POPULATION SYNCHRONY IN MAYFLIES: A PREDATOR SATIATION HYPOTHESIS

The larvae of mayflies (Order: Ephemeroptera) inhabit most permanent freshwater environments (rivers, lakes, ponds). The duration of the mayfly life cycle varies from a few weeks to several years depending on the response of each species to regional climate and nutrition (Needham et al., 1935). Most of the mayfly life cycle is spent in either the egg or larval stage because almost none of the adults feed and, depending on the species, live only about 10 min to 48 h before reproduction and death. Adults of many species appear in flight in large numbers during a brief period each year and form vast mating swarms over the water surface. Both the size and periodicity of mayfly swarms have been conspicuous enough to stimulate detailed descriptions by early naturalists (see Needham et al., 1935 for review) as well as more recent speculation concerning the ecological and evolutionary significance of swarm periodicity (Corbet, 1964; Edmunds and Edmunds, 1980). Population synchronization in aquatic insects, especially mayflies, refers to a tendency for the adult reproductive stage of individuals in a given population to be active in the environment during a short, well defined period of the year (i.e., seasonal synchronization within a time frame >24 h but usually less than 2-4 wk) and/or the day (i.e., time frame <24 h and usually about 1-2 h in duration). For our purpose here, each 24-h period represents a unit of observation and the sum of such units describes the emergence period for each species under consideration. It has been suggested that the adaptive value of adult synchronization for an insect species resides largely with increasing the probability of finding a mate (Corbet, 1964). If so, synchronization should be developed best in species having short-lived adults that are spatially dispersed. This hypothesis is difficult to test because spatial dispersion cannot be adequately determined. In addition, no one has demonstrated for any aquatic species that individuals emerging very early or late in the emergence period fail to mate successfully. Here we argue that mating success is unlikely to have been the only factor selecting for adult synchronization in mayflies. We hypothesize that predator satiation provides a better conceptual framework for assessing adult emergence patterns in mayflies and perhaps other aquatic insects. "Predator satiation" was used initially to describe the interaction between periodical cicadas (Insecta: Homoptera) and their predators (see Lloyd and Dybas, 1966 for review). Predator satiation occurs when the quantity of a particular prey item (e.g., cicada) at a given point in time far exceeds the potential number that can be taken by a fixed density of local predators (e.g., birds). The predators are satiated and the remaining prey survive to reproduce providing prey densities remain above the level necessary for satiation. Thus, a predator satiation hypothesis for any predator-prey system predicts an inverse relationship between prey mortality due to predation and prey availability above the level needed to satiate predators as long as the number of predators remains more or less fixed for a given time interval. In this paper we present quantitative data on adult mortality during each day of the adult emergence period for the mayfly Dolania americana Edmunds and Traver to demonstrate how emergence