Comparative functional responses of introduced and native ladybird beetles track ecological impact through predation and competition
Recent advances in invasion research has highlighted that differences in the feeding ecology of native and non-native species manifest through differences in their functional responses (FRs) and that FRs track ecological impact. Further, as with plant competition studies, differential resource use patterns may illuminate competition mechanisms among animal taxa. Ladybirds are a diverse family of beetles that possess substantial variation in body mass across taxa. Further, some ladybird species have been widely introduced into novel habitats, where they may pose significant risks of ecological and economic harm. By controlling for allometric scaling, we tested the hypothesis that introduced Asian multicoloured ladybirds (Harmonia axyridis) are more efficient predators on aphids than trophically-analogous, native convergent ladybirds (Hippodamia convergens). We utilized a prey-replacement FR design, and assessed rates of pea aphid consumption as a function of initial prey density. H. axyridis possessed significantly higher absolute and mass-adjusted FRs (Type II) than the native species, consistent with expectations. The higher FR was mediated by reduced handling time, suggesting that the ability of H. axyridis to manipulate and digest prey exceeds that of H. convergens. Our feeding results may explain, in part, the increasing occurrence and abundance of the introduced species, and documented declines of native species of prey and competitor in invaded habitats.
KeywordsFunctional response Invasion Non-indigenous species Limiting similarity Niche Sympatry
We thank Drs. Dan Pritchard and Steven Marshall for analytical and field advice and Dr. Trevor Pitcher with experimental setup and execution. We are also indebted to the comments made by two referees that enhanced the manuscript. We are grateful for funding from an NSERC Discovery Grant and Canada Research Chair to HJM.
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