Editing Ecological niche (section)

===Relative significance of the mechanisms===
Both paradigms acknowledge a role for all mechanisms (except possibly for that of random selection of niches in the first paradigm), but emphasis on the various mechanisms varies. The first paradigm stresses the paramount importance of interspecific competition, whereas the second paradigm tries to explain many cases which are thought to be due to competition in the first paradigm, by reinforcement of reproductive barriers and/or random selection of niches. – Many authors believe in the overriding importance of interspecific competition. Intuitively, one would expect that [[interspecific competition]] is of particular importance in all those cases in which sympatric species (i.e., species occurring together in the same area) with large population densities use the same resources and largely exhaust them. However, Andrewartha and Birch (1954,1984)<ref>Andrewartha, H.G. and Birch, L.C. 1954. The distribution and abundance of animals. University of Chicago Press, Chicago.</ref><ref>Andrewartha, H.G. and Birch, L.C. 1984. The ecological web. University of Chicago Press. Chicago and London.</ref> and others have pointed out that most natural populations usually don't even approach exhaustion of resources, and too much emphasis on interspecific competition is therefore wrong. Concerning the possibility that competition has led to segregation in the evolutionary past, Wiens (1974, 1984)<ref>Wiens, J.A. 1974. Habitat heterogeneity and avian community structure in North American grasslands. American Midland Naturalist 91,195-213.</ref><ref>Wiens, J.A. 1984. Resource systems, populations, and communities. In: Price, P.W., Slobodchikoff, C.N. and Gaud, W.S. Eds. A new ecology. Novel approaches to interactive systems. John Wiley & Sons, New York, Chichester, Brisbane, Toronto, Singapore, pp. 397–436.</ref> concluded that such assumptions cannot be proven, and Connell (1980)<ref>Connell, J.H. 1980. Diversity and the coevolution of competitors, or the ghost of competition past. Oikos 35, 131–138.</ref> found that interspecific competition as a mechanism of niche segregation has been proven only for some pest insects. Barker (1983),<ref>Barker, J.S.F. 1983. Interspecific competition. In: Ashburner, M., Carson, H.L. and Thompson, jr., J.N. Ed. The genetics and biology of Drosophila. Academic Press, London, pp. 285–341.</ref> in his review of competition in ''Drosophila'' and related genera, which are among the best known animal groups, concluded that the idea of niche segregation by interspecific competition is attractive, but that no study has yet been able to show a mechanism responsible for segregation. Without specific evidence, the possibility of random segregation can never be excluded, and assumption of such randomness can indeed serve as a null-model. – Many physiological and morphological differences between species can prevent hybridization. Evidence for niche segregation as the result of reinforcement of reproductive barriers is especially convincing in those cases in which such differences are not found in allopatric but only in sympatric locations. For example, Kawano (2002)<ref>Kawano, K. 2002. Character displacement in giant rhinoceros beetles. American Naturalist 159, 255–271.</ref> has shown this for giant rhinoceros beetles in Southeast Asia. Two closely related species occur in 12 allopatric (i.e., in different areas) and 7 sympatric (i.e., in the same area) locations. In the former, body length and length of genitalia are practically identical, in the latter, they are significantly different, and much more so for the genitalia than the body, convincing evidence that reinforcement is an important factor (and possibly the only one) responsible for niche segregation. - The very detailed studies of communities of Monogenea parasitic on the gills of marine and freshwater fishes by several authors have shown the same. Species use strictly defined microhabitats and have very complex copulatory organs.  This and the fact that fish replicas are available in almost unlimited numbers, makes them ideal ecological models.  Many congeners (species belonging to the same genus) and non-congeners were found on single host species. The maximum number of congeners was nine species. The only limiting factor is space for attachment, since food (blood, mucus, fast regenerating epithelial cells) is in unlimited supply as long as the fish is alive. Various authors, using a variety of statistical methods, have consistently found that species with different copulatory organs may co-occur in the same microhabitat, whereas congeners with identical or very similar copulatory organs are spatially segregated, convincing evidence that reinforcement and not competition is responsible for niche segregation.<ref>Rohde, K. 1991. Intra- and interspecific interactions in low density populations in resource-rich habitats.  Oikos 60, 91–104.</ref><ref>Rohde, K. 1994. Niche restriction in parasites: proximate and ultimate causes. Parasitology 109, S69-S84.</ref><ref>Simkova, A., Desdevises, Y., Gelnar, M. and Morand, S. (2000). Co-existence of nine gill ectoparasites (Dactylogyus: Monogenea) parasitising the roach Rutilus rutilus ( L.): history and present ecology. International Journal for Parasitology 30, 1077–1088.</ref><ref>Simkova, A., Gelnar, M. and Morand, S. (2001). Order and disorder in ectoparasite communities: the case of congeneric gill monogeneans (Dactylogyrus spp.). International Journal for Parasitology 31, 1205–1210.</ref><ref>Simkova, A., Gelnar, M. and Sasal, P. (2001). Aggregation of congeneric parasites (Monogenea: Dactylogyrus). Parasitology 123, 599–607.</ref><ref>Simkova, A., Desdevises, Y., Gelnar, M. and Morand, S. 2001. Morphometric correlates of host specificity in Dactylogyrus species (Monogenea) parasites of European Cyprinid fish. Parasitology 123, 169–177.</ref>

For a detailed discussion, especially of competition and reinforcement of reproductive barriers, see<ref name="Nonequilibrium Ecology(1992)"/>