HABITAT SEGREGATION AND INTERACTIVE EFFECTS OF MULTIPLE PREDATORS ON A PREY ASSEMBLAGE

1. In a field experiment we examined the interactive effects of two co mmon predators of zooplankton, bluegill sunfish (Lepomis macrochirus) and Chaoborus spp. on the growth rate and habitat use of three congene ric prey species (Daphnia). Bluegill and Chaoborus both consume Daphni a, but bluegill also prey on Chaoborus. The prey species, Daphnia puli caria, D, rosea and D, retrocurva, differed in body size and vertical distribution. We expected the largest species, D. pulicaria, to be mos t vulnerable to fish predation and the smallest species, D, retrocuroa , to be most vulnerable to Chaoborus predation. 2. As we expected, the population growth rate of D. pulicaria was significantly reduced by f ish. However, Chaoborus also significantly reduced the growth rate of this species. No significant interaction effect was detected, indicati ng that the effect of these predators was additive. The growth rates o f D. rosea and D. retrocurva were significantly reduced by Chaoborus, but a significant interaction effect indicated that the effect of Chao borus was stronger in the absence of fish than when fish were present. Therefore the impact of Chaoborus and fish on D. rosea and D. retrocu rva was non-additive. The interactive effect of the two predators on D . retrocurva was greater in magnitude than on D. rosea. 3. In the abse nce of predators, the three Daphnia species showed no differences in m ean habitat depth between day and night. Both predators significantly affected diel habitat use of D. pulicaria and D. rosea. Fish caused bo th of these Daphnia species to move deeper during the day, whereas Cha oborus caused Daphnia to move into shallower water at night. Daphnia r etrocurva tended to migrate upwards at night in all predator treatment s, but no significant differences in migration were observed among the predator treatments. The effects of predators on habitat use were not interactive for any prey species. 4. Our results suggest that body si ze, habitat use and the diel migratory response to predators are impor tant factors mediating the interactive effects of multiple predator ty pes on zooplankton.