focused on two

[jigesmiliy]

Relating resource availability and herbivore escape opportunity to plant community invasibility

Plant invasions are causing negative environmental, economic, and social impacts worldwide. It is necessary to understand the mechanisms behind their success in order to prevent future invasion and to control existing ones. This thesis adds needed empirical evidence to the current debate on what factors drive biotic invasions. More specifically, it examines the respective importance in the invasion process of the release from natural enemies and the ability to exploit resource opportunities in a new environment. Biological control aims to reduce enemy escape opportunity and is widely developed for many target organisms, whereas little is currently done to manipulate resource availability as a means to manage weed invasions. In this thesis, we used an integrated approach with a controlled glasshouse experiment, two http://www.valentinoshoessale.co.uk/ manipulative field experiments, and two common garden experiments. We focused on two environmental weeds: Alternanthera philoxeroides (Alligator weed) and Phyla canescens (Lippia). Both invasive plants originate from Argentina, coexist in parts of both their native and introduced ranges, and invade riparian habitats in Australia. In a field fertilisation experiment, we investigated which nutrient is primarily limiting the growth of P. canescens and studied the short term response of the invaded community to fertilisation (Chapter 2). Nitrogen (N) was identified as a nutrient limiting the growth of P. canescens. In a second experiment, we examined how varying concentrations of the limiting nutrient N differently affect the two invasive species when compared with the response of two native congeners (Chapter 3). In a controlled glasshouse experiment, we compared the allocation patterns, morphological and physiological traits and phenotypic plasticity of two pairs of congeners (A. philoxeroides and Alternanthera denticulata, and P. canescens and Phyla nodiflora). Invaders and native species had similar morphology and phenotypic plasticity, but invasive species had higher relative growth rates. Comparison of leaf trait relationships showed that the invasive P. canescens was more responsive to N variability than the native P. nodiflora. Both invaders used different strategies to persist at low N supply: A. philoxeroides had a high phenotypic plasticity in resource allocation whereas P. canescens had a high photosynthetic rate. This study shows that environments with high N levels favour the growth of both invasive plants and that reducing N inputs is likely to decrease their growth but may not alter the competitive outcome between native and invasive species. In a third experiment, we investigated the role of enemy release by comparing the response of two invasive plants to herbivore exclusion in both their native and introduced ranges, alone and in the presence of two native competitors (Chapter 4). A novel method was developed to test the enemy release hypothesis; the IRIC (InterRange IntraCommunity) study examined whether the difference in the effects of enemies on plant performance in the native range between invasive species and cooccurring species is less than the difference in enemy effects between invasive and native species in the introduced range. In the native range, the difference in enemy effects between a native and A. philoxeroides was less than the difference in enemy effects between a native and A. philoxeroides in the introduced range. However, this was not evident for P. canescens communities, indicating that A. philoxeroides experiences a release from natural enemies while P. canescens does not. The IRIC study, because it takes the community interactions into consideration, is more accurate than the traditional biogeographic study in determining whether enemy release is a mechanism that allows a plant to become invasive. In a fourth experiment, we showed that combining topdown (herbivory, biological control) and bottomup (nutrient management) methods provides better control than each method separately (Chapter 5). We used a combination of herbivory simulation (clipping) and resource availability manipulation in the presence of native enemies in the introduced range, and we examined the response of both invasive species compared with that of native competitors. Low soil fertility and clipping of the invasive decreased their growth and reproduction. This method should be used more widely to predict the effectiveness of management strategies prior to implementation. The research discussed in this thesis has theoretical and practical applications. First, it identifies several mechanisms behind the success of invasive species. Enemy release and resource availability play different roles in the invasion success depending on the species. Second, the development of the IRIC method provides a tool to test whether enemy release is one of the mechanisms explaining the success of an invasive species and predicts the efficacy of biological control. Finally, it offers a method to test whether the combination of topdown and bottomup control methods is more efficient than each method separately.