The role of competition in ecology and evolution is hotly debated. Many biologists claim that it is of paramount importance in shaping ecological communities, others doubt this. Indeed, the assumption of a major role of competition is one of the pillars of Darwin’s theory of evolution by natural selection. Some authors have distinguished two ecological paradigms, in one of these, intra- and interspecific competition are of paramount importance leading to coevolution of species by optimization processes, whereas in the other coevolution by short-term optimization processes is thought to be impossible because of great environmental variability. All these aspects were discussed in detail in several chapters of Nonequilibrium Ecology (Rohde 2005) .
Crawley (1986)  defines competition (between species) as the increase of the population density of one species at the expense of the reproductive rate and population density of another. However, because it is not species but individuals that interact, competition is better defined as an interaction between individuals,usuallyarising from shared requirements for a limiting resource. More concisely we can say thatcompetition is the interaction between individuals of the same or different species leading to an increase of the fitness of one at the expense of the other.Types of competitionIn intraspecific competition, individuals of the same species, ininterspecific competition, individuals of different species interact. Inexploitation competition, competing individuals use the same limited resource, ininterference competition, one competitor directly or indirectly reduces the access of the other to a resource (Park 1954) . Incontest competition, the limited resource is completely utilized by one of the competitors (i.e., it is all or nothing), inscramble competition, the competitors share a resource in various proportions (Nicholson 1954) . Barker (1983)  pointed out that both contest and scramble competition may lead to interference.
Factors leading to competition
Competition is usually for a resource that is in limited supply, such as food or space. However, Levin (1970) has shown that theonly important criterion for species coexistence is that limiting factors (whether a food resource, predation, etc.) differ and are independent. For example, two species cannot indefinitely coexist if they are limited by the same predator, even if they utilize different resources.
Ecological/evolutionary significance of competition
There can beno doubt that competition between individuals of the same and of different species is widespread and important. Any gardener will frequently observe that of two plants (of the same or different species) only one will survive if planted too close together, and the same applies to animals living in crowded conditions: only the competitively superior plant or animal will survive. However, the question asked in evolutionary biology is not about such possibly transient effects, but about the effects competitive interactions have in evolutionary history. Do they, for instance, lead to competitive exclusion of one species by another, or do they lead to character displacement (divergence) and coevolution? (see knols”Competitive exclusion” and “Niche restriction and segregation”).That an answer to such questions is of very great importance, is suggested by the fact that thebelief in the major role of competition is at the basis of most ecological modelling.For example. the concepts of ESS (evolutionarily stable strategy) (see knol”Evolutionarily stable strategies” ), CSS (continuously stable strategy), and NIS (neighbourhood invader strategy), are based to a large degree on such an assumption (e.g., Apaloo 2003, further references therein ). Among the major ecologists, Hutchinson (1959) , like many others, believed that competition is the major factor determining species diversity and patterns in ecological communities. However, evidence has accumulated which puts the predominant role of interspecific competition in doubt.- The discussion on the ecological/evolutionary importance of interspecific competition has beencentral to the equilibrium versus nonequilibrium debate. It is intuitively likely that under equilibrium conditions and in saturated niche space, competition will be more common and more intense than under nonequilibrium conditions and in nonsaturated niche space (Rohde 2005)  (see knol “Vacant niches”).
How do we prove the evolutionary role of competition?
Lawton (1984a)  pointed out that a search for competition should follow demonstration of interspecific resource limitation, which – in most cases – has not been done. According to Connell (1980) , most tests for competition are inadequate, and many studies claim competition without any evidence. He established the following criteria for demonstrating divergence of competitors: 1) there has been divergence in resource use between competitors; 2) competition and not some other mechanism was reponsible for the divergence; and 3) divergence is not simply phenotypic, but has a genetic basis. Because these criteria are very difficult to apply, many authors use plausibility arguments. Colwell (1984)  and Cooper (1993)  emphasized the usefulness of plausibility arguments based on background information. However, if such arguments are not applied very critically, they may, for instance, lead to confusion of interspecific competition with other factors such as reinforcement of reproductive barriers responsible for species segregation (see knol “Niche restriction and segregation”).
Evidence for competition
Connell (1980)  stressed that divergence between extant species has not been demonstrated except for some pests of crop plants; for some fossil sequences divergence has been demonstrated, but it is not clear whether interspecific competition was responsible. Connell concludes that there is little support for the coevolutionary divergence of competitors, and that is is probable only in low diversity communities (because in high diversity habitats a species may have many others as “neighbours” and there is insufficient opportunity for coevolution with one or a few particular species). Wiens (1984)  points out, with regard to “ghost of competition past”, i.e. the explanation of present abundance patterns by past competition, that such hypotheses are not testable, and that demonstration of patterns of resource partitioning does not explain the processes leading to such partitioning. Lawton (1984a)  refers to several studies which suggest that certain ecological patterns may be entirely or partly determined by interspecific competition, but he concludes that each of these patterns can also be explained by other hypotheses. Various models that have been established to fit species abundance patterns are based on the assumption that species’ abundances are determined by resource allocations (e.g., Tokeshi 1990 , 1999 ). One of these models, the Random Assortment Model (which assumes that resource allocations are not important), was fitted successfully to data for three parasite communities. In other words, interactions between species are insignificant, and these parasites are likely to live in unsaturated communities (Mouillot et al. 2003) . For ectoparasite communities of 45 species of marine fish, Gotelli and Rohde (2002) , using a null model analysis, demonstrated that patterns are random, i.e. not structured by interspecific competition. – Species ofDrosophilaand related genera are among the best known groups of animals. They have been used in many field and laboratory experiments, some dealing with niche segregation and its causes. Barker (1983)  has reviewed some of these studies and concluded that, although the idea of interspecific competition leading to niche segregation and differential adaptation is attractive, and although studies have demonstrated niche segregation, they provide no evidence for the mechanism that has led to segregation.On the other hand, Brown (1975)  concludes that competition is a major force in the structuring of desert rodent communities: coexisting species subdivide food resources by foraging in different microhabitats and by using differently sized seeds. Character displacement in body size is a consequence of selection of different seed sizes. In the absence of competitors, species use a much wider range of seed sizes. Brown et al. (1979)  used an experimental approach to show that competition for seeds is indeed important in determining community structure of seed-eating desert rodents. Also experimentally, Pimm (1978)  has demonstrated that, in hummingbirds, competition occurs when resources are predictable, but decreasingly so when resources became less predictable. One of the best examined examples for character displacement is that of Darwin’s finches on the Galapagos Islands. Grant and Schluter (1984)  examined beak morphology of various species and demonstrated that species co-occurring on the same island differ more strongly in beak morphology than the same species occurring singly on different islands. They concluded that interspecific competition was responsible for the greater differences in sympatric species. Size differences of the species was considered to correspond to size differences in food particles. Studies of birds in New Guinea and elsewhere also were used to support this hypothesis ( Diamond 1973 ,1975 , discussion in Roughgarden 1989 ).However, different interpretations are possible. Thus, physical and vegetational differences between islands, and reinforcement of reproductive isolation have also been held responsible, and null models (that species are independent of each other) apparently suggest that interspecific competition is not important in bringing about segregation of birds of the Galapagos Islands (although the validity of the null models has also been questioned (references in Rohde 2005 ).Schoener (1983)  reviewed evidence for interspecific competition in the past literature: in 90% of the studies and 76% of the speciessome degreeof competition was found. However, Connell (1983) , who also reviewed such evidence, found indication of competition in not more than 43% of species. The percentages may be misleading because study groups were not selected randomly (see section on “General arguments against the importance of competition”).
Vacant niches and competition
There is strong evidence that niche space may be unsaturated for various reasons (for a discussion see the knol “Vacant niches”). Many species, such as ectoparasites of fishes, live in habitats without or with few potential competitors (discussion and references in ).
Non-linear dynamics and competition
Even under strong interspecific competition and because of it, species may coexist. This has, for example, been demonstrated for some plankton species (see knol “The paradox of the plankton”).Interactive and non-interactive (isolationist) communitiesSome authors distinguish interactive and non-interactive communities. In the former, competition between members of the community are strong, in the latter they are weak (although they may still occur). For a discussion and references see ). Rohde (1991) reviewed findings on fish parasites as a general model for evaluating intra- and interspecific interactions in low density populations in resource-rich habitats and extended his findings to animals in general, concluding that most animal species are likely to live in low-density populations in resource-rich habitats. In such habitats, many potential niches are vacant and interspecific competition is of little importance although it may occur. In other words, non-interactive communities are the rule rather than the exception. A justification for this generalization is the fact that there are many more rare species than common ones. As shown by Rosenzweig (1995) , most assemblages of species can best be described by log series or lognormal distributions , i.e., most species are rare and have little potential for interactions with other species. New (rare) species could therefore be added without much effect on species already present.
Does competition play a role in the formation of species? Such a role was, for example, postulated by Rosenzweig (1995) . Evidence for and against this hypothesis is the same as that for the importance of competition in general. A possible mechanism is that populations diverge because of competition for resources and finally become reproductively segregated as different species. It is uncertain how common such a mechanism is.
General arguments against the importance of competition
A general objection to the approach explaining differences between species by competition for limiting resources resulting in character displacement was made by Andrewartha and Birch (1984) , who point out that one should expect to find similar species in similar habitats. One should rather ask: why can species with different ecological requirements live together? They further emphasize (and provide much evidence) that most natural populations never become numerous enough to use a substantial proportion of the resources needed by them.According to White (1993) , it is important to see that selection is a negative process, i.e. the unfit are selected against. Nitrogen is the most limiting chemical for plants, and competition is a consequence of and not a cause of its limitation. In other words, “competition does not decide the distribution and abundance of organisms. That is decided by the inadequacy of the environment.” Evidence is growing that interspecific (in contrast to intraspecific) competition is relatively uncommon, and of debatable significance in evolution.Price (1980) , in particular, has emphasized thatnegative results are rarely reported (which editor accepts a paper that presents purely negative findings?), and that communities are not randomly selected for study.Groups are selected for which positive results are likely! This skews the evidence significantly in favour of competition.Importantly, in many cases the outcome of competition, where it occurs, cannot be predicted because of environmental stochasticity, i.e., optimization (optimal adaptations to a particular niche) is impossible. This is discussed in the next section. Also, most studies on competition have failed to formulate null hypotheses (what differences between species can we expect even in the absence of competition?) against which results can be tested.
Two mutually exclusive paradigms in ecology?
Hengeveld and Walter (1999)  and Walter and Hengeveld (2000)  distinguishtwo mutually exclusive paradigmsin current ecology, i.e., the generally accepted demographic and the less developed autecological paradigm. In the former, intra- and interspecific competition are of paramount importance, leading to coevolution of species by short-term ecological optimization processes, thought to be possible because the abiotic component of the environment is believed to be on average constant, and nature is balanced (i.e., populations are in equilibrium and communities are saturated with species). In the latter, optimization is impossible because the environment is very variable in space and time, communities are not balanced and unsaturated. This distinction is similar to the distinction of equlibrium and nonequilibrium conditions by Rohde (2005) , who – however – does not consider the distinctions as mutually exclusive paradigms. He concludes that certain characteristics of species (large body and/or population size, great vagility) will make them more likely to occur in equilibrium, others (small body and/or population size, low vagility) will make them more likely to occur in nonequilibrium (see knol “Niche restriction and segregation”).ConclusionIn conclusion we can say that much of the evidence for ecological/evolutionary effects of interspecific competition is not convincing. A major problem in demonstrating such effects is the difficulty in formulating valid null hypotheses. Altough interspecific competition occurs in certain taxa and under certain conditions, the outcome may be largely unpredictable.The uncertain outcome of interspecific competition due to chaos, environmental stochasticity or other factors, reduces the likelihood that it has significant evolutionary effects.Overall, occurrence of competition is not evidence for its evolutionary significance.
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