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| Keystone Species and the Diversity-Stability Hypothesis |
Not all species are equal with respect to their effects on other species. Starfish feeding in the intertidal zone clean an area free of barnacles and mussels. These barnacles and mussels, without predation by the starfish, would come to dominate the community. In a classic 1966 study, Robert Paine removed starfish from enclosures. In those enclosures where the starfish were removed, the number of species in the community dropped from fifteen to eight. Paine called starfish a keystone species, one whose presence has a dramatic effect on species diversity.
Prior to 1973 most ecologists thought that more diverse ecosystems would be more stable than would ones with fewer species. This general belief, what has become known as the diversity-stability hypothesis, was based on a variety of observations but not really tested. One such observation was that cultivated land that had simplified ecological communities was more subject to species invasions than similar areas that hadn't had human influence. In addition, insect outbreaks are much more common in the less diverse boreal forests than they are in tropical forests.
In 1973 Robert May published a theoretical study that challenged the intuitive ideas that ecologists had about the diversity-stability hypothesis. May analyzed randomly constructed communities and found that communities with more species tended to be less, not more, stable. May's study, like more theoretical studies of the 1970s, assumed that population numbers of each species were at equilibria. This assumption was made, not because it reflected reality, but because it made the mathematics more tractable. More recent studies have shown that if there is some degree of flux in the population numbers, the community can maintain more species than in equilibrium. This variability may allow different species to respond differently to the environment, and can result in fewer species being lost due to competitive exclusion. When theoretical ecologists relax the equilibrium assumption and allow for population fluxes, they have found results consistent with the diversity-stability hypothesis: communities with more species are more stable.
Several lines of evidence now support the diversity-stability hypothesis. The studies conducted by David Tilman and his colleagues provide some of the strongest evidence for the hypothesis. In 1982 Tilman divided grassland fields in Minnesota's Cedar Creek National History Area into more than 200 plots. He and his colleagues monitored the species richness and community biomass (the total mass of all plants) in each of those fields over the next two decades. They found that diversity within a community is positively correlated with plant community stability, as defined by the extent of variation in community biomass. Various other studies at different scales have found similar results: stability increases with diversity.4