Ecological impacts of invasive non-native species vary across habitats and over time, owing to the influence of local environmental variables. This context dependency poses a major challenge for risk assessment. Here, we explicitly incorporate contrasting habitat conditions in an experimental study that aims to understand the trophic impacts of an invasive fish, the tench (Tinca tinca), which is currently spreading in the St. Lawrence River and likely to colonize the Great Lakes. We compared the growth rate and functional response of tench exposed to combinations of two physical habitat variables: substrate size (rocks versus sand) and temperature (18°C versus 26°C, the latter being among the maximum mean summer nearshore temperatures projected for the lower Great Lakes). As tench are reported to thrive in habitats dominated by fine sediments, we hypothesized that they are better adapted to foraging in such habitats and so we tested the prediction that prey consumption efficiency and growth would be reduced on coarse substrates. Secondly, we hypothesized that their prey consumption would be higher at 26°C, which is within the upper limit of the reported preferred temperature range for tench, compared to a temperature below the preferred range.
Contrary to published reports suggesting rocky habitats are suboptimal for the species, tench did not exhibit impaired rates of consumption or growth on rocks compared to sand. Although tench require vegetated fine-sediment habitat for spawning and are known to take refuge from predation in macrophyte beds, their efficient foraging on coarse substrate suggests that habitat plasticity contributes to their ability to rapidly colonize large heterogeneous waterbodies. Furthermore, tench functional response shifted from a stabilizing Type III to a de-stabilizing Type II relationship as temperature increased from 18°C to 26°C on both substrate types. Thus, climate warming could erode the ability of prey to exploit refugia against tench predation, possibly resulting in enhanced competition for food resources with native benthivorous fishes. Our results demonstrate the value of explicitly incorporating different environmental contexts into experimental comparisons of performance to inform invasive species risk assessment.