A test of community saturation: Comparing vegetated plot data and null communities at multiple spatial scales in Southeastern U.S. forests

Background/Question/Methods

Community saturation predicts that exotic species should be most frequent in low diversity communities due to variation in community structuring processes, such as reduced competition and increased resource availability. Fridley et al. (2004) determined that native-exotic relationships demonstrated by actual data could be replicated by random assignments of species origins. Their null model results indicated that communities are not explicitly structured according to species interactions or community processes. We examined (1) how exotic species richness changes as native species richness changes, and (2) how the relationships between exotic richness and native richness compare to a null model of randomly assigned species origins.

We calculated native and exotic species richness at 5 spatial grains in over 4600 vegetated plots across North and South Carolina.  We developed linear models of exotic richness as a function of native richness at each scale. Following Fridley et al. (2004), we randomly re-assigned species origins, developed a new model for each permutation, and repeated this random re-assignment process 999 times to develop a “null” distribution of the data. The original model was then compared to the permuted data, and one-tailed P values were calculated to determine statistical significance.

Results/Conclusions

Across most scales, exotic richness is positively correlated with native richness.  This contradicts community saturation, suggesting that species rich communities are not as resistant to invasion as originally hypothesized. At the very smallest scale examined, 0.01 m², exotic richness is not correlated with native richness.  This corroborates and expands on conclusions of previous research, and suggests that neighborhood crowding may limit the available space for exotic establishment at very small scales.  Each model was also significantly different from the null model, suggesting that the observed communities are structured at least in part by species interactions or other community assembly processes.