Pollinators and predators at home and away: do they determine invasion success for Australian Acacia in New Zealand?

Abstract

Aim: Interactions with pollinators and pre-dispersal seed predators are important determinants of reproductive output and could influence the success of plant species introduced to areas outside their native range. We identified the role of these interactions in determining reproductive output and invasion outcomes for species of Australian Acacia introduced to New Zealand. Location: Australia and New Zealand. Methods: We studied three species of Australian Acacia with different invasion success in New Zealand. In both Australia and New Zealand, we measured pollination success as the number of pods per inflorescence and the proportion of aborted seeds per pod, determined losses to pre-dispersal seed predators, and measured overall seed output. For each species, we compared performance in New Zealand with that in Australia, then examined whether there was any variation among species in their relative performance in each country. Results: The number of pods per inflorescence and proportion of seeds aborted were similar in each country and among species. There was little difference in pre-dispersal seed predation rate between Australia and New Zealand for Acacia dealbata, an invasive species, and Acacia baileyana, a species widely naturalized in New Zealand. However, pre-dispersal seed predation rate was lower in New Zealand for Acacia pravissima, currently considered to be a casual species there. Both the invasive A. dealbata and the casual A. pravissima produced more seeds per tree in New Zealand than Australia. Main conclusions: Differences in reproductive success between the native and introduced range could not explain the differences in invasion success among the three Acacia species. Although per capita reproductive output was higher in New Zealand for two species, neither mutualistic interactions with pollinators nor antagonistic interactions with pre-dispersal seed predators explained those differences. The high seed output of A. pravissima suggests it has the potential to become invasive. These findings highlight the value of broad comparative studies in elucidating the drivers of invasion.