Local adaptation in trait-mediated trophic cascades.

Predator-induced changes in prey foraging can influence community dynamics by increasing the abundance of basal resources via a trait-mediated trophic cascade. The strength of these cascades may be altered by eco-evolutionary relationships between predators and prey, but the role of basal resources has received limited attention. We hypothesized that trait-mediated trophic cascade strength may be shaped by selection from trophic levels above and below prey. Field and laboratory experiments used snails (Nucella lapillus) from two regions in the Gulf of Maine (GoM) that vary in basal resource availability (e.g. mussels), seawater temperature, and contact history with the invasive green crab, Carcinus maenas. In field and laboratory experiments, Nucella from both regions foraged on mussels in the presence or absence of green crab risk cues. In the field, Nucella from the northern GoM, where mussels are scarce, were less responsive to risk cues and more responsive to seawater temperature than southern Nucella. In the lab, however, northern Nucella foraged and grew more than southern snails in the presence of risk, but foraging and growth were similar in the absence of risk. We suggest that adaptation to basal resource availability may shape geographical variation in the strength of trait-mediated trophic cascades.