Competitive Exclusion Principle

Origins: mathematical models and Gause's experiments

Lotka (1925) and Volterra (1926) formulated competition equations in which two species suppress each other's population growth. In these models two species can coexist only if competition between species is weaker than competition within each species. The Russian biologist Georgy Gause (1934) tested this experimentally. Two species of Paramecium grew well when cultured separately, but when grown together in the same medium on the same food, one disappeared, whereas species that fed in different parts of the culture coexisted. In Park's (1948) flour beetle experiments, which species won depended on temperature and humidity. Hardin (1960) summed up the idea as complete competitors cannot coexist and popularized the name competitive exclusion principle.

Niches and resource partitioning

Hutchinson (1957) defined the niche as an n-dimensional hypervolume of environmental variables and distinguished the fundamental niche a species could occupy without competitors from the realized niche it actually occupies under competition. Studying two barnacle species on the Scottish coast, Connell (1961) used removal experiments to show that the upper-shore barnacle was kept from moving lower by a stronger competitor. MacArthur (1958) reported resource partitioning among five warbler species that forage in the same trees but use different heights and parts of them. Such work led to the view that competition drives niche differentiation, which in turn allows coexistence.

The paradox and modern coexistence theory

Hutchinson (1961) posed the paradox of the plankton, asking how dozens of phytoplankton species coexist in lakes while competing for a few nutrients, and suggested that the environment keeps changing before equilibrium is reached. Tilman (1982) used resource ratio theory to identify conditions under which species limited by different resources coexist. Chesson (2000) organized modern coexistence theory by separating equalizing mechanisms, which reduce fitness differences between species, from stabilizing mechanisms, which make competition within species stronger than competition between them. Hubbell's (2001) neutral theory, by contrast, argued that diversity patterns can be explained even if species are ecologically equivalent, while Levine and HilleRisLambers (2009) showed experimentally that niche differences do matter for maintaining diversity. Together with research on metapopulations and keystone species, the principle underpins our understanding of how species diversity is maintained.

FAQ

What is the competitive exclusion principle in simple terms?

Two species that use exactly the same food and space in the same way cannot live together in one place for long; the slightly better competitor eventually wins.

Then how do similar species coexist?

Through resource partitioning, such as using different feeding places, times or resources, through environmental fluctuation, and through stabilizing mechanisms that make competition within species stronger than between species.

Sources reviewed: 2026-10-26

References

1. Gause, G. F. (1934). The Struggle for Existence. Baltimore: Williams & Wilkins.
2. Lotka, A. J. (1925). Elements of Physical Biology. Baltimore: Williams & Wilkins.
3. Volterra, V. (1926). Fluctuations in the abundance of a species considered mathematically. Nature, 118(2972), 558–560.
4. Hardin, G. (1960). The competitive exclusion principle. Science, 131(3409), 1292–1297.
5. Park, T. (1948). Experimental studies of interspecies competition. I. Competition between populations of the flour beetles, Tribolium confusum Duval and Tribolium castaneum Herbst. Ecological Monographs, 18(2), 265–307.
6. Connell, J. H. (1961). The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus. Ecology, 42(4), 710–723.
7. Hutchinson, G. E. (1957). Concluding remarks. Cold Spring Harbor Symposia on Quantitative Biology, 22, 415–427.
8. MacArthur, R. H. (1958). Population ecology of some warblers of northeastern coniferous forests. Ecology, 39(4), 599–619.
9. Hutchinson, G. E. (1961). The paradox of the plankton. American Naturalist, 95(882), 137–145.
10. Tilman, D. (1982). Resource Competition and Community Structure. Princeton: Princeton University Press.
11. Chesson, P. (2000). Mechanisms of maintenance of species diversity. Annual Review of Ecology and Systematics, 31, 343–366.
12. Hubbell, S. P. (2001). The Unified Neutral Theory of Biodiversity and Biogeography. Princeton: Princeton University Press.
13. Levine, J. M., & HilleRisLambers, J. (2009). The importance of niches for the maintenance of species diversity. Nature, 461(7261), 254–257.