A dominant view in ecology is that fine-scale environmental heterogeneity is expected to enhance species diversity within communities through niche partitioning. For sessile species such as plants, the importance of spatial heterogeneity to diversity is expected to be particularly acute. Observational studies have provided some evidence of positive heterogeneity-diversity (H-D) relationships, but, surprisingly, field experiments have failed to find good evidence for positive H-D relationships. This experimental evidence may be sensitive to field manipulations that primarily rely on fertilizer addition to create differences in soil patches. Specifically, fertilizer may undermine plant-soil interactions in ways that obscure H-D relationships. To address this bias, we avoided fertilizers and manipulated the soil directly to create soil heterogeneity in a grassland community assembly experiment. Heterogeneous plots were established by removing three strata from the vertical soil profile and randomly allocated to 0.5x0.5 m patches within 2.4x2.4 m plots. Homogenous plots were created by combining the three soil types before redistribution into the patches comprising the plots. Seeds of thirty-four native prairie species were sown once, prior to the first growing season, and species presence and density were collected in each patch over five years.
Results/Conclusions
Species richness was higher in heterogeneous than homogeneous plots in four out of five years and, in most years, this difference was driven by greater species turnover among patches in heterogeneous than homogeneous plots. Additionally, species sorting among patch types was higher in heterogeneous plots corresponding to the increased species turnover. In year five, species diversity did not differ among plots due to greater extinction in heterogeneous than homogeneous plots. This difference in year five may reflect the effects of a dormant season, prescribed fire that occurred between year four and five. Currently, it is unclear whether this temporal variation in the heterogeneity-diversity pattern reflects year-to-year variation, disturbance, or a weakening of the relationship as communities continue to assemble. These alternatives will be discussed along with additional tests necessary to unravel the direction, strength, and stability of these patterns.