Title page for ETD etd-08102016-001026

Type of Document Dissertation
Author Allen, Warwick
Author's Email Address warwick.j.allen@gmail.com
URN etd-08102016-001026
Title Biological Invasions: Biogeography and Multitrophic Interactions
Degree Doctor of Philosophy (Ph.D.)
Department Biological Sciences
Advisory Committee
Advisor Name Title
Cronin, James Committee Chair
Aschehoug, Erik Committee Member
Geaghan, James Committee Member
Stout, Michael Committee Member
Rouse, Lawrence Dean's Representative
  • invasive species
  • biogeography
  • latitude
  • species interactions
  • Phragmites australis
Date of Defense 2016-07-29
Availability unrestricted
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. Using field surveys combined with common garden and greenhouse experiments, I investigated 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 escaped from introduced Lipara gall-flies, attributed to greater vertebrate predation on Lipara infesting the invasive than the native lineage. A complementary common garden experiment revealed that enemy release of the invasive P. australis lineage from Lipara was driven by local environmental conditions rather than genetic differences between the two lineages. Importantly, 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 the interactive effects of rhizosphere soil biota, interspecific competition, and nutrient availability on performance of P. australis and native smooth cordgrass, 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 P. australis lineage negatively impacted S. alterniflora, whereas soil biota from the native lineage had a positive impact. This indirect spillover of pathogens and mutualists interaction may have 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.

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