Biological invasions: biogeography and multitrophic interactions

Abstract

Species interactions play a prominent role in the establishment and spread of many invasive species. However, rarely are invasions studied in more than a direct pairwise species context, or with consideration to how species interactions can vary biogeographically. I used field surveys combined with common garden and greenhouse experiments to investigate how multitrophic above- and belowground interactions influence plant invasions at large spatial scales. I focused on comparisons between sympatric native and invasive lineages of Phragmites australis, a wetland grass distributed throughout North America. I conducted a field survey to examine support for the enemy release hypothesis in a tritrophic framework. In North America, the invasive lineage of P. australis benefitted from strong local enemy release from introduced Lipara gall-flies relative to the native lineage, attributed to greater vertebrate predation on Lipara infesting the invasive lineage. A complementary common garden experiment revealed that local enemy release of the invasive P. australis lineage from Lipara was driven by local environmental conditions rather than genetic differences between the native and invasive lineages. Moreover, local enemy release was strongest at northern latitudes, generated by genetically based non-parallel latitudinal gradients in Lipara herbivory for the native and invasive lineages. This phenomenon could translate to biogeographic variation in invasion success and is worthy of investigation across a range of invaded systems and species interactions. I also conducted a greenhouse experiment to examine interactive effects of rhizosphere soil biota, interspecific competition, and nutrient availability on performance of P. australis and native Spartina alterniflora. All lineages of P. australis suffered negative impacts from soil biota, suggesting this interaction does not directly facilitate the success of invasive P. australis. However, the most interesting result from this experiment was that soil biota from the invasive lineage negatively impacted S. alterniflora, whereas soil biota from the native lineage had the opposite effect. This type of indirect interaction may result in important implications for invasion success and restoration. In summary, my dissertation highlights the importance of examining biological invasions in a biogeographic and multitrophic context and has broad implications for the understanding and management of biological invasions.