) interspecific competition [21] 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 [22] ). 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 [23] ). 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; [24] Kraft et al. 2007 [25] ). In their study [ citation needed ], they have shown that traits are convergent rather than conserved. While, in another study [ citation needed ], 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. [26] For example, hunter-gatherer groups surrounded by other hunter-gatherer groups in the same ecological niche will fight, at least occasionally, while hunter-gatherer groups surrounded by groups with a different means of subsistence can coexist peacefully. [26]

Another recent application: in his work Historical Dynamics, Peter Turchin developed the so-called meta-ethnic frontier theory, wherein both rise and eventual fall of empires derives from geographically and or -politically colliding populations. [27] Accordingly, boundary regions, in which the competitive exclusion principle applies, are supposed to be key to human ethnogenesis. Summarizing its more wide-ranging predictions all in one:

Asabiya is a concept from the writings of Ibn Khaldun which Turchin defines as “the capacity for collective action” of a society. The Metaethnic Frontier theory is meant to incorporate asabiya as a key factor in predicting the dynamics of imperial agrarian societies - how they grow, shrink, and begin. Turchin posits that multi-level selection can help us identify the dynamics of asabiya in groups. He follows by noting three ways in which the logic of multi-level selection can be relevant in understanding change in “collective solidarity”: intergroup conflict, population and resource constraints, and ethnic boundaries. For small groups, intergroup conflict can increase asabiya as people need to band together to survive as a group. Conversely (again for small groups), a large population with respect to available resources can decrease asabiya as individuals compete for limited resources. For larger groups, Turchin proposes that ethnic boundries can influence how bands of small groups with moderate ethnic differences can band together against people who are even more “ethnically distanced” - more “Other”. In this process of small groups banding together against peoples more Other than themselves, they can form what Turchin calls a Metaethnic Frontier … Turchin notes that the this ethnic boundry dynamic which generates asabiya in a large group (composed of smaller groups) is weak because as the size of the group grows larger, the central regions are less exposed to intergroup conflict and asabiya decreases, leading to greater internal division. Finally, Turchin notes that all three aforementioned possiblities occur at regions which constitute imperial and metaethnic frontiers (imperial and metaethnic frontiers often coincide, he notes). It is in these regions of intense dynamics where asabiya is forged which are most prone to ethnogenesis. [28]

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">Biological interaction</span> Effect that organisms have on other organisms

In ecology, a biological interaction is the effect that a pair of organisms living together in a community have on each other. They can be either of the same species, or of different species. These effects may be short-term, or long-term, both often strongly influence the adaptation and evolution of the species involved. Biological interactions range from mutualism, beneficial to both partners, to competition, harmful to both partners. Interactions can be direct when physical contact is established or indirect, through intermediaries such as shared resources, territories, ecological services, metabolic waste, toxins or growth inhibitors. This type of relationship can be shown by net effect based on individual effects on both organisms arising out of relationship.

Georgy Frantsevich Gause, was a Soviet and Russian biologist and evolutionist, who proposed the competitive exclusion principle, fundamental to the science of ecology. Classic of ecology, he would devote most of his later life to the research of antibiotics.

<span class="mw-page-title-main">Character displacement</span>

Character displacement is the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur, but are minimized or lost where the species' distributions do not overlap. This pattern results from evolutionary change driven by biological competition among species for a limited resource. The rationale for character displacement stems from the competitive exclusion principle, also called Gause's Law, which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche; without differentiation, one species will eliminate or exclude the other through competition.

<span class="mw-page-title-main">Intermediate disturbance hypothesis</span> Model proposing regional biodiversity is increased by a moderate level of ecological disturbance

The intermediate disturbance hypothesis (IDH) suggests that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent. At low levels of disturbance, more competitive organisms will push subordinate species to extinction and dominate the ecosystem. At high levels of disturbance, due to frequent forest fires or human impacts like deforestation, all species are at risk of going extinct. According to IDH theory, at intermediate levels of disturbance, diversity is thus maximized because species that thrive at both early and late successional stages can coexist. IDH is a nonequilibrium model used to describe the relationship between disturbance and species diversity. IDH is based on the following premises: First, ecological disturbances have major effects on species richness within the area of disturbance. Second, interspecific competition results in one species driving a competitor to extinction and becoming dominant in the ecosystem. Third, moderate ecological scale disturbances prevent interspecific competition.

Colonisation or colonization is the spread and development of an organism in a new area or habitat. Colonization comprises the physical arrival of a species in a new area, but also its successful establishment within the local community. In ecology, it is represented by the symbol λ to denote the long-term intrinsic growth rate of a population.

<span class="mw-page-title-main">Competition (biology)</span> Interaction where the fitness of one organism is lowered by the presence of another organism

Competition is an interaction between organisms or species in which both require a resource that is in limited supply. Competition lowers the fitness of both organisms involved since the presence of one of the organisms always reduces the amount of the resource available to the other.

<span class="mw-page-title-main">Interspecific competition</span> Form of competition

Interspecific competition, in ecology, is a form of competition in which individuals of different species compete for the same resources in an ecosystem. This can be contrasted with mutualism, a type of symbiosis. Competition between members of the same species is called intraspecific competition.

<span class="mw-page-title-main">Community (ecology)</span> Associated populations of species in a given area

In ecology, a community is a group or association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis, biotic community, biological community, ecological community, or life assemblage. The term community has a variety of uses. In its simplest form it refers to groups of organisms in a specific place or time, for example, "the fish community of Lake Ontario before industrialization".

A guild is any group of species that exploit the same resources, or that exploit different resources in related ways. It is not necessary that the species within a guild occupy the same, or even similar, ecological niches.

<span class="mw-page-title-main">Paradox of the plankton</span> The ecological observation of high plankton diversity despite competition for few resources

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 two species compete for the same resource, one will be driven to extinction.

Limiting similarity 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.

The term phylogenetic niche conservatism has seen increasing use in recent years in the scientific literature, though the exact definition has been a matter of some contention. Fundamentally, phylogenetic niche conservatism refers to the tendency of species to retain their ancestral traits. When defined as such, phylogenetic niche conservatism is therefore nearly synonymous with phylogenetic signal. The point of contention is whether or not "conservatism" refers simply to the tendency of species to resemble their ancestors, or implies that "closely related species are more similar than expected based on phylogenetic relationships". If the latter interpretation is employed, then phylogenetic niche conservatism can be seen as an extreme case of phylogenetic signal, and implies that the processes which prevent divergence are in operation in the lineage under consideration. Despite efforts by Jonathan Losos to end this habit, however, the former interpretation appears to frequently motivate scientific research. In this case, phylogenetic niche conservatism might best be considered a form of phylogenetic signal reserved for traits with broad-scale ecological ramifications. Thus, phylogenetic niche conservatism is usually invoked with regards to closely related species occurring in similar environments.

<span class="mw-page-title-main">Coexistence theory</span> Framework explaining how competitor traits can maintain species diversity

Coexistence theory is a framework to understand how competitor traits can maintain species diversity and stave-off competitive exclusion even among similar species living in ecologically similar environments. Coexistence theory explains the stable coexistence of species as an interaction between two opposing forces: fitness differences between species, which should drive the best-adapted species to exclude others within a particular ecological niche, and stabilizing mechanisms, which maintains diversity via niche differentiation. For many species to be stabilized in a community, population growth must be negative density-dependent, i.e. all participating species have a tendency to increase in density as their populations decline. In such communities, any species that becomes rare will experience positive growth, pushing its population to recover and making local extinction unlikely. As the population of one species declines, individuals of that species tend to compete predominantly with individuals of other species. Thus, the tendency of a population to recover as it declines in density reflects reduced intraspecific competition (within-species) relative to interspecific competition (between-species), the signature of niche differentiation.

The R* rule is a hypothesis in community ecology that attempts to predict which species will become dominant as the result of competition for resources. The hypothesis was formulated by American ecologist David Tilman. It predicts that if multiple species are competing for a single limiting resource, then whichever species can survive at the lowest equilibrium resource level can outcompete all other species. If two species are competing for two resources, then coexistence is only possible if each species has a lower R* on one of the resources. For example, two phytoplankton species may be able to coexist if one is more limited by nitrogen, and the other is more limited by phosphorus.

<span class="mw-page-title-main">Biological rules</span> Generalized principle to describe patterns observed in living organisms

A biological rule or biological law is a generalized law, principle, or rule of thumb formulated to describe patterns observed in living organisms. Biological rules and laws are often developed as succinct, broadly applicable ways to explain complex phenomena or salient observations about the ecology and biogeographical distributions of plant and animal species around the world, though they have been proposed for or extended to all types of organisms. Many of these regularities of ecology and biogeography are named after the biologists who first described them.

<span class="mw-page-title-main">Kill the Winner hypothesis</span> Microbiological population model hypothesis

The "Kill the Winner" hypothesis (KtW) is an ecological model of population growth involving prokaryotes, viruses and protozoans that links trophic interactions to biogeochemistry. The model is related to the Lotka–Volterra equations. It assumes that prokaryotes adopt one of two strategies when competing for limited resources: priority is either given to population growth ("winners") or survival ("defenders"). As "winners" become more abundant and active in their environment, their contact with host-specific viruses increases, making them more susceptible to viral infection and lysis. Thus, viruses moderate the population size of "winners" and allow multiple species to coexist. Current understanding of KtW primarily stems from studies of lytic viruses and their host populations.

<span class="mw-page-title-main">Competition–colonization trade-off</span>

In ecology, the competition–colonization trade-off is a stabilizing mechanism that has been proposed to explain species diversity in some biological systems, especially those that are not in equilibrium. In which case some species are particularly good at colonizing and others have well-established survival abilities. The concept of the competition-colonization trade-off was originally proposed by Levins and Culver, the model indicated that two species could coexist if one had impeccable competition skill and the other was excellent at colonizing. The model indicates that there is typically a trade-off, in which a species is typically better at either competing or colonizing. A later model, labelled The Lottery Model was also proposed, in which interspecific competition is accounted for within the population.

Jeannine Cavender-Bares is Professor of Organismic and Evolutionary Biology at Harvard University and Director of the Harvard University Herbaria. She is also adjunct professor in the Department of Ecology, Evolution & Behavior at the University of Minnesota, where she served on the faculty for over two decades. Her research integrates evolutionary biology, ecology, and physiology by studying the functional traits of plants, with a particular focus on oaks.

<span class="mw-page-title-main">Phylogenetic signal</span>

Phylogenetic signal is an evolutionary and ecological term, that describes the tendency or the pattern of related biological species to resemble each other more than any other species that is randomly picked from the same phylogenetic tree.

References

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