Biotic resistance refers to the ability of species in a resident community to restrict invasion. This research investigates determinants of biotic resistance to invasion. Its overall objectives were to identify the characteristics of species and communities making them more or less resistant to species invasion and to quantify the contribution of other biotic and abiotic factors to the regulation of biotic resistance. I hypothesized that (1) functional group identity of wetland species would be a good predictor of their biotic resistance; (2) mixtures of species would be more invasion resistant than monocultures; (3) abiotic constraints (flooding in this case) would influence biotic resistance both through direct effect on invaders and indirect effect on resident wetland species, and (4) propagule pressure of invading species would interact with wetland plant density to influence biotic resistance.
I chose an introduced lineage of Phragmites australis as a model invasive species to test biotic resistance. I conducted a series of rigorous community assembly experiments both in pots and in wetland to simulate a situation where P. australis seeds land on bare soil along with other wetland species, a common occurrence in the field after disturbances or wetland restoration.
Results/Conclusions
Strong resistance of short fast-growing annual plants to restrict P. australis emergence was one of the most consistent findings across several experiments. Regarding the diversity-invasibility relationship in community assembly, combining certain functional groups in specific ratio led to complementarity diversity effect which strengthened biotic resistance. This result implies species interactions between functional groups are key mechanisms generating diversity effect. Structural equation model supported abiotic constraint and biotic resistance worked synergistically or antagonistically in controlling invasion depending on the fitness of the wetlands species involved. Finally, propagule pressure increased invasion success up to a threshold beyond which additional P. australis seeds did not increase invasion proportionally.
This study can be an important step toward predicting invasion risk and impact as well as designing native community assembly for invasive plant management.