Document: BRI-3-12-7

Plant diversity and ecosystem functioning: The importance of species evenness.

WILSEY, B.J.* ¹, G.STIRLING ² and H.W.POLLEY ³

Texas A+M University-Commerce, Commerce, Texas ¹
McGill University, Montreal, Quebec ²
USDA-ARS, Grassland, Soil and Water Research Lab, Temple, Texas ³

Abstract:
Changes in land use, habitat fragmentation, and pollution often lead to reduced plant diversity. Diversity has two components: species richness (S), or the number of plant species in a given area, and species evenness (equitability, J), or how well distributed abundance or biomass is among species within a community. Most studies on relationships between biodiversity and ecosystem functioning use S as the sole index of diversity, usually under the implicit assumption that S, J, and diversity (e.g. H') are all positively correlated. We conducted a literature review on relationships between these variables and found that S and J were positively correlated in vertebrate (r = 0.54, P<0.001) and invertebrate (r = 0.51, P<0.001) animal communities, and that both were excellent predictors of H' (0.58 < r² < 0.82, all P's<0.001), as expected. However, for plants, S was negatively related to J (r = -0.43, P<0.001) and J accounted for more of the variation (r² = 0.53, P<0.001) in H' than did S (r² = 0.06, P<0.05). In a field experiment in a Texas grassland, we experimentally varied species evenness and the identity of the dominant species to test the hypotheses that reduced community J would lead to reductions in productivity and resource capture, and increases in the spread of spittle bug infestation of Solidago altissima. Plant density and S were kept constant. Initial (first year) results suggest that aboveground resource capture was largely unaffected, but that the spread of spittle bugs was increased by experimentally reducing plant community J (P<0.03). These results, when taken together, suggest that plant species evenness is more important than previously thought and that it should be taken into account in studies of biodiversity and ecosystem functioning.

Keywords: diversity, evenness, species richness, ecosystem functioning

This abstract is being presented at: 10:15 AM in session:
Oral Session #2: Conservation Ecology.