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Vacant niche
The issue of what exactly defines a vacant niche and whether they exist in ecosystems is controversial. The subject is intimately tied into a much broader debate on whether ecosystems can reach equilibrium, where they could theoretically become maximally saturated with species. Given that saturation is a measure of the number of species per resource axis per ecosystem, the question becomes: is it useful to define unused resource clusters as niche 'vacancies'?
==History of the concept==
Whether vacant niches are permissible has been both confirmed and denied as the definition of a niche has changed over time. Within the pre-Hutchinsonian niche frameworks of Grinnell (1917)〔Grinnell, J. (1917). The niche relationship of the California Thrasher. Auk, 34, 427-433.〕 and Elton (1927)〔Elton, C. (1927). Animal Ecology. Sidgwick and Jackson, London.〕 vacant niches were allowable. In the framework of Grinnell, the species niche was largely equivalent to its habitat, such that a niche vacancy could be looked upon as a habitat vacancy. The Eltonian framework considered the niche to be equivalent to a species position in a trophic web, or food chain, and in this respect there is always going to be a vacant niche at the top predator level. Whether this position gets filled depends upon the ecological efficiency of the species filling it however.
The Hutchinsonian niche framework, on the other hand, directly precludes the possibility of there being vacant niches. Hutchinson defined the niche as an n-dimensional hyper-volume whose dimensions correspond to resource gradients over which species are distributed in a unimodal fashion. In this we see that the operational definition of his niche rests on the fact that a species is needed in order to rationally ''define'' a niche in the first place. This fact didn't stop Hutchinson from making statements inconsistent with this such as: “The question raised by cases like this is whether the three ''Nilghiri Corixinae'' fill all the available niches...or whether there are really empty niches.. . .The rapid spread of introduced species often gives evidence of empty niches, but such rapid spread in many instances has taken place in disturbed areas.” Hutchinson (1957).〔Hutchinson, G. E. (1957). Concluding remarks. Cold Spring Harbour Symposium on Quantitative Biology 22, 415-427.〕
The concept of the “vacant” or “empty niche” has been used regularly in the scientific literature. Some of the many examples are Elton (1958, pp. 135–136),〔Elton, C.S. 1958. The ecology of invasions by animals and plants. Chapman and Hall, London, UK. 181 pp. ).〕 Rohde (1977, 1979, 1980)〔Rohde, K. (1977). A non-competitive mechanism responsible for restricting niches. Zoologischer Anzeiger 199, 164-172.〕〔Rohde, K. (1979). A critical evaluation of intrinsic and extrinsic factord responsible for restricting niches. American Naturalist 114, 648-671.〕〔Rohde, K. (1980 ). Warum sind ökologische Nischen begrenzt? Zwischenartlicher Antagonismus oder innerartlicher Zusammenhalt?. Naturwissenschaftliche Rundschau, 33, 98-102. ,〕 Lawton (1984),〔Lawton, J.H. (1984). Non-competitive populations, non-convergent communities, and vacant niches: the herbivores of bracken. In: Strong, D.R. Jr., Simberloff, D., Abele, L.G. and Thistle, A.B. eds. Ecological communities: conceptual issues and the evidence. Princeton University Press, Princeton, N.J., pp. 67-101.〕 Price (1984),〔Price, P.W. (1984). Alternative paradigms in community ecology. In: Price, P.W., Slobodchikoff, C.N. and Gaud, W.S. eds. (1984). A new ecology. Novel approaches to interactive systems. John Wiley & Sons, New York, Chichester, Brisbane, Toronto, Singapore, pp.353-383.〕 Compton et al. (1989),〔Compton, S.G., Lawton, J.H. and Rashbrook, V.K. (1989). Regional diversity, local community structure and vacant niches: the herbivorous arthropods of bracken in South Africa. Ecological Entomology 14, 365-373.〕 Begon et al. (1990)〔Begon, M.J., Harper, L. and Townsend, C.R. (1990). ''Ecology: Individuals, populations and communities''. 2.ed. Blackwell Scientific, Boston.〕 and Cornell (1999).〔Cornell, H.V. (1999). Unsaturation and regional influences on species richness in ecological communities: a review of the evidence. Ecoscience 6, 303-315.〕 Further examples, some of them in great detail, are discussed in Rohde (2005b).〔K. Rohde: Nonequilibrium Ecology, Cambridge University Press, Cambridge, 2005b, 223 pp. auf http://www.cambridge.org/9780521674553〕
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