Abstract
The spread of invasive species continues to reduce biodiversity across all regions and habitat types globally. However, invader impact prediction can be nebulous, and approaches often fail to integrate coupled direct and indirect invader effects. Here, we examine the ecological impacts of an invasive higher predator on lower trophic groups, further developing methodologies to more holistically quantify invader impact. We employ functional response (FR, resource use under different densities) and prey switching experiments to examine the trait- and density-mediated impacts of the invasive mosquitofish Gambusia affinis on an endemic intermediate predator Lovenula raynerae (Copepoda). Lovenula raynerae effectively consumed larval mosquitoes, but was naïve to mosquitofish cues, with attack rates and handling times of the intermediate predator unaffected by mosquitofish cue-treated water. Mosquitofish did not switch between male and female prey, consistently displaying a strong preference for female copepods. We thus demonstrate a lack of risk-reduction activity in the presence of invasive fish by L. raynerae and, in turn, high susceptibility of such intermediate trophic groups to invader impact. Further, we show that mosquitofish demonstrate sex-skewed predator selectivity towards intermediate predators of mosquito larvae, which may affect predator population demographics and, perversely, increase disease vector proliferations. We advocate the utility of FRs and prey switching combined to holistically quantify invasive species impact potential on native organisms at multiple trophic levels.
Highlights
Invasive species incursions and proliferations are accelerating and present an enormous threat to environments and economies globally[1,2]
We examine the responsiveness of an intermediate predator, endemic to South Africa, the open-water calanoid copepod Lovenula raynerae Suárez-Morales, Wasserman and Dalu to water-borne mosquitofish cues, using mosquito larvae of the Culex pipiens complex as a basal prey
Attack rates of L. raynerae did not differ significantly between cue-free and G. affinis cue treatments, and there was no significant difference within cue treatments
Summary
Invasive species incursions and proliferations are accelerating and present an enormous threat to environments and economies globally[1,2]. The application of comparative FRs can be informative in terms of relative consumer impacts, and directly enables the derivation of emergent context-dependencies that modulate consumer-resource interaction strengths[34,36] These effects can be both abiotic (e.g. temperature/structural complexity37) and biotic (e.g. higher predators[21]). The effectiveness of mosquitofish in biological control has been fundamentally questioned[47], and their application has been recorded to, perversely, increase mosquito proliferations due to interguild predation upon intermediate trophic groups such as notonectids[48] This has resulted in calls to cease the use of such non-native fish in biological control efforts[49]. We currently lack holistic impact quantifications of such invasive species upon ecosystems outside of their native range
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