The nature of the relationship between species richness and community productivity and invasibility remains contentious. Past research efforts have failed to account for the effects of potentially contributory factors such as individual density, propagule pressure, and abiotic conditions, and have been inhibited by problematic experimental designs that lack adequate protections from the effects of sampling bias. Additionally, there remains a glaring lack of experimental evidence from natural communities other than herbaceous and grassland communities. Our greenhouse experiment consisted of a fully crossed design comparing the effects of individual density, species richness, and invader propagule pressure at field-observed low and high levels in planted communities of randomly selected assortments of 12 species of woody, native South Florida dry forest seedlings that had been grown from seed. Additional monoculture plantings of each species used complemented our experiment to account for potential sampling effects. Seeds of Ardisia elliptica, a noxious invader of southern Florida’s tropical dry forests, were sown in varying densities over time in our experimental seedling communities. We tested the hypothesis that species richness and density would have discernable effects on community productivity and invasibility. Species-rich plots were predicted to achieve greater evenness among replicates in terms of productivity and reduced mean invader biomass than species-poor plots. Additionally, high-density plots were predicted to achieve greater productivity and reduced invader biomass than low-density plots.
Our preliminary results indicate that community invasibility is influenced by species richness, seedling density, and invasive propagule pressure. Increased propagule pressure predictably resulted in increased invader biomass, but increased species richness also significantly increased Ardisia elliptica establishment and biomass production. With respect to native community productivity, the main effect of propagule pressure was significant and negatively correlated with native root biomass production, while the main effect of density was significant and positively correlated with native shoot biomass production. For native shoot biomass production, the interaction effect between species richness and density was also significant; density effects on productivity appear to be moderated by increasing species richness. Number of fruits produced per capita by one native species, Callicarpa americana, was positively correlated with increasing species richness and negatively correlated with seedling density. These results have important implications for community and restoration ecology theory. Additionally, Ardisia elliptica survival and biomass production results gleaned from this experiment potentially suggest previously undocumented characteristics which may contribute to its invasion success in South Florida and beyond.