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Vacant niches

This post is based on my earlier entry in Wikipedia. This seems necessary because Wikipedia permits changes made by anybody, whether with the necessary knowledge or not.

The concept of a vacant or empty niche has been controversial in ecology. It is at the basis of any discussion of whether equilibrium or nonequilibrium conditions prevail in ecological systems. A vacant niche can be defined as the possibility that in ecosystems or habitats more species can exist than are present at a particular point in time, because many possibilities are not used by potentially existing species (K. Rohde 2005 Nonequilibrium Ecology, Cambridge University Press, Cambridge, http://www.cambridge.org/9780521674553).

1. History of the concept

Hutchinson, G. E. (1957) Concluding remarks. Cold Spring Harbour Symposium on Quantitative Biology 22, 415-427 was apparently the first who considered the possibility of vacant niches. He writes: (p.424): “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.”

Since then, the concept “vacant niche” or “empty niche” has been used regularly in the scientific literature. Some of the many examples are Elton, C. (1958) The ecology of invasions by animals and plants. Chapman and Hall, London, UK. 181 pp.; 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, 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, 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, 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, M.J., Harper, L. and Townsend, C.R. (1990). Ecology. Individuals, populations and communities, second edition. Blackwell Scientific, Boston; and 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 K. Rohde (2005) Nonequilibrium Ecology, Cambridge University Press, Cambridge.

2. Causes of vacant niches

Vacant niches can have several causes.

One cause is radical disturbances in a habitat (biotop). For example, draughts or forest fires can destroy a flora and fauna partially or completely. However, in such cases species suitable for the habitat usually survive in the neighbourhood and colonize the vacated niches, leading to a relatively fast reestablishment of the original conditions.

Further causes of vacant niches are radical and long-lasting changes in the environment, such as ice ages.

Vacant niches can also be due to evolutionary contingencies: suitable species did not evolve for usually unknown reasons.

3. Demonstration of vacant niches

Vacant niches can best be demonstrated by comparing 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, 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.

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 K. Rohde (2005) Nonequilibrium Ecology, Cambridge University Press, Cambridge.

Using a similar way of reasoning, Walker, T.D. und Valentine, J.W. (1984). Equilibrium models of evolutionary diversity and the number of empty niches. American Naturalist 124, 887-899. estimated that 12-54% of niches for marine invertebrates are empty.
The ground breaking theoretical investigations of Kauffman, S.A. (1993). The origins of order. Self-organization and selection in evolution. Oxford University Press, New York Oxford and Wolfram, S. (2002). A new kind of science. Wolfram Media Inc. Champaign, Il. 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.

The packing rules of Ritchie, M. und Olff, H. (1999). Spatial scaling laws yield a synthetic theory of biodiversity. Nature 400, 557-562 can be used as a measure of the filling of niche space. They apply to savanna plants and large herbivorous mammals, but not to any of 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, K. (2001). Spatial scaling laws may not apply to most animal species. Oikos 93, 499-503).

That niche space may be nonsaturated, is also shown by introduced pest species. Such species lose almost without exception all or many of their parasites (Torchin, M.E. and Kuris, A.M. (2005). Introduced parasites. In: Rohde, K. (Ed.) Marine Parasitology. CSIRO Publishing Melbourne und CABI Wallingford, Oxon., pp. 358-366). Species that could occupy the vacant niches either do not exist or, if they exist, have not had the chance to invade these niches.

Diversity of marine benthos, interrupted by some collapses and plateaus, has increased from the Cambrian to the Recent, and there is no evidence that saturation has been reached (Jablonski, D. (1999) The future of the fossil record, Science 284, 2114-2116).

Simulations of latitudinal gradients in species diversity using the Chowdhury ecosystem model, have shown that results are closest to reality when many niches are kept empty (Rohde, K. and Stauffer, D . 2005 Simulation of geographical trends in the Chowdhury ecosystem model. Advances in Complex Systems 8, 451-464).

4. Consequences of the nonsaturation of niche space

The view that niche space is largely or completely saturated with species is widespread. It is thought that new species are accommodated mainly by subdivision of niches occupied by previously existing species, although an increase in diversity by colonization of large empty living spaces (such as land in the geologic past) or by the formation of new bauplans also occurs. It is also recognized that many populations never completely reach a climax state (i.e., they may come close to an equilibrium but never quite reach it). However, altogether the view prevails that individuals and species are densely packed and that interspecific competition is of paramount significance. According to this view, nonequilibria are generally caused by environmental disturbances.

Many recent studies (above and Rohde, K. (2005) Eine neue Ökologie. Aktuelle Probleme der evolutionären Ökologie. Naturwissenschaftliche Rundschau, 58, 420-426; Rohde, K. 2005: Nonequilibrium Ecology, Cambridge University Press, Cambridge support the view that niche space is largely unsaturated, i.e. that numerous vacant niches exist. As a consequence, competition between species is not as important as usually assumed. Nonequilibria are caused not only by environmental disturbances, but are widespread because of nonsaturation of niche space. Newly evolved species are absorbed into empty niche space, that is, niches occupied by existing species do not necessarily have to shrink.

5. Relative frequency of vacant niches in various groups of animals and plants

Available evidence suggests that vacant niches are more common in some groups than in others. Using SES values (standardized effect sizes) for various groups, which can be used as approximate predictors of the filling of niche space, Gotelli, N.J. and Rohde, K. (2002). Co-occurrence of ectoparasites of marine fishes: null-model analysis. Ecology Letters 5, 86-94 have shown that SES values are high for animal populations which occur in large population densities and/or are of large body size and are vagile, they are low for animal groups which occur in small population densities and/or are of small body size and have little vagility. In other words, more vacant niches can be expected for the latter.

6. Criticisms of the concept

The concept of vacant niche is not accepted by all. The reason given is that a niche is a property of a species and does therefore not exist if no species is present. In other words, the term is thought to be “illogical”. However, some authors who have contributed most to the formulation of the modern niche concept (Hutchinson, Elton) apparently saw no difficulties in using the term. If a niche is defined as the interrelationship of a species with all the biotic and abiotic factors affecting it, there is no reason not to admit the possibility of additional potential interrelationships. So, it seems logical to refer to vacant niches.

Furthermore, it seems that authors most critical of the concept vacant niche really are critical of the view that niche space is largely empty and can easily absorb additional species. They instead adhere to the view that everything is much of the time in equilibrium (or at least close to it), resulting in a continual strong competition for resources. This view, indeed, is the basis of Darwinian natural selection. Many recent studies, some empirical , some theoretical, have provided support for the alternate view that nonequilibrium conditions are widespread (see above and the recent review in Rohde K. Rohde 2005) Nonequilibrium Ecology, Cambridge University Press, Cambridge.

In the German literature, an alternate term for vacant niches has found some acceptance. It is that of “freie ökologische Lizens” (free ecological license) (Sudhaus,W. und Rehfeld, K. 1992. Einführung in die Phylogenetik und Systematik. Gustav Fischer Verlag Jena. It has the disadvantage that it does not convey immediately and easily what is meant, and it indeed does not correspond exactly to the term vacant niche. The usefulness of a term should be measured on the basis of its pregnancy and easy understandability, and on how fertile it is in promoting future research. The term vacant niche appears to fulfill these requirements.

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