Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2008
Publication Date: 6/30/2009
Citation: Isbell, F., Polley, H.W., Wilsey, B.J. 2009. Species interaction mechanisms maintain grassland plant species diversity. Ecology. 90:1821-1830.
Interpretive Summary: Human activities are reducing plant diversity on many grasslands by reducing the number of plant species present and by creating greater inequity in abundances of species that remain. Declines in diversity may, in turn, be reducing the capacity of grasslands to meet human needs, yet we have limited understanding of processes that increase or maintain diversity. We develop a method to distinguish among three types of processes that may maintain plant diversity. These include processes that reduce the loss of plant species, processes that minimize differences in abundances among species, and factors that minimize differences in growth rates among species. We measured aboveground biomass in field plots in central Texas in which we had varied the number and relative abundances of grassland species at planting. We used changes in biomass yields of each species over 7 years to determine which of the three processes that maintain plant diversity predominated in our plots. Diversity was maintained mostly by processes that minimize differences in abundances among species. Our results indicate that grassland diversity may be maintained by adopting management practices that tend to equalize plant species abundances. Among these practices are those that benefit the least productive species in grassland communities by promoting grazing of highly-productive species.
Technical Abstract: Theory has outpaced empirical research in pursuit of identifying mechanisms maintaining species diversity. Here we demonstrate how data from diversity-ecosystem functioning experiments can be used to test maintenance of diversity theory. We predict that grassland plant diversity can be maintained by three classes of mechanisms: 1) richness stabilizing mechanisms, which increase niche partitioning and positive species interactions (and thus maximize the complementarity effect), 2) evenness stabilizing mechanisms, which favor the yielding behavior of the least productive species (and thus minimize the selection effect), and 3) equalizing mechanisms, which minimize average fitness differences among species (and thus minimize the coefficient of variation in monoculture biomass production among species in mixture). We tested these predictions with long-term data from an ongoing study where grassland plant species evenness and richness were experimentally varied. We used path analysis to model the effects of these mechanisms on annual and multiyear changes in diversity. We found that diversity was frequently maintained by evenness stabilizing mechanisms, was infrequently maintained by equalizing mechanisms, and was not maintained by richness stabilizing mechanisms. Evenness stabilizing mechanisms maintained diversity by allowing the species that produced the least biomass in monoculture to benefit the most from species interactions in mixture. Equalizing mechanisms infrequently maintained diversity because these pecies interactions prevented asymmetric competition from resulting in competitive exclusion. We recommend applying this analysis to existing data sets from other diversity-ecosystem functioning experiments. We propose that this theoretical mechanistic context can be used to determine the processes driving the recent declines in diversity.