Estimating cicada population size and predation rates by birds.

Predator Satiation by Periodical Cicadas Periodical cicadas, Magicicada spp., emerge as adults once every 13 years in the southern part of their range in North America and once every 17 years in the northern part of their range. Though these insects emerge only once every 13 or 17 years in any particular area, virtually every year sees a brood emerging somewhere in eastern North America. An emergence of periodical cicadas produces a sudden flush of singing insects whose density can approach 4 X 10^6 individuals per hectare, which translates into a biomass of 1,900 to 3,700 kg of cicadas per hectare, the highest biomass of a natural population of terrestrial animals ever recorded.  Periodical cicadas are insects of the order Homoptera, which includes the leafhoppers and aphids. Like their relatives, cicadas make their living by sucking the fluids of plants and spend either 13 or 17 years of their life as nymphs underground, where they feed on the xylem fluids in roots. When mature, nymphs dig their way to the soil surface, where they shed their nymphal skin and emerge as winged adults. Among periodical cicadas this emergence is so synchronized that millions of adults emerge over a period of only a few days. Following emergence males fly to the treetops, where they sing the mating songs to which females are attracted. After they mate, females lay their eggs in living twigs of shrubs and trees. When the nymphs hatch in about six weeks, they immediately drop to the ground and burrow down to a root, where they begin to feed, moving around very little for the next 13 or 17 years. A mass emergence of periodical cicadas, one of the most memorable biological phenomena nature has to offer, appears aimed at predator satiation.  Kathy Williams and her colleagues (1993) tested the effectiveness of predator satiation in a population of 13-year periodical cicadas in northwest Arkansas. They monitored emergence of cicadas using conical emergence traps constructed of plastic mesh and inverted their traps to measure predation rates (fig. 14.24). Nymphs emerging from the ground below the traps could be counted to estimate the numbers of emerging nymphs. Then, as adult cicadas died from a variety of factors, including physical factors, senescence, and pathogens, they fell from the trees to the ground, where some were caught in the inverted traps. Because the major predators were birds, predation rates could be estimated because birds discard the wings of cicadas as they feed upon them. The wings falling into the inverted traps gave an estimate of predation rates.