Enemy release or enemy reduction? Biogeographical and community approaches for two environmental weeds.

C. CLECH-GOODS; S. SCHOOLER; A.J. SOSA

Brisbane

Congreso; The 10th International Congress of Ecology; 2009

Invasive species are causing negative environmental, economic and social impacts worldwide. It is widely assumed that the competitive advantage displayed by environmental weeds over native species is due to the release from top-down regulation by specialist herbivores (Enemy Release Hypothesis or ERH). Despite the demonstrated importance of natural enemies to host population dynamics, studies testing the ERH have diverging conclusions. Biogeographical studies comparing the herbivore populations occurring on theinvasive species between native and exotic ranges show a reduction in the diversity of natural enemies in the introduced range when compared with the native range. However studies comparing herbivory on native and invasive species co-occurring in the same community in the exotic range, indicate that the invasive and native species are similarly affected by natural enemies. Both types of studies have not been carried out for the same species. In this study, we used biogeographical and community-based approaches to examine the effect of natural enemies on plant invasiveness of two environmental weeds with similar growth habit, originating from Argentina and coexisting in Australia: Alternanthera philoxeroides and Phyla canescens. Using garden experiments in both the native and introduced ranges, we compared herbivore loads and plant population vital rates between plants protected from and exposed to herbivores in a community or growing individually. The ability of enemy escape to drive invasions is primarily due to the reduced impact of natural enemies on the exotic species relative to that on native species  competitors. In this study, we examined whether the difference in regulation in the native range between invasive species and co-occurring species is less than the difference in regulation between invasive species and Australian native species. In this presentation, we present the results of these experiments in the context of the ERH and we discuss their implications for biological control.