Consumer communities are being re-arranged through unprecedented rates of human-mediated invasions and extinctions. Such changes in consumer diversity potentially alter community function and impact on resource populations. Although insect herbivore invasions are increasingly common, the infl uence of such species additions on native resident herbivore guilds, along with their individual and combined eff ects on native plant resources, are rarely investigated. Here, we used site-to-site and plant-to-plant variation in herbivore composition to examine how the addition of an invasive exotic weevil, Rhinocyllus conicus , combines with a guild of native fl oral herbivores (tephritid fl ies, pyralid moths) to infl uence two key components of herbivore community function – aggregate herbivore densities and cumulative levels of seed destruction – on a native thistle, Cirsium canescens. Invasion of a site by R. conicus more than doubled aggregate herbivore density, result- ing in increased levels of seed destruction and a halving of seed production by the native thistle. Further, herbivore function was signifi cantly higher on individual plants attacked by R. conicus , compared to plants attacked only by native herbivores. Insect densities and levels of seed destruction on plants attacked by multiple herbivore taxa never exceeded those observed for plants attacked by R. conicus alone, suggesting that increases in herbivore community function with invasion resulted from the inclusion of a functionally dominant insect rather than any complementarity eff ects. Some evidence for interfer- ence between insects emerged, with a trend towards reduced moth and weevil densities in two and three taxon mixtures compared with plants attacked by each taxon alone. However, density compensation was limited so that, overall, the addi- tion of a novel herbivore to the fl oral guild was associated with a signifi cant increase in herbivore community function and impact on seed production. T e results suggest that invasion of a functionally dominant herbivore into an unsaturated recipient community can augment function within a resource guild.
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