Evidence for a shift in defence driving the invasion success of Acacia longifolia in Australia

The enemy release hypothesis (ERH) outlines the most widely tested and accepted invasion mechanism. Within the ERH there are two hypotheses, the evolution of increased competitive ability hypothesis (EICA) and the shift in defence hypothesis (SDH), which describe how this mechanism may work. To our knowledge these two hypotheses are yet to be comprehensively tested in a single study. Therefore, the aim of this study was to test them for three Acacia species (A. cyclops, A. longifolia and A. saligna) that have become highly invasive in their introduced ranges. We grew these species in a controlled glasshouse experiment from seed collected from multiple populations in their native and introduced ranges in Australia. We then measured their growth (proxy for competitive ability) as well as their qualitative (terpenes) and quantitative (phenolics) defences. We found that A. longifolia plants grown from introduced range seed produced more biomass (marginally non-significant) and terpenes and less phenolics than their native range counterparts, providing strong evidence for the SDH. In contrast, there was no difference in growth or chemical defences for A. cyclops and A. saligna between ranges. This may be because these two species were introduced to eastern Australia ~ 50 years prior to A. longifolia being introduced to western Australia, giving more chance for natural enemies to adapt to them in their introduced range. If this is true, we can conclude that A. longifolia currently poses the greatest invasion risk of our Acacia study species and therefore warrants the closest management attention.