Patterns of host promiscuity among legumes varying in invasiveness differ between global and regional scales
Acacia species have been introduced outside their native range of Australia, and a subset of these species has become invasive in multiple parts of the world. Within specific regions, the invasive status of these species may vary. Our study examines whether a key mechanism that has been shown to drive invasiveness of Acacia species, the legume-rhizobia symbiosis, influences the invasiveness of these species on both global and regional scales. We first grew 12 Acacia species that range from invasive to non-invasive globally with 12 rhizobial strains to determine whether species in different invasive categories can associate with a wider diversity of rhizobial strains (i.e. are more promiscuous hosts). We then examined host promiscuity on a regional scale, growing seven Acacia species that vary in invasiveness in California with ten different soil inoculants hosting different rhizobial communities. For both experiments we measured and compared plant performance, including aboveground biomass, nodulation response, and survival. This research allowed us to assess whether acacias varying in invasiveness globally and in California are more promiscuous rhizobial hosts, and also whether patterns of host promiscuity are dependent on the scale at which they are evaluated.
Our results indicate that acacia host promiscuity with rhizobia is an important mechanism influencing their invasion capacity on a global scale, but on a regional scale this pattern is not maintained. Acacia species that have become invasive in at least one area outside their native range had a more positive growth response with a wider number of rhizobial symbionts than naturalized or non-invasive species (six of 12 strains). However, when examined on a regional scale (i.e. within California), Acacia species that were invasive, naturalized, or non-invasive differed in aboveground biomass for only one soil and did not differ in nodulation or survival within individual soils. This indicates that non-native Acacia species that have been introduced to California are all equally promiscuous hosts, and that host promiscuity per se does not explain the observed differences in invasiveness on a regional scale. Our results highlight the importance of considering the scale at which invasiveness is categorized when examining mechanisms of species invasions abroad.