Global change and predator-prey interactions on a woody perennial
Hentley, William Thomas
The impacts of global change on ecosystems from climate change and invasive species are likely to be complex. Rising atmospheric CO2 concentrations, the associated climate forcing and greater frequency of extreme weather are serious challenges to natural ecosystems. In tandem with climate change, globalisation has led to the spread of invasive alien species around the globe that threaten to interrupt food web dynamics. Advancing understanding of the effects of global change on trophic interactions therefore requires study of interspecific and multi-trophic interactions. \ud The aim of this thesis was to examine how host-plant heterogeneity, native–invasive species interactions and climate change effects (elevated atmospheric CO2 (eCO2) or drought) influence trophic interactions. An experimental approach was used which centred on a study system comprising the European raspberry (Rubus idaeus), the herbivorous large raspberry aphid (Amphorophora idaei) and coccinellid beetle predators (native species: Adalia bipunctata, Coccinella septempunctata; invasive alien species Harmonia axyridis).\ud Under eCO2, R. idaeus resistance to A. idaei was unchanged for two cultivars (Glen Clova, Glen Ample) partially susceptible to A. idaei, but significantly reduced for another (Octavia) with complete resistance in ambient climatic conditions. The inclusion of a coccinellid predator, however, mitigated the reduction in the resistance of Octavia by reducing aphid abundance. Behavioural responses to predation by A. idaei were also impaired under eCO2 after feeding on Glen Ample. The role of natural enemies in controlling herbivore abundance in future climates is therefore crucial. Native coccinellid species are currently declining in much of Europe, attributed to the occurrence of the invasive species, H. axyridis. Despite the declines in native coccinellid species, it was found that behavioural modification to feeding by both native and invasive coccinellid species can, theoretically, result in coexistence.\ud Plant resistance in a future climate is likely to be modified significantly. Reduced resistance to aphid herbivory demonstrated here mirrors previous studies, highlighting the future importance of natural enemies to control aphid abundance. Changes to the abundance and behaviour of aphid prey and intraguild predators will modify the effectiveness of native and invasive natural enemies. Further mechanistic research is required to understand multi-trophic interactions in dynamic environments.