Density-dependent life history differences in a stream mayfly (Deleatidium) inhabiting permanent and intermittent stream reaches

A life-history response by the mayfly Deleatidium (Ephemeroptera: Leptophlebiidae) from a permanent-fl owing and an intermittent-flowing reach of the Selwyn River, New Zealand, to failing water conditions was examined using field populations and laboratory experiments in January 2006. Field populations had similar densities under flowing water conditions, but size distributions were skewed towards larger larvae in the intermittent reach relative to the permanent reach. Under field conditions of falling water at the intermittent reach, more larvae were caught in the drift during the day and more adults at dusk at the intermittent than at the permanent reach. A desiccation study in the laboratory showed that larvae survived up to 3 days under moist conditions, whereas larvae died within 3 h under dry conditions. Another laboratory experiment revealed that both populations increased emergence under falling water in the high density treatment but not in the ambient density treatment. We conclude that a density increase caused by falling water cues late instar larvae to hasten emergence, an adaptive trait that may reduce population mortality from stream drying.