Why do plants become invasive?: the role of phylogeny, herbivores and time

Invasive plants are a global problem, with significant environmental and economic consequences. Reducing the risk that introduced species become invasive is a high biosecurity priority. Much research has been directed at identifying particular biological traits associated with invasion success. Despite some progress in identifying mechanisms and traits associated with successful invasive species, we are still a long way from making generalisations about invasion success among species and communities. The idea that phylogenetic relationships between species may be important in determining the establishment and naturalisation of non-native species was first put forward by Charles Darwin as his 'naturalisation' hypothesis. This idea proposes that upon introduction, nonnative species are less likely to naturalise if they have native near relatives due to competition for similar resources or alternatively because they are more likely to be attacked by pre-adapted native natural enemies. This study was aimed at investigating whether an understanding of time since introduction, and phylogenetic relatedness of introduced species relative to the community into which they are introduced, can be used to predict their invasive risk due to the likelihood of control by natural enemies. -- I examined patterns of herbivore and pathogen damage, and the abundance and richness of invertebrate communities on introduced and native plants in eastern Australia. Comparisons were made at two taxonomic levels, above and below genus. At genus level and above, 37% of variation in leaf damage across 14 invasive plants was predicted by the phylogenetic distance to the nearest native relative. As phylogenetic distance between the introduced and native plants increased, leaf damage decreased. This result indicates that phylogenetic relatedness of invasive plants to species in native communities is a useful tool for assessing the likelihood that plants will accumulate a community of natural enemies from that community which may influence their invasive potential (Chapter 2). Withingenus predictions of success of invasive species could not be based on phylogeny Chapter 3; Chapter 4). Damage to non-native Senecio species was less than on natives, but no difference in damage was found between invasive Senecio madagascariensis Poir. and other non-invasive Senecio species. Senecio madagascariensis had greater herbivore loads than native Senecio species (Chapter 3; 4). Herbivore loads increased over time on S. madagascariensis, but this did not equate to enemy release at early stages of invasion (Chapter 4). High leaf damage occurred where S. madagascariensis had recently colonised, even though this was also where there was a low herbivore load. At the introduction site damage was low, and herbivore loads high (Chapter 4). A complex relationship exists between an invader and invertebrate community response at fine taxonomic scales. This may be a result of changing selective pressures on an invasive species over time. Phylogenetic distance between non-native species and native recipient communities is recommended as a management tool for predicting the likelihood that species will become invasive at broad phylogenetic scales.