History of the concept Introduced species, such as the common brushtail possum, are often free of many of their normal parasites. 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 URL Despite the large scale currency that the term has gained as a "catch all" in scientific literature, surprisingly little effort has been made to come up with a coherent definition. The most notable attempt is that of the ecologist K. Rohde, who has suggested that a vacant niche can be defined as the possibility that in ecosystems or habitats more species could exist than are present at a particular point in time, because many possibilities are not used by potentially existing species (Rohde 2005b). For a systematic review see: (Lekevičius, 2009). Lekevičius , E. (2009). Vacant niches in nature, ecology, and evolutionary theory: a mini-review. Ekologija 55, 165-174. 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) 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 (1993) Kauffman, S.A. (1993). The origins of order. Self-organization and selection in evolution. Oxford University Press, New York Oxford. and Wolfram (2002) 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. 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) Ritchie, M. and 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 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). Rohde, K. (2001). Spatial scaling laws may not apply to most animal species. Oikos 93, 499-503. 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). 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, 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). D.Jablonski: The future of the fossil record, Science 284, 2114-2116, 1999. However, many recent studies (above and Rohde 2005a,b) Rohde, K. (2005a) Eine neue Ökologie. Aktuelle Probleme der evolutionären Ökologie. Naturwissenschaftliche Rundschau, 58, 420-426. 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. 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 and Rohde (2002) 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 large and vagile species or for those which occur in large population densities, and that they are low for animal species 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. In the German literature, an alternate term for vacant niches has found some acceptance - that of freie ökologische Lizens (free ecological license) (Sudhaus und Rehfeld 1992) .Sudhaus, W. und Rehfeld, K. Einführung in die Phylogenetik und Systematik. Gustav Fischer Verlag Jena . It has been argued that this conceptualization has a disadvantage in that it does not convey immediately and easily what is meant, furthermore the concept does not correspond exactly to the term "vacant niche". The usefulness of a term should be assessed on the basis of its understandability and on its capacity to promote future research. The term "vacant niche" appears to fulfill these requirements.
History of the concept Introduced species, such as the common brushtail possum, are often free of many of their normal parasites. 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) and Elton (1927) 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). 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), Rohde (1977, 1979, 1980) Lawton (1984), Price (1984), Compton et al. (1989), Begon et al. (1990) and Cornell (1999). Further examples, some of them in great detail, are discussed in Rohde ( 2005). Despite the large scale currency that the term has gained as a "catch all" in scientific literature, surprisingly little effort has been made to come up with a coherent definition. The most notable attempt is that of the ecologist K. Rohde, who has suggested that a vacant niche can be defined as the possibility that in ecosystems or habitats more species could exist than are present at a particular point in time, because many possibilities are not used by potentially existing species (Rohde 2005b). For a systematic review see: (Lekevičius, 2009). 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). However, many recent studies (above and Rohde 2005a,b) 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. 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 and Rohde (2002) have shown that SES values are high for large and vagile species or for those which occur in large population densities, and that they are low for animal species 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. In the German literature, an alternate term for vacant niches has found some acceptance - that of freie ökologische Lizens (free ecological license) (Sudhaus und Rehfeld 1992) . It has been argued that this conceptualization has a disadvantage in that it does not convey immediately and easily what is meant, furthermore the concept does not correspond exactly to the term "vacant niche". The usefulness of a term should be assessed on the basis of its understandability and on its capacity to promote future research. The term "vacant niche" appears to fulfill these requirements.
D
Grinnell, J. (1917). The niche relationship of the California Thrasher. Auk, 34, 427-433.
D
Elton, C. (1927). Animal Ecology. Sidgwick and Jackson, London.
D
Hutchinson, G. E. (1957). Concluding remarks. Cold Spring Harbour Symposium on Quantitative Biology 22, 415-427.
R
“vacant” or “empty niche”
"vacant" or "empty niche"
D
Elton, C.S. 1958. The ecology of invasions by animals and plants. Chapman and Hall, London, UK. 181 pp. ).
D
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. ,
D
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.
D
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
D
Sons, New York, Chichester, Brisbane, Toronto, Singapore, pp.353-383.
D
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.
R
Begon, M.J., Harper, L. and Townsend, C.R. (1990). Ecology: Individuals, populations and communities. 2.ed. Blackwell Scientific, Boston. and
and
D
Cornell, H.V. (1999). Unsaturation and regional influences on species richness in ecological communities: a review of the evidence. Ecoscience 6, 303-315.
R
2005b). K. Rohde: Nonequilibrium Ecology, Cambridge University Press, Cambridge, 2005b, 223 pp. auf URL
2005).
D
Lekevičius , E. (2009). Vacant niches in nature, ecology, and evolutionary theory: a mini-review. Ekologija 55, 165-174.
D
Walker, T.D. und Valentine, J.W.(1984). Equilibrium models of evolutionary diversity and the number of empty niches. American Naturalist 124, 887-899.
R
Kauffman, S.A. (1993). The origins of order. Self-organization and selection in evolution. Oxford University Press, New York Oxford. and
and
D
Wolfram, S. (2002). A new kind of science. Wolfram Media Inc. Champaign, Il.
D
Ritchie, M. and Olff, H. (1999). Spatial scaling laws yield a synthetic theory of biodiversity. Nature 400, 557-562.
D
Rohde, K. (2001). Spatial scaling laws may not apply to most animal species. Oikos 93, 499-503.
D
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.
D
D.Jablonski: The future of the fossil record, Science 284, 2114-2116, 1999.
D
Rohde, K. (2005a) Eine neue Ökologie. Aktuelle Probleme der evolutionären Ökologie. Naturwissenschaftliche Rundschau, 58, 420-426.
D
Gotelli, N.J. and Rohde, K. (2002). Co-occurrence of ectoparasites of marine fishes: null-model analysis. Ecology Letters 5, 86-94.
D
.Sudhaus, W. und Rehfeld, K. Einführung in die Phylogenetik und Systematik. Gustav Fischer Verlag Jena