Invasive non-native species place high pressures on native communities and can result in ecological impacts often associated with differences in key functional behaviours that mediate top-down and bottom-up forces. In this thesis, I use two model systems, the UK Coccinellidae system and the UK freshwater amphipod system, to quantify per-capita differences between native and invasive non-native species. I scale these studies up to more complex ecological communities and attempt to account for additional environmental pressures (e.g. pathogenic infection).

First, I present a laboratory experiment to quantify the per-capita differences in predatory behaviour between native and invasive non-native Coccinellidae with a pathogen (Beauveria bassiana) exposure treatment. H. axyridis was the most effcient predator and pathogenic infection reduced the forage ability in all species.

Second, I used existing H. axyridis distribution and aphid abundance data to quantify H. axyridis’ impact through top-down forces. The arrival of H. axyridis is correlated with significant changes in aphid abundance and, of the 14 species studied, five declined in abundance, four increased, while the remaining five showed no significant change.

Third, I measured the per-capita differences in detrital processing rates between native and invasive freshwater amphipods when provided with three diets of differing resource quality and maintained at three temperatures. The rates of detrital processing varied between the native and invasive non-native species and between the temperature and resource quality treatments.

Fourth, I applied native and invasive amphipods at two density treatments (high and low) to a field mesocosm experiment to measure how the per-capita differences impacted more complex ecological systems. The presence of invasive amphipods changed the macroinvertebrate community composition and ecosystem functioning.

I finish by highlighting that our understanding as to how the pressures of invasive non-native species interact with additional environmental stressors remains limited and an area that warrants further investigation.