Predation-driven geographical isolation of broods in periodical cicadas.

Periodical cicadas are remarkable for their incredibly long, prime-numbered life cycles and almost perfectly synchronized mass emergence. Synchronized emergence is a generally localized event, referred to as a "brood". Broods are separated in space and time, or parapatric; adjacent broods emerge on different schedules, whereas some cicadas emerge off schedule, called stragglers. Straggling can potentially erode brood boundaries; thus, the mechanism allowing broods to maintain nonoverlapping distributions is highly puzzling. Here, we propose that predation may allow broods to exclude each other. Our model and numerical simulations show that predation could act as an important factor for maintaining the nonoverlapping distributions. The proposed mechanism is most effective in the vicinity of the critical strength of predation, beyond which the population is doomed to extinction. An increase in predation intensity increases resistance to settlement of a minority brood, while suppressing the main brood population.