Competitive exclusion principle
In ecology, the competitive exclusion principle, sometimes referred to as Gause's law, is a proposition named for Georgy Gause that two species competing 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 can not coexist".
History
The competitive exclusion principle is classically attributed to Georgii Gause, 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 and experimental truth.
Experimental basis
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". 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. 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 consistently outcompeted Schizosaccharomyces kefir by producing a higher concentration of ethyl alcohol.
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 ecosystems, and many biological communities appear to violate Gause's law. The best-known example is the so-called "paradox of the plankton". 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 and Darwin's finches, 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, trophic interactions, multiple resource competition, competition-colonization trade-offs, and lag may prevent exclusion. 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 community is the assembly of species which is maintained by ecological and evolutionary process. These two processes play an important role in shaping the existing community and will continue in the future. 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 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
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 or phylogenetic limiting similarity hypothesis 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. 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. 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, the baseline assumption is that phylogenetically related species are also ecologically similar. 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. 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.