Since Elton proposed the Biotic Resistance Hypothesis many empirical investigations have supported a negative relationship between species richness and invasibility in line with this theory, while many observational studies contend that these community characters are positively correlated. In particular, these observational studies have been critiqued for their inability to isolate covarying factors and because they are often conducted on a landscape, as opposed to a community, scale. We have attempted to circumvent some of these shortcomings by focusing on assembling vegetative community data from small (15m2) quadrats from a single community type within a nested spatial data collection framework. We focused our study on tropical hardwood hammock communities in South Florida, a type of tropical dry forest that occurs in naturally discrete patches and is primarily threatened by habitat destruction and invasive species. Overall, we sampled four 3x5-meter quadrats along each of three randomly-set 40-meter transects in each of 13 separate hammocks along the Miami Rock Ridge that roughly corresponds with a rural-urban gradient. We tested the hypothesis that the relationship between species richness and invasibility would vary across spatial scales with a negative correlation at local scales trending positive at some higher level of spatial organization.
Results/Conclusions
Our preliminary results indicate that community invasibility is influenced by species richness differentially at varying spatial scales. A negative relationship between species richness and invasibility was detected at local scales, however, at the stand level a positive relationship developed. Non-native species richness increased along the rural-urban gradient; however, invasive species richness presented no relationship along this gradient. Therefore, we may consider benign non-native species richness as an artifact of past land use and useful in our analyses as a proxy for anthropogenic disturbance to compare against patterns of invasive species distributions. These comparisons revealed that community invasibility in hardwood hammocks by aggressive invader species is driven by biotic resistance rather than disturbance at local levels, but this effect is swamped at higher spatial levels of analysis. These results have important implications for community and invasion ecology theory as well as practical application in restoration efforts.