Sun, Slope, and Butterflies: Topographic Determinants of Habitat Quality for Euphydryas Editha

Abstract

Thermal environments in a large, topographically diverse serpentine soil—based grassland were quantified and ranked using a computer model of clear sky insolation and shading on different slopes to determine the effects of microclimate on the rates of development of each of the life stages of the butterfly Euphydryas editha bayensis. Larvae developed to pupation earlier on warm slopes than on progressively cooler slopes. Availability of sunlight can be limiting for larvae, which bask indirect sun to raise body temperature. Larvae can disperse >10 m/d, allowing them to transfer between microclimates. Pupae on warmer slopes also developed faster than those on cooler slopes. Microclimate also affects the phenology of host plants of larvae and nectar sources of adults. Larval and pupal development and host—plant phenology determine the phase relationship between adult butterfly flight and host—plant senescence, which in turn determines mortality rates of prediapause larvae. Adult females that eclosed early in the season could have their offspring survive on almost all slopes, survivorship of offspring from adults that eclosed in the middle of the flight season was restricted to cooler slopes in the habitat. Some butterflies eclosed too late for their offspring to survive on any slope. Topographic diversity on several scales is a prime indicator of habitat quality for this butterfly. Areas of high local topographic diversity on a scale of tens of metres appear particularly important for long—term population persistence under variable climatic conditions.