THE ECOLOGICAL CONSEQUENCES OF CHANGES IN BIODIVERSITY: A SEARCH FOR GENERAL PRINCIPLES¹⁰¹

This paper uses theory and experiments to explore the effects of diversity on stability, productivity, and susceptibility to invasion. A model of resource competition predicts that increases in diversity cause community stability to increase, but population stability to decrease. These opposite effects are, to a great extent, explained by how temporal variances in species abundances scale with mean abundance, and by the differential impact of this scaling on population vs. community stability. Community stability also depends on a negative covariance effect (competitive compensation) and on overyielding (ecosystem productivity increasing with diversity). A long-term study in Minnesota grasslands supports these predictions. Models of competition predict, and field experiments confirm, that greater plant diversity leads to greater primary productivity. This diversity–productivity relationship results both from the greater chance that a more productive species would be present at higher diversity (the sampling effect) and from the better “coverage” of habitat heterogeneity caused by the broader range of species traits in a more diverse community (the niche differentiation effect). Both effects cause more complete utilization of limiting resources at higher diversity, which increases resource retention, further increasing productivity. Finally, lower levels of available limiting resources at higher diversity are predicted to decrease the susceptibility of an ecosystem to invasion, supporting the diversity–invasibility hypothesis. This mechanism provides rules for community assembly and invasion resistance. In total, biodiversity should be added to species composition, disturbance, nutrient supply, and climate as a major controller of population and ecosystem dynamics and structure. By their increasingly great directional impacts on all of these controllers, humans are likely to cause major long-term changes in the functioning of ecosystems worldwide. A better understanding of these ecosystem changes is needed if ecologists are to provide society with the knowledge essential for wise management of the earth and its biological resources.