Deep impact:in situfunctional responses reveal context‐dependent interactions between vertically migrating invasive and native mesopredators and shared prey

Abstract

Summary The ecological effects of invasive species depend on myriad environmental contexts, rendering understanding problematic. Functional responses provide a means to quantify resource use by consumers over short timescales and could therefore provide insight into how the effects of invasive species vary over space and time. Here, we use novel in situ microcosm experiments to track changes in the functional responses of two aquatic mesopredators, one native and the other an invader, as they undergo diel vertical migrations through a lake water column. The Ponto–Caspian mysid, Hemimysis anomala, a known ecologically damaging invader, generally had higher a functional response towards cladoceran prey than did a native trophic analogue, Mysis salemaai. However, this differential was spatiotemporally dependent, being minimal during the day on the lake bottom, and increasing at night, particularly inshore. Because the functional response of the native predator was spatiotemporally consistent, the above pattern was driven by changes in the invader functional response over the diel cycle. In particular, the functional response of H. anomala was significantly reduced on the lake bottom during the daytime relative to night, and predation was especially pronounced in shallow surface waters. We demonstrate the context dependency of the effects of an invasive predator on prey populations and emphasise the utility of functional responses as tools to inform our understanding of predator–prey interactions. In situ manipulations integrate experimental rigour with field relevance and have the potential to reveal how impacts manifest over a range of spatiotemporal scales.

Predator‐free space, functional responses and biological invasions

“…; Williams, Flaherty & Flaxman ) and, recently, have yielded insights into how invasive species affect recipient communities by, for example revealing how impacts on prey populations vary over space and time (Barrios‐O'Neill et al . ). Historically, ecologists have adopted a categorical approach to defining functional responses; Type I functional responses are linear; Type II functional responses are characterized by a decreasing proportion of prey consumed with increasing prey density; whilst Type III functional responses describe an initial increase of proportional prey consumption, followed by a decrease (Juliano ).…”

Increasing temperature decreases the predatory effect of the intertidal shannyLipophrys pholison an amphipod prey

“…It is important to build experimental datasets to quantify and predict predator effects under different contexts and using a range of different organisms (Dell et al., ; Gilbert et al., ). Comparative functional responses have been pioneered as a method for quantifying the effect of invasive species (Dick et al., ; Alexander et al., , ; Barrios‐O'Neill et al., ; Laverty et al., ). In the context of these studies, functional responses were applied to compare the effect of invasive species against a native analogue under various abiotic contexts.…”

A trophic interaction framework for identifying the invasive capacity of novel organisms

“…In situ measurements and manipulations provide realistic settings, but tend to allow poor control of confounding factors (but see Barrios‐O'Neill, Dick, Ricciardi, et al., ). Also, they cannot be carried out if the focal organism is not (yet) present in the target environment.…”

“…Laboratory‐derived functional responses typically isolate an individual predator and single prey species (e.g. Barrios‐O'Neill, Dick, Ricciardi, et al., ; Dick et al., ; Xu et al., ; but see Alexander et al., ; Medoc, Spataro, & Arditi, ; Wasserman et al., ), and thus rarely account for prey switching or interference among predators which could affect the outcome of an interaction (Amarasekare, ; Tschanz, Bersier, & Bacher, ; van Leeuwen, Brännström, Jansen, Dieckmann, & Rossberg, ). The degree of spatio‐temporal overlap of habitat use by the focal organism and its interaction partners is another important consideration (Polis, Anderson, & Holt, ).…”

“…Invading predators with high ecological impact on their prey populations often have elevated functional responses compared to ecologically similar native species (Dick et al., ). Using comparative functional responses as an empirical screening method is thus rapidly gaining popularity among invasion biologists (Alexander, Dick, Weyl, Robinson, & Richardson, ; Barrios‐O'Neill, Dick, Ricciardi, MacIsaac, & Emmerson, ; Dick et al., ; Rosewarne et al., ; Xu et al., ). The method typically infers invasion success and/or potential impacts from a limited number of prey species (frequently just one).…”