Does enemy damage vary across the range of exotic plant species? Evidence from two coastal dune plant species in eastern Australia

Release from natural enemies is often cited as a key factor for understanding the success of invasive plant species in novel environments. However, with time invasive species will accumulate native enemies in their invaded range, with factors such as spread distance from the site of introduction, climate and leaf-level traits potentially affecting enemy acquisition rates. However, the influence of such factors is difficult to assess without examining enemy attack across the entire species’ range. We tested the significance of factors associated with range expansion (distance from source population and maximum population density), climatic variables (annual temperature and rainfall) and leaf-level traits [specific leaf area (SLA) and foliar nitrogen concentration] in explaining variation in enemy damage across multiple populations of two coastal invasive plants (Gladiolus gueinzii Kunze and Hydrocotyle bonariensis Lam.) along their entire introduced distribution in eastern Australia. We found that for H. bonariensis, amount of foliar damage increased with distance from source population. In contrast, for G. gueinzii, probability and amount of foliar damage decreased with decreasing temperature and increasing rainfall, respectively. Our results show that patterns of enemy attack across species’ ranges are complex and cannot be generalised between species or even range edges.