Do mutualists matter? The role of pollinators, seed dispersers and belowground symbionts in the invasion success of Acacia

Abstract

Plant species introduced to new locations may lose their natural enemies but can also leave behind important mutualists. Here, I take a novel comparative approach to identify the potential role of mutualistic interactions in determining invasion outcomes. I examine the strength of pollination, seed dispersal and belowground symbioses with nitrogen-fixing bacteria (rhizobia) across three species that vary in invasion success in both their introduced and native range. I used species of Australian Acacia introduced to New Zealand. I hypothesised that if interactions with mutualists are important for plant invasion then species would vary in the strength of interactions with one or more of the groups of mutualists I examined, and that the pattern of variation would correlate with the degree to which they have established and spread in New Zealand. At each stage I also consider the potentially mediating influence of natural enemies. For A. dealbata, a highly invasive species, and A. baileyana, a species that is widely naturalised in New Zealand, I found no differences in any of the variables I examined in relation to pollination and predispersal seed predation. However, for A. pravissima, currently considered a casual species in New Zealand, pre-dispersal seed predation was lower in New Zealand and overall seed production was much higher relative to conspecifics in Australia, and relative to the other two species. In relation to seed dispersal I found that the three species, which are all adapted for dispersal by ants (myrmecochory), were able to form dispersal mutualisms in New Zealand, potentially to the same degree as in Australia. Seed predation following seed fall was also lower for species in New Zealand than in Australia. There was no variation between the three species in seed removal associated with either dispersal or predation. By examining species’ growth and nodulation with rhizobia in both Australia and New Zealand I found that their ability to spread away from introduction sites could be limited by the availability of rhizobia in New Zealand, relative to Australia. However, there were again no differences between species. This is the first study to have directly measured mutualistic interactions across species that vary in invasive success in both their native and introduced range. I demonstrated that species introduced to new locations are able to establish mutualistic interactions with pollinators and dispersers to the same degree as in their native range. I also found the first direct evidence that the availability of rhizobia could limit species’ abilities to colonise new sites in the introduced range. However, mutualistic associations could not explain the variable invasive success of each species. Overall, these findings suggest that mutualistic interactions may be important for alien plant establishment, but alone cannot explain invasion outcomes. Instead, it is likely that invasive success is determined by a combination of biotic, abiotic and human factors, with the ability to establish mutualistic interactions just one component necessary for successful establishment and spread. These findings underline the importance of such broad geographical and comparative studies in attempts to elucidate drivers of invasion.