Differential competitive interactions and the included niche: An experimental analysis with grasshoppers

The included niche is a situation where one species' niche is included within that of a more generalized species. The species whose niche is included must be a superior competitor for the shared resources on a per capita basis in order ro coexist. In addition to per capita effects that one species has on the other for shared resources, the overall competitive effect a generalist can have on a specialist is determined by the density that it can maintain on exclusive resources while still having a consumptive effect on the shared resources. This 'numerical effect' occurs independently of the per capita effects for the shared resources. Thus, coexistence and the relative abundances of species within an included niche depend upon per capita effects of both species for the shared resources and the ratio of exclusive:shared resources available to the generalist. I examined the dynamics of two grass-feeding grasshoppers, the larger, Arphia pseudoneitana, and the smaller Trachyrhachys kiowa, whose feeding niches were included within that of the generalist, Melanoplus sanguinipes. While both specialists were better competitors for the shared resources than was M. sanguinipes, their overall competitive effects were different. When M. sanguinipes was with T. kiowa, its exclusive resources consisted of forbs as well as a proportion of grasses. By having a relatively high carrying capacity on exclusive resources, M. sanguinipes was able to exert a greater overall competitive effect on T. kiowa than the reverse. When M. sanguinipes was with A. pseudoneitana. its exclusive resources consisted of only forbs, such that M. sanguinipes exhibited a smaller overall effect on A. pseudoneitana than the reverse.