Invasibility and compositional stability in a grassland community: relationships to diversity and extrinsic factors

We present results from an ongoing field study conducted in Kansas grassland to examine correlates of invasibility and community stability along a natural gradient of plant diversity. Invasibility was evaluated by sowing seeds of 34 plant species into 40 experimental plots and then measuring colonization success after two growing seasons. Compositional stability, defined as resistance to change in species relative abundances over two growing seasons and in response to experimental disturbance, was measured in a separate set of 40 plots. We found that community susceptibility to invasion was greatest in high diversity microsites within this grassland. Multiple regression analyses suggested that the positive correlation between invasibility and plant diversity was due to the direct influences of the extrinsic factors that contribute to spatial variation in diversity (soil disturbances; light availability), not to any direct impact of diversity. In addition, we found that compositional stability in response to disturbance was greatest within low diversity microsites and was strongly related to the dominance (evenness) component of diversity.