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Diversity-stability hypothesis Gregory M. Mikkelson McGill School of Environment and Department of Philosophy 3534 University Street Montréal, Québec H3A 2A7 [email protected] In: Callicott, J. B. and R. Frodeman, editors. Encyclopedia of Environmental Ethics and Philosophy. Thomson Gale. Farmington Hills, MI. The idea that biological diversity enhances ecological stability has inspired a huge body of scientific research, from the 1950's and before to the 2000's and beyond. It has also played an important role in environmental ethics, e.g., in Leopold's land ethic. In his view, biodiversity is essential for "a food chain aimed to harmonize the wild and the tame in the joint interest of stability, productivity, and beauty." (1949, p. 199) Then, as now, potential links between diversity and stability helped to whet the impetus for conservation. However, ecologists have at times shown what McNaughton called a "marked instability of attitudes regarding diversity-stability relationships". He attributed this to a "low diversity of empirical tests of the hypothesis" (1977, p. 523). In this respect, scientists are now in a much better position to assess the diversity-stability hypothesis (henceforth below, "the hypothesis"), with over 40 direct experimental trials on the books already, and presumably more in progress. But another reason for changing attitudes has been that "stability" is a multi-vocal concept, and evidence suggests that not all types of it vary positively with biodiversity. Thus, the plausibility of the hypothesis has varied as different kinds of ecological stability have come into vogue. Three inter-related trajectories are discernible over the past 55 years or so. For one, as intimated above, scientists' confidence in the hypothesis plummeted in the early 1970's; but then it steadily rose again, beginning in the mid-1990's. Two other trends help to explain this recovery: emphasis has shifted from the stability of individual populations to that of entire communities or ecosystems; and ecologists have come to focus on forms of stability that are both empirically measurable and theoretically tractable. Below, I will trace these and related developments. In the 1950's, three of the 20th Century's most influential ecologists each argued that the hypothesis was plausible enough to warrant further study (Odum 1953, MacArthur 1955, Elton 1958). They reasoned that if a given species preys on several others, its population size will fluctuate less in response to environmental variation affecting one of its prey, than it would if the species in question ate fewer prey species. Similarly, if a species has many predators, its population will vary less in response to exogenous changes in one predator's population size. As empirical support for these ideas, Odum and Elton cited the dramatic oscillations experienced by many populations in the arctic, but not in the far more species-rich tropics; and the tendency for pest populations to undergo more frequent and severe "outbreaks" in simplified agricultural systems than in complex natural systems. While MacArthur's paper was more conceptual than empirical, he also construed the hypothesis in terms of population stability. The few empirical studies of diversity-stability relations conducted in the 1960's and early 1970's yielded a confusing mix of positive, negative, and ambiguous results (Goodman 1975). But the coup de grace came from a purely theoretical exercise, on the basis of which May claimed that "simple mathematical models with many species are in general less stable than the corresponding simple mathematical models with few species." (1973, p. 49; italics added) I say "claimed", because this result actually depends on particular, and indeed quite debatable, assumptions about whether and how certain other variables (e.g., the number and strength of interactions between species pairs) change as the number of species increases (McCann 2000). Nevertheless, the idea that diversity de-stabilizes ecological systems quickly became a "new paradigm", despite a continued dearth of empirical proofs for or against the hypothesis (Loreau et al. 2002). Direct experimental tests finally got under way in the mid-1990's, as evidence came to light that when it comes to diversity-stability relations, the whole may not predictable from the parts. (By "direct", I mean experiments in which the number of species is manipulated independently of other "independent" variables, such as soil fertility.) In some grasslands where diversity decreases the stability of individual plant populations, it nevertheless increases the stability of the ecosystem as a whole. This is because in the more diverse ecosystems, upsurges in some populations are more often offset by declines in others, and vice versa (Tilman et al. 2006). Along the way to this discovery, a key conceptual stumbling block was overcome, namely the mathematically convenient but empirically inapplicable definitions of stability used by May and many others. Such definitions often assume infinitely small perturbations of population size, from equilibria that most actual populations never reach – among other potential problems (Mikkelson 1997, Justus in press). Ecologists eventually settled on two empirically meaningful types of stability, in the great majority of direct experimental tests of the hypothesis: resistance to invasion by new species, and temporal stability – the mean value of a variable (usually biomass or productivity), divided by its standard deviation, both calculated over time. The latter measure respects the pre-1970's emphasis on variability; and can be applied to entire ecosystems as well as to component populations, and explored through a broad range of theoretical tools (Lehman and Tilman 2000). The upshot of the direct experimental tests performed so far is that diversity does indeed enhance both invasion resistance and the temporal stability of ecosystem biomass or productivity (Ives and Carpenter 2007, Table S1). Although these authors did not review direct experimental tests of the diversity-population-stability hypothesis, my sense is that the evidence about it is more mixed (see, e.g., Romanuk et al. 2006). One possibility is that Odum, MacArthur, and Elton were right about the positive effects of prey and predator diversity on population stability. But they did not take into account the fact that having numerous prey and predator species allows numerous competitor species to also "fit" into the ecosystem (Chase and Leibold 2003). If competitor diversity, in turn, has negative effects on population stability, then the overall diversity-populationstability relationship may depend upon a highly contingent balance between the effects of diversity at these different trophic levels. To sum up then, a period of relatively unconstrained theorizing led most ecologists to reject the diversity-stability hypothesis by the 1980's. A much more experimentally-driven research program, along with a shift in focus from population to ecosystem stability, then led to the rehabilitation of the hypothesis, starting in the mid1990's. Hopefully, knowledge of diversity-stability relations will help us to mitigate the current, human-induced mass extinction. As this knowledge improves, however, it is important to keep in mind that effects of diversity on stability and other aspects of ecosystem function are only some of the many reasons – e.g., moral, intellectual, and aesthetic reasons – for protecting and promoting the wondrous variety of life on Earth. As Elton put it, "conserving the variety of nature" is also simply "a right relation between man and living things" (1958, p. 145). References Chase, J. M. and M. A. Leibold. 2003. Ecological Niches: Linking Classical and Contemporary Approaches. University of Chicago Press. Chicago, IL. Elton, C. S. 1958. The Ecology of Invasions by Animals and Plants. Methuen. London, England. Goodman, D. 1975. The theory of diversity-stability relationships in ecology. The Quarterly Review of Biology 50:237-266. Ives, A. R. and S. R. Carpenter. 2007. Stability and diversity of ecosystems. Science 317:58-62. Justus, J. In press. Ecological and Lyapunov stability. Philosophy of Science. Lehman, C. L. and D. Tilman. 2000. Biodiversity, stability, and productivity in competitive communities. The American Naturalist 156:534-552. Leopold, A. 1949. A Sand County Almanac. Oxford University. New York, NY. Loreau, M., A. Downing, M. Emmerson, A. Gonzalez, J. Hughes, P. Inchausti, J. Joshi, J. Norberg, and O. Sala. 2002. A new look at the relationship between diversity and stability. In: Loreau, M., S. Naeem, and P. Inchausti, editors. Biodiversity and Ecosystem Functioning: Synthesis and Perspectives. Oxford University. New York, NY. Pp. 79-91. MacArthur, R. H. 1955. Fluctuations of animal populations and a measure of community stability. Ecology 36:533-536. May, R. M. 1973. Stability and Complexity in Model Ecosystems. Princeton University. Princeton, NJ. McCann, K. S. 2000. The diversity-stability debate. Nature 405:228-233. McNaughton, S. J. 1977. Diversity and stability of ecological communities: A comment on the role of empiricism in ecology. The American Naturalist 111:515-525. Mikkelson, G. M. 1997. Methods and metaphors in community ecology: The problem of defining stability. Perspectives on Science 5:481-498. Odum, E. P. 1953. Fundamentals of Ecology. W. B. Saunders. Philadelphia, PA. Romanuk, T. N., R. J. Vogt, and J. Kolasa. 2006. Nutrient enrichment weakens the stabilizing effect of species richness. Oikos 114:291-302. Tilman, D., P. B. Reich, and J. M. H. Knops. 2006. Biodiversity and stability in a decade-long grassland experiment. Nature 441:629-632.