Patchiness, Density, and Aggregative Behavior in Sympatric Allochronic Populations of 17‐Year Cicadas

Abstract

Densities of two allochronic, geographically overlapping populations of periodical cicadas were censused in the eggnest stage in order to test the hypothesis that larger populations can be supported when the root—feeding nymphs are of different ages. In a frequently burned chest—high stand of scrub oak (Quercus ilicifolia) on Long Island, Magicicada septendecim emerged in 1974 (Brood XIV), then again in 1978 (Brood I) from under the same individual plants over an area of about 1.2 x 3.5 km. Adjacent to this, in the same vegetation, periodical cicadas emerged in 1974 but not in 1978. A nested scheme of sampling revealed significantly higher total eggnest densities when Broods I and XIV occupied the same ground than when Brood XIV existed by itself. The major component of variation in density was among quadrats within plots. That is, cicadas exhibited patchiness on a small scale (metres) which was not reflected in patchiness on a larger scale (tens of metres). Microspatial distributions were thus patchy in both Brood I and XIV, yet dense patches of one brood were usually not correlated with dense patches of the other. Apparently the vegetation appears about as uniform to the cicadas as it does to the human observer, the primary cause of the patchy oviposition pattern being either mutual attraction among oviposition females or lack of dispersal from the chorusing and mating centers. The degree of patchiness is inversely density—dependent, in that the most patchy distributions were invariably those with lowest density. However, several plots with low density were not very patchy. This pattern makes biological sense, remembering that mutual attraction cannot operate unless the animals perceive one another, and at low densities they may sometimes do so and sometimes not. Further analysis, plotting mean crowding against mean density (following Iwao 1968) suggests that mutual attraction among females is weakened at high densities. As modes of data analysis, both kinds of plots–mean crowding vs. mean density and patchiness vs. mean density–bring out patterns that would not otherwise be apparent. In this connection, we offer an empirical definition for the concept, density—independent aggregation."