“Competition and ‘stragglers’ as mediators of developmental synchrony in periodical cicadas”

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Julie C. Blackwood, Jonathan Machta, Alexander D. Meyer, Andrew E. Noble, Alan Hastings, and Andrew M. Liebhold (Oct 2018)

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Periodical cicadas are enigmatic organisms: broods spanning large spatial ranges consist of developmentally synchronized populations of 3-4 sympatric species that emerge as adults every 13 or 17 years. Only one brood typically occupies any single location, with well-defined boundaries separating distinct broods. The cause of such synchronous development remains uncertain but it is known that synchronous emergence of large numbers of adults in a single year satiates predators, allowing a substantial fraction of emerging adults to survive long enough to reproduce. Competition among nymphs feeding on tree roots almost certainly plays a role in limiting populations. However, due to the difficulty of working with such long-lived subterranean life stages, the mechanisms governing competition in periodical cicadas have not been identified. A second process that may affect synchrony among periodical cicadas is their ability to delay or accelerate their emergence as adults by one year and accelerate it by four years (“stragglers”). We develop a non-linear Leslie-type matrix model that describes cicada dynamics accounting for predation, competition and stragglers. Using numerical simulations, we identify conditions that generate dynamics in which a single brood occupies a given geographical location. Our results show that while stragglers have potential for introducing multiple sympatric broods, the interaction of interbrood competition with predation-driven Allee effects creates a system resistant to such invasions and populations maintain developmental synchrony.