Evidence for enemy release and increased seed production and size for two invasive Australian acacias
Invasive plants are hypothesized to have higher fitness in introduced areas due to their release from pathogens and herbivores and the relocation of resources to reproduction. However, few studies have tested this hypothesis in native and introduced regions. A biogeographical approach is fundamental to understanding the mechanisms involved in plant invasions and to detect rapid evolutionary changes in the introduced area. Reproduction was assessed in native and introduced ranges of two invasive Australian woody legumes, Acacia dealbata and A. longifolia. Seed production, pre-dispersal seed predation, seed and elaiosome size and seedling size were assessed in 7-10 populations from both ranges, taking into account the effect of differences in climate. There was a significantly higher percentage of fully developed seeds per pod, a lower proportion of aborted seeds and the absence of pre-dispersal predation in the introduced range for both Acacia species. Acacia longifolia produced more seeds per pod in the invaded range, whereas A. dealbata produced more seeds per tree in the invaded range. Seeds were bigger in the invaded range for both species, and elaiosome: seed ratio was smaller for A. longifolia in the invaded range. Seedlings were also larger in the invaded range, suggesting that the increase in seed size results into greater offspring growth. There were no differences in the climatic conditions of sites occupied by A. longifolia in both regions. Minimum temperature was higher in Portuguese A. dealbata populations, but this difference did not explain the increase in seed production and seed size in the introduced range. It did have, however, a positive effect on the number of pods per tree. Synthesis. Acacia dealbata and A. longifolia escape pre-dispersal predation in the introduced range and display a higher production of fully developed seeds per fruit and bigger seeds. These differences may explain the invasion of both species because they result in an increased seedling growth and the production of abundant soil seedbanks in the introduced area.