A MODEL FOR SEASONAL SYNCHRONY IN STREAM MAYFLIES

Many mayfly species have synchronous univoltine life histories over broad geographic ranges, but the life-history adaptations underlying their seasonality remain unidentified. We investigated whether simple adaptations in the response of development rate to temperature might account for the observed phenology of adult emergence and the 1- and 2-year life histories of eight mayfly species studied in Piedmont streams of eastern North America between 34-degrees-N and 50-degrees-N latitude. We present a model consisting of two sequential life-history stages. Development rate in each stage is a linear function of streamwater temperature above a lower threshold but, in the first stage, a developmental quiescence occurs whenever a maximal temperature is exceeded. The five model parameters are assumed not to vary with latitude. Using daily field temperatures, the model simulated several successive generations beginning from an arbitrary day of the year. With parameters fitted from the field data, the model could reproduce both the univoltine life history and the latitudinal variation in the timing of adult emergence for six species. For two additional species, the model reproduced a northward transition to a semivoltine life history. The simulations suggest that nearly all development occurs in spring and autumn during periods of roughly equivalent thermal regime at all latitudes.