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Before we get into this concept, here are some things to know:

  • Interspecific competition : when populations of two or more species in a community rely on similar limiting resources; i.e., competition between species.

  • Carrying Capacity : the maximum population size that can be supported by the available resources

  • Logistic population growth : this model of population growth incorporates the effect of population density on r, allowing it to vary from rmax under ideal conditions to zero as carrying capacity is reached. In other words, if a populations size is below the carrying capacity, population growth is rapid. But as N approaches K, population growth is slow.
    r : population growth rate
    rmax : the maximum population growth rate, or the intrinsic rate of increase
    N : population size
    K : carrying capacity

  • Protozoan : (pl. protozoa), a protist that lives primarily by ingesting food

In the early twentieth century, two mathmetician/bioligist people by the names of A.J. Lotka and V. Volterra independently modified the logistic model of population growth to incorporate interspecific competition.

According to these guys, two species with similar requirements could not coexist in the same community, as one species would inevitable harvest resources and reproduce more efficiently, driving the other to local extinction. They must have similar needs and be in the same community, or else their niches would not overlap and therefore there would be no competition.

In 1934, a Russian man named G.F. Gause tested this hypothesis in the lab using two closely related species of protozoa, Paramecium aurelia and P. caudatum. When these two were in separate containers under constant conditions, and were provided with a supply of bacteria to nibble on, each population grew until it leveled off at the assumed carrying capacity.

However, when he put the two populations together into one container under the same conditions with the same food, P. caudatum, apparently unable to compete with P. Aurelia, was driven to extinction in the container. This supported the hypothesis that two species with similar needs for the same limiting resources cannot coexist in the same place. This was deemed the competitive exclusion principle.

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