Examining the Species-Diversity-Productivity Relationship: Plant Species Richness and Light Heterogeneity Along Productivity Gradients in Southwestern Michigan
Coppinger, James Peter
Among terrestrial plant communities, the relationship between species diversity and productivity is often unimodal; species diversity is often greatest at intermediate levels of productivity. This phenomenon has often been attributed to changes in resource heterogeneity resulting from changes in productivity. Resource-based competition theory predicts that resource heterogeneity promotes species diversity in plant communities by preventing competitive exclusion through niche partitioning. Applying this theory to light remains controversial, as the relationship between light heterogeneity, productivity, and species diversity remains enigmatic. In this study, the relationship between light heterogeneity, species richness, and productivity was studied across plant communities in the Allegan State Game Area in southwestern Michigan. Plant species richness was sampled in grassland, oak/savanna, and forest community types, which encompassed a broad gradient of productivity. Litter mass and herbaceous biomass provided an index of annual net primary productivity. Light heterogeneity was measured and analyzed using geostatistics to determine variance and patch size-two variables used to assess the magnitude and scale of spatial heterogeneity. The relationship between species richness, light heterogeneity, and productivity was unimodal. Intermediate productivity communities (oak/savanna) exhibited both highest species richness and greatest heterogeneity of light, indicated by large patch size and variance. Light heterogeneity and species richness was lowest at both low and high levels of productivity, suggesting that change in light heterogeneity with productivity may account for the unimodal nature of the species richness-productivity relationship. These results support a positive relationship between light heterogeneity and species richness, consistent with the contemporary theories on resource heterogeneity and species diversity.
vii, 40 p.
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