Lack of evidence for indirect effects from stonefly predators on primary production under future climate warming scenarios

Consumptive and non-consumptive interactions of predators and prey can have strong direct and indirect effects on primary producers, such as stream algae. Increasing water temperatures may alter these interactions and thus influence productivity in streams. For each of 3 temperature treatments (‘ambient’, +2°C and +4°C), we measured the amount of algal biomass removed by grazing mayflies from 91 mesocosms after a 24-hour test period under 3 grazing treatments: lethal predators, non-lethal predators, and no predators. At all temperatures, grazers reduced algal biomass (p < 0.01), and the presence of lethal predators effectively dampened mayfly consumption of algae (p < 0.01). However, differences in algal biomass between lethal and non-lethal predator treatments were not significant, indicating that predators had no indirect behaviorally mediated effects on grazer consumption. Grazer removal of algal biomass marginally increased with increasing temperature (p = 0.051). We analyzed video data for changes in grazer foraging and drift behavior. Mayflies increased drift in the presence of lethal predators (p < 0.01) but not non-lethal predators, and no behavioral changes were seen with temperature increases. Mesocosms can help elucidate possible future shifts in trophic interactions due to climate warming. Yet, we found no evidence of indirect stonefly predator effects on grazing mayflies under these warming scenarios.