Living on the edge: ecological and evolutionary mechanisms of range expansion in invasive species

Despite ongoing research into traits of successful invasive species, few studies have examined how selection on such traits change during the course of an invasion. This is despite increasing recognition that populations in the invaded range are generally not at equilibrium and many invasive species are still undergoing range expansion. As a species expands in its range from its range core where it was first introduced, populations at the edge of the range may experience different ecological (e.g. fewer enemies) and evolutionary conditions (i.e. selection for increased dispersal, growth and reproduction). Changes in these conditions towards range edges may facilitate continual range expansion and examining how such selective pressures vary across the range of invasive species may increase our understanding of factors determining species' ranges. This thesis explores the ecological and evolutionary mechanisms of range expansion in exotic invasive species using two coastal exotic dune plants (Gladiolus gueinzii Kunze (Iridaceae) and Hydrocotyle bonariensis Lam. (Apiaceae)) occurring along eastern Australia. In particular, it examines variation in enemy attack across ranges (Chapter 2) as well as selection for increased growth (Chapter 3), dispersal (Chapter 4) and reproduction (Chapter 5) towards range edges to facilitate further range expansion. Both H. bonariensis and G. gueinzii did not show consistent responses in terms of enemy release and trait shifts towards range edges. Hydrocotyle bonariensis experienced increased enemy attack and growth towards range edges however there were no such differences towards range edges for G. gueinzii. These inconsistent responses highlight the complexity of understanding factors influencing range expansion within and across species and provide a rich avenue for continued research.