Facilitation between herons and smallmouth bass foraging on common prey

Understanding how multiple predators affect one another and their shared prey is an increasingly important goal for ecologists examining predator–prey dynamics and food-web structure. In a field experiment, we examined the outcome of interactions between terrestrial and freshwater predators foraging for the same prey in two temperate North American streams. We used a factorial design to examine the combined foraging effects of herons and smallmouth bass on striped shiners and central stonerollers. We found that there was facilitation between the two predators, resulting in risk-enhancement for the prey species, with particularly pronounced effects on the smallest (<70 mm) size classes. Facilitation is the least well-documented predator–predator interaction and has not been quantitatively demonstrated for freshwater and terrestrial predators. Our results indicate that bass may gain a net benefit from the presence of wading birds such as herons and egrets, and that concerns about the negative effects of birds on fish stocks through competition may be unwarranted.