Plant communities are commonly described by the number (richness) and relative abundances (evenness) of species in a given locale.  However, these metrics do not necessarily account for species distributions in space, which may affect local species interactions and subsequent invasion.  With this study we tested if species aggregation in space affects species diversity and invasion within experimental grassland plots. We test the hypotheses that (1) native species diversity will decline over time in plots with small-single species patches, and (2) invader abundance will be highest in plots with large single-species patches.  In May 2006 we planted 24 – 4 m² plots with transplants of four prairie species to test the effects of plant distribution on diversity and invasion. Four common grassland species were planted as transplants in one of three spatial arrays (low, medium, or high aggregation), while keeping species richness and abundances constant at the plot scale.

Results/Conclusions

Two years after establishment, plot-scale native species diversity and invader relative abundance were similar among plots planted in different spatial arrays.  Mean patch extent was maintained through time with each spatial array, but mean patch size decreased in high aggregation arrays and increased in low aggregation arrays.  This convergence in patch size among spatial arrays corresponded with a universal decline in the proportion of the plots covered by grasses and an increase in forbs.  As predicted, Schizachyrium scoparium went locally extinct more frequently and the highly clonal Monarda fistulosa spread more extensively through low aggregation arrays.  However, these changes in fine-scale site occupancy did not result in reduced diversity in low aggregation arrays.  We also found that invader abundance was similar among the spatial arrays, which is likely attributable to vegetative spread of M. fistulosa resulting in the homogenization of available microsites for establishment among arrays.  Our findings suggest that larger scale diversity is unaffected by fine-scale site occupancy dynamics.  Future studies need to investigate if these patterns hold for more species-rich systems and under different abundance scenarios.