IMPACTS OF PREDATION AND INTRACOHORT CANNIBALISM IN THE WATER STRIDERGERRIS-BUENOI (HETEROPTERA, GERRIDAE)

Despite the growing appreciation of intraguild predation as a factor i n the population dynamics of predatory invertebrates, few empirical st udies exist, especially for cannibalism among individuals of the same age. We evaluate the relative role of such processes for the water str ider Gerris buenoi with a field experiment. Field enclosures were used to rear water striders from the first instar to the adult stage. Each enclosure had compartments for three treatments: group rearing with f ree access for predators, group rearing under predator exclusion, and individual rearing under predator exclusion. Overall survival was simi lar in the group and individual rearings under predator exclusion, but was substantially lower where predators were present, indicating that predation by non-gerrids influences juvenile survival more than intra cohort cannibalism. Regular censuses showed that most mortality occurr ed in the first instar. Moreover, survival in group rearings was lower than in individual rearings during this period, indicating that intra cohort cannibalism is a relevant factor in the first few days of larva l life. The bugs from individual rearings had the lowest average adult weight and longest development time, whereas the fastest-maturing and heaviest adults were those from group rearings under predator exclusi on. This indicates that factors other than competition for food are re sponsible for variation in growth performance.