Vacant Niche - Demonstration of Vacant Niches

Demonstration of Vacant Niches

Vacant niches can best be demonstrated by considering the spatial component of niches in simple habitats. For example, Lawton and collaborators compared the insect fauna of the bracken Pteridium aquilinum, a widely distributed species, in different habitats and geographical regions and found vastly differing numbers of insect species. They concluded that many niches remain vacant (e.g., Lawton 1984).

Rohde and collaborators have shown that the number of ectoparasitic species on the gills of different species of marine fishes varies from 0 to about 30, even when fish of similar size and from similar habitats are compared. Assuming that the host species with the largest number of parasite species has the largest possible number of parasite species, only about 16% of all niches are occupied. However, the maximum may well be greater, since the possibility cannot be excluded that even on fish with a rich parasite fauna, more species could be accommodated (recent review in Rohde 2005b). Using similar reasoning, Walker and Valentine (1984) estimated that 12-54% of niches for marine invertebrates are empty.

The ground breaking theoretical investigations of Kauffman (1993) and Wolfram (2002) also suggest the existence of a vast number of vacant niches. Using different approaches, both have shown that species rarely if ever reach global adaptive optima. Rather, they get trapped in local optima from which they cannot escape, i.e., they are not perfectly adapted. As the number of potential local optima is almost infinite, the niche space is largely unsaturated and species have little opportunity for interspecific competition. Kauffman (p. 19) writes: “...many conceivable useful phenotypes do not exist” and: (p. 218) “Landscapes are rugged and multipeaked. Adaptive processes typically become trapped on such optima”.

The packing rules of Ritchie and Olff (1999) can be used as a measure of the filling of niche space. They apply to savanna plants and large herbivorous mammals, but not to all the parasite species examined so far. It seems likely that they do not apply to most animal groups. In other words, most species are not densely packed: many niches remain empty (Rohde 2001).

That niche space may not be saturated is also shown by introduced pest species. Such species lose, almost without exception, all or many of their parasites (Torchin and Kuris 2005). Species that could occupy the vacant niches either do not exist or, if they exist, cannot adapt to these niches.

The diversity of marine benthos, i.e. the organisms living near the seabed, though interrupted by some collapses and plateaus has increased from the Cambrian to the Recent. Furthermore there is no evidence to suggest that saturation has been reached (Jablonski 1999).