Submitted to: Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/22/2004
Publication Date: 5/19/2004
Citation: Wilsey, B.J., Polley, H.W. 2004. Realistically low species evenness does not alter grassland species-richness-productivity relationships. Ecology. 85(10):2693-2700. Interpretive Summary: Humans are reducing the number of plant species that are present in many biological systems and are creating greater disparity in the abundances of remaining plants with some species becoming very abundant and others becoming rare. Recent studies have shown that the loss of plant species may reduce plant productivity in grasslands. Whether the increasing disparity in species abundances also is affecting the capacity of grasslands to meet human needs is not known. To determine effects of both species number and species abundances on grasslands, we experimentally varied the number of plant species and their abundances in small plots. Both plant biomass and net uptake of carbon by plants declined when the number of species was reduced. Effects of species number on plant production were the same, however, whether species were equally abundant or whether some were very abundant and others were rare. Over the two years of this experiment, some species were lost or became extinct from plots. Importantly, extinctions were greater in plots in which some species were very abundant and others were rare than in plots in which all species were equally abundant. Our results indicate that the productivity of grasslands may decline as plant species are lost independently of how abundances are distributed among species. However, activities that increase the disparity in abundances among plants may accelerate species loss and the accompanying decline in grassland productivity.
Technical Abstract: Biodiversity is declining world-wide from reductions in both species richness and evenness. Field experiments have shown that primary productivity is often reduced when richness of plant species is lowered. However, experiments testing richness effects have used evenness levels that are much higher than normally encountered in plant communities and have been based on the assumption that species extinctions are random. We experimentally varied, for the first time, both species richness (1-8 perennial species per m2) and species evenness (near maximal vs. realistically low) in grassland plots. Net primary productivity and ecosystem CO2 uptake declined when richness was reduced, and reductions were similar between evenness treatments. Richness effects were associated more with a selection effect than with complementarity (found only with high evenness). Importantly, extinctions in plots during the second year were not random but were greater at low than at high evenness (i.e. with increased rarity) and in species with low aboveground growth rates. Our results indicate that richness studies may not be biased by using mixtures with artificially high evenness levels, but also demonstrate that results from these studies are directly applicable only to communities in which plant extinctions are random.