Historical development of the niche theory is very closely related to one of the most important topics of ecology, that is, the problem of species competition and coexistence. Since the beginning of the ecological niche concept, it has been assumed that no two species sharing a single niche could locally coexist. Originally, the later Volterra-Gause principle states that ''under constant conditions, no two species utilizing, and limited by, a single resource can coexist in a limited system'' and was formulated and proved by Vito Volterra, while Alfred Gause showed experimental evidence of competitive exclusion in an undiversified environment. The explanation ofthe competitive exclusion lies in the fact that utilization of a limited resource leads to its depletion, and the population growth, therefore, necessarily leads to a moment when the resource level is insufficient for further growth. If only one population utilizes the resource, this situation leads to simple negative feedback, causing the decrease of population growth rate and thus a release of resource consumption, stabilizing the population size. However, in the case of two species sharing the resource, there will likely exist a resource level when the first species population can still grow up even if the second cannot, leading to further decrease ofpopulation growth rate of the second species, and eventually to its extinction. Even if two species sharing several resources have exactly the same requirements and ability to utilize them, the coexistence of such species is not stable in a stochastic environment (if their total population density is limited): one of the species would ultimately become extinct by chance over infinite time (unless there is an advantage for the less abundant species).
The 'competitive exclusion principle' is the core principle in community ecology, and much of this field has been devoted to study how species with similar ecological requirements can coexist. This question has transformed into the problem of 'limiting similarity': how similar can ecological niches be to still ensure local coexistence.
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