When necessary resources are limited, competition can occur among individuals of the same species or of different species. A. Competition can eliminate a species, but since most species do not depend on a single resource, competition is often reduced by switching to alternative resources. B. Competition between individuals of the same species is usually for food whereas competition between species is usually for habitat. C. Investigation of the ecological role of competition is difficult because ordinarily the competition cannot be observed directly and must be inferred from its presumed effects. D. Experiments and field observations have established that competition between species is strong enough to prevent two species from occupying the same ecologica niche. E. Competition between a pair of species tends to lessen over time because the species tend to evolve to occupy different ecological niches and ranges. F. Competition is usually strongest when the density of the competing populations is the same.