Predation Risk and The Foraging Behavior of Competing Stream Insects

Abstract

Invertebrates that graze periphyton growing on stones in coldwater streams in Michigan are at greatest risk of predation from a benthic-feeding fish (the mottled sculpin, Cottus bairdi) while on the top surface of substrates, where periphyton is most abundant, and during the day, when sculpin feed most actively. We conducted laboratory experiments with larvae of two strong exploitative competitors for periphyton (the mayfly Baetis tri- caudatus and the caddisfly Glossosoma nigrior) to test if and how their foraging behavior was affected by presence of sculpin when sculpin were not allowed to attack prey. We incorporated four experimental factors in a factorial design: sculpin presence/absence, the abundance (low, high) of food that was patchily distributed on the top surface of substrates, grazer hunger level (fed, starved), and time of day (day, night). In feeding trials, Baetis was much more vulnerable than Glossosoma to sculpin pre- dation. Similarly, only the foraging behavior of Baetis was strongly affected by the presence of sculpin. Baetis larvae responded to the presence of sculpin by: (1) significantly reducing the time spent on the top surface of substrates, and their movement rate while moving within and between food patches, and (2) significantly increasing the proportion of time spent in food patches, and the proportion of individuals that left the substrate by drifting downstream. Baetis larvae accepted greater risk of predation (by spending more time on top) when food abundance on top was high and when larvae had been starved for 27 h. Also, Baetis behavior was strongly affected by sculpin presence during the day but not at night. Collectively, these results suggest that Baetis larvae made adaptive compromises between feeding and avoiding sculpin. In contrast, Glossosoma behavior was not affected by the presence of sculpin. Glossosoma responded most strongly to alterations in food density and their own hunger level. Our results suggest that the presence of sculpin, even though the fish consumed no larvae, might indirectly affect the outcome of competitive interactions between Baetis and Glossosoma through the grazers' differential behavioral responses to the predator.