Competition is the all‐pervasive interaction between organisms in which each reduces the performance of the other, either by depleting mutually required resources or by directly inflicting damage. Competition is an important factor that controls the distribution and abundance of many if not most plants and animals. More generally, it has been a potent force driving natural selection and is a major factor (along with stress, disturbance, predation and mutualism) in shaping the structure of biological communities. Too often, competition is assumed to be occurring without sufficient evidence. Carefully designed experiments are one of the best sources of evidence for competition. Different kinds of experiments uncover different views of competition. In general, plants are organised into competitive hierarchies in which light is often the key resource. Mathematical models allow us to explore how competition might lead to either coexistence or exclusion.

Key Concepts

  • The contest between organisms for access to essential resources is called competition.

  • Competition is an important factor controlling the distribution and abundance of many if not most plants and animals.

  • Kinds of competition can be described by the mechanism of interaction, the type of organisms involved and the relative impacts of the competitors.

  • Mathematical models, based on assumptions about organism interactions, are sometimes used to explore the consequences of competition.

  • Except in relatively trivial cases of aggression, competition between organisms can be detected and measured only through carefully designed experiments.

  • Competition can be either symmetrical (competitors equally matched) or asymmetrical (a clear winner and loser) depending upon the species and the habitat.

  • Competition may lead to resource partitioning (particularly among motile animals) or may be counterbalanced by natural disturbances (in plants and among sessile animals).

Keywords: communities; dominance; hierarchies; evolution; interactions; resources

Figure 1. The Lotka–Volterra model is one traditional way of exploring competition between two populations, in this case labelled 1 and 2. As the text describes, the two axes are the population size of the two species and the lines show isoclines where population growth is zero. Depending on the nature of interspecific competition, four outcomes are possible. Cases (a) and (b) top show competitive dominance, where one species can predictably eliminate the other. In case (c), the two species reach a stable equilibrium, which allows long‐term coexistence. In case (d), competitive dominance again occurs, but the winning species is entirely dependent on the starting size of the two populations, which is sometimes called contingent competition. The arrows show changes in population size with time. The solid dots represent the equilibrium points (expected outcomes) of these pairwise interactions. The open circle is an unstable equilibrium point.
Figure 2. Resource partitioning occurs when a group of species (a) through (g) harvests different sizes or kinds of resources. At one time, in a simplistic way, the amount of overlap in resource use was thought to measure the amount of competition between each pair of species. The amount of overlap is, however, shaped by other factors including evolutionary history sometimes called ‘the ghost of competition past’, intensity of present‐day competition and patterns in the availability of resources.
Figure 3. Weak competitors (green) may survive by escaping to habitat patches that are not occupied by stronger species (orange). Four possible combinations of seedlings are shown (a), and the outcome of adults is given (b) (from Keddy, in press after Pielou, 1975 and Skellam, 1951).
Figure 4. (a,b) Effect of competition upon two common species of desert shrubs was measured by comparing the water potential of plants having many neighbours line designated ‘control’ with plants where all neighbours of both species had been removed line designated ‘all removed’. For both species, the removal of neighbours significantly improved their water potential ((a/b) after Fonteyn and Mahall, 1981).
Figure 5. Effects of above‐ and belowground competition from hardwood trees upon slash pine were assessed in four types of plots, from left to right, C = controls, S = reduced shading, T = reduced root competition and ST = reduced shading and reduced root competition. See text for more details. As reduced shading S did not increase pine growth, but the reduced root competition T allowed plants to nearly double in size, belowground competition appears far more important than aboveground competition. Corroborating evidence comes from the treatment ST, where reduced shading combined with reduced root competition was no different from merely reduced root competition (after Putz, 1992).
Figure 6. The importance of different ecological factors probably changes along environmental gradients. Total competition likely increases from left to right, as biomass accumulates and small plants are increasingly shaded. The relative importance of root and shoot competition, that is, below‐ and aboveground competition, probably shifts as light becomes increasingly limited. Mutualism may be important at the far left where plants may ameliorate harsh conditions for their neighbours. Thus, the importance of mutualism, and competition, and above‐ and belowground competition, may depend on the location of an organism (after Keddy, 2001a).


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Further Reading

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Dawkins R (1976) The Selfish Gene. Oxford, UK: Oxford University Press.

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Keddy PA (2007) Plants and Vegetation. Cambridge, UK: Cambridge University Press.

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Wisheu IC and Keddy PA (1992) Competition and centrifugal organization of plant communities: theory and tests. Journal of Vegetation Science 3: 147–156.

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Keddy, Paul A(Jan 2015) Competition. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0003162.pub2]