Submitted to: Ecology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 20, 2002
Publication Date: May 1, 2003
Citation: Polley, H.W., Wilsey, B.J., Derner, J.D. 2003. Do species evenness and plant density influence the magnitude of selection and complementarity effects in annual plant species mixtures? Ecology Letters. 6:248-256.
Interpretive Summary: The diversity of plant species in many grasslands is declining as a result of poor management and other human activities. This decline in diversity threatens the productivity of grasslands, but our understanding of which aspects of plant diversity are most critical to sustaining grassland function is rudimentary. We examined the role of species richness (number of plant species present) and species relative abundances in determining biomass production of experimental plots with three species of annual plants that co-occur in central Texas grasslands. Contrary to expectation, biomass production was smaller in plots in which species were grown together than in plots in which species were grown separately. For these species, increasing diversity by increasing the number of species present in experimental plots actually reduced productivity because species interacted negatively and the most successful species in mixtures was the least productive of the plants studied. Biomass was no greater in mixtures in which species were equally abundant than in those in which one of the three species dominated. Our results demonstrate that effects of diversity depend on the nature of species interactions in grasslands. Increasing diversity may not provide a benefit to plant production when interactions among plant species are largely negative or antagonistic.
Diversity is a function of species richness and of species evenness. Plant species richness is thought to influence primary productivity and other ecosystem processes via mechanisms that (1) favor species with particular traits (non-complementarity) and (2) promote niche differentiation or facilitation between species (complementarity). Influences of evenness on productivity are poorly defined, but may depend on which of the two richness effects dominate in species mixtures. We grew three species of annuals that co-occur in grazed grasslands in central Texas (Gaillardia pulchella, Lolium perenne, and Monarda citriodora) in monocultures and in 3-species mixtures. Evenness and identity of the dominant species were varied in replicated mixtures at each of 3 density levels to test predictions concerning effects of species relative abundances and competitive intensity (density) on expression of the dominant mechanism determining biomass production of plant mixtures. Non-complementary or antagonistic interactions clearly governed production of these assemblages. Biomass of most mixtures was smaller than the average biomass of species monocultures, and mixtures consistently produced less biomass than expected from monocultures (Relative Yield Total or RYT < 1). These trends reflected an inverse correlation among species between competitive ability and size at maturity. Contrary to expectation, RYT of mixtures was insensitive to both competitive intensity and species evenness. The grass Lolium performed relatively better in mixtures than in monocultures at high density, but the large disparity in size between forbs and Lolium masked expression of this trend in mixtures. This experiment apparently is the first to demonstrate that non-complementary or antagonistic interactions may be insensitive to species evenness. Our results also illustrate the central role that size differences among plants may play in mediating the influence of species richness on productivity.