Increased Reproductive Success at High Densities and Predator Satiation For Periodical Cicadas

The reproductive success of periodical cicadas, measured as the number of offspring produced per adult, is found to increase as adult density increases. The inverse density—dependent pattern at the adult stage occurred over the entire range of adult densities encountered. This result could have been caused by (1) predator satiation, (2) increased fecundity at higher densities, (3) more efficient mating at higher densities, and (4) movement of adults from sites of low initial densities to sites of high densities. The hypothesis of predator satiation was explored in depth. Predation at each site was crudely estimated by collecting cicada wings, which are discarded by avian predators. If predator satiation accounts for the inverse density—dependent mortality, then each cicada's risk of capture will decrease as cicada density increases. The data suggest that this prediction is true; predation did not increase as cicada density increased so that the probability of escaping predation is greater at high—density sites. Estimated predator densities are independent of cicada densities, which indicates that predators cannot respond numerically to the cicada emergence. As cicada density increases, the number of cicada wings recovered per bird does not significantly increase, suggesting that birds are satiated. Significantly more cicada wings were recovered at sites with more birds. Total cicada predation was independent of cicada density but dependent on the density of predators. Some evolutionary consequences of these results are discussed.