Regulation of local diversity via diversity-dependent invasion and species loss

The stability of community composition is a poorly understood concept in ecology. Invasive species and mass extinctions are rapidly changing the composition of many of our world’s natural communities. In understanding the processes contributing to community assembly we may be able to better understand these detrimental processes. Here we look at the changes in composition of established experimental plant communities of varying diversity after cessation of weeding practices. We found evidence to support the proposition that greater diversity leads to fewer invaders, as well as a greater number of local species extinctions. We also found that these two factors interact to create a stabilizing effect on diversity. In plots of initially high diversity the number of species lost due to extinction was greater than the number of species gained due to invasion on average. In the plots of low initial diversity the number of species gained due to invasion events was greater than the number of species lost due to extinctions events on average. At an intermediate level of diversity the number of species lost and number of species gained over time of abandonment was approximately equivalent, leaving the plot with no net change in richness. These results suggest that local communities may have an optimal level of diversity, and perturbations from that level through experimental manipulation or disturbance may be overcome naturally.