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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.