Doctor of Philosophy (PhD)


Biological Sciences

Document Type



Comparing species establishment and expansion across regional and global scales and comparing congeners that differ in invasive capacity can provide novel insights into mechanisms driving species invasions. Acacia species introduced from Australia around the globe constitute an ideal group for comparing drivers of establishment across different regions and species. Acacias form mutualistic associations with nitrogen-fixing root-nodule bacteria (i.e. rhizobia), which may be a key factor in invasion success. In this dissertation, I examine the acacia-rhizobia mutualism across a suite of host species to explore a key mechanism influencing patterns and processes of acacia invasion. I used three complementary approaches to examine the role of rhizobial mutualisms in the invasiveness of Australian acacias. First, I examined acacia host promiscuity with rhizobia on a regional scale, comparing plant performance and rhizobial symbiont diversity among hosts grown in novel soils. This approach allowed me to examine whether acacias that are invasive in California are more promiscuous rhizobial hosts. Second, I examined acacias that range in invasiveness globally, pairing them with multiple rhizobial strains to examine whether global invasiveness is influenced by promiscuity with rhizobia. Third, I examined the provenance of rhizobia nodulating with acacias in their native and introduced ranges, which allowed me to assess whether acacias were introduced to California concurrent with their native rhizobia. These approaches comprise a comprehensive examination of a key mutualism that may influence acacia invasion in a multi-species, large-scale framework. Results from this dissertation suggest that host promiscuity with rhizobia delineates acacia invasiveness on a global, but not regional scale. Acacia species categorized as globally invasive were more promiscuous hosts than naturalized or non-invasive species. On a regional scale, acacias differing in invasiveness within California did not differ in host promiscuity. Regardless of invasive status in California, acacias appear to have been co-introduced with their native Australian rhizobia. This project contributes to clarifying the driving forces of exotic species invasion success abroad. Mechanistic approaches to understanding the causes of species invasions are important for informing management, control, and abatement of further introduction of species that have proven records of, or the capacity to become, invasive when introduced abroad.



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Committee Chair

Harms, Kyle E.