The Periodical Cicada Problem. I. Population Ecology

par LLOYOD Monte & DYBAS Henry S., 1966

Abstract

It is argued that populations of non-periodical cicadas, some of which appear above ground every year, are governed by their natural enemies because populations of predators can build up from year to year at the expense of successive year classes of cicadas. This is not possible in the case of periodical cicadas, because the benefit to the predator populations will have become dissipated during the 16 (or 12) years between emergences-years in which virtually no periodical cicadas are to be found above ground. Periodical cicadas, we think, are governed by some other ecological mechanism, at a level far higher than the level at which predators govern populations of non-periodical cicadas. Predators below ground, especially moles, undoubtedly do affect cicada numbers, but we do not think that moles can govern periodical cicada populations, in Nicholson's (1954) sense, because the very young nymphs of periodical cicadas are probably too small for the moles to feed on, and remain so for several years. Like predation above ground, the benefit to the mole population does not extend over more than one generation of cicadas. We discuss several other mechanisms that might be capable of governing periodical cicada populations. No synchronized predator appears to exist, but the fungus disease, Massospora cicadina, has resting spores which apparently carry it over from one generation of cicadas to the next, so that under certain circumstances this fungus may perhaps provide a governing mechanism. If for some reason the cicadas are able to escape control by the fungus then, we think, the number of underground feeding sites on the tree roots may set the limit to the population, excess nymphs being eliminated by starvation and by fighting underground. Mortality of eggs, resulting secondarily from overcrowding in the twigs, could theoretically govern the cicada population. We doubt that it does so, because the limit set by egg mortality would probably be much higher than the limit set already by the number of feeding sites underground. Even so, egg mortality may affect cicada numbers in local patches within a woods. Another possible mechanism entails the assumption that some of the trees may be resistant to cicadas, but there is as yet too little evidence to give an opinion about this hypothesis, one way or the other.

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