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.

Phenological asynchrony between herbivorous insects and their hosts: signal of climate change or pre-existing adaptive strategy?

“…In fact, they did not. Weiss et al (1988) found that small differences in air temperatures in the field (68C on average between northand south-facing slopes) were enough to alter the relative timing of butterfly egg-laying and host-plant senescence, such that the window for feeding by prediapause larvae could be as long as 17 days (for north-facing slopes) or could collapse to nothing, with the average egg mass being laid after host senescence, on south-facing slopes. Singer (1971) found that the time difference between peak oviposition and 50 per cent host senescence was not constant among years, varying by 6 days across 4 years of study.…”

“…How much fecundity would she have gained by the same decision? We can make an educated guess by putting together the known mean weight of eggs (0.23 mg; Moore & Singer 1987) with the measured rates of weight gain by larvae in the field (Weiss et al 1988(Weiss et al , 1993. In late January, last instar larvae gained an average of 35 mg d 21 on a 158 south-facing slope and 8 mg d 21 on a 158 northfacing slope (Weiss et al 1993).…”

Extracting useful data from imperfect monitoring schemes: endangered butterflies at San Bruno Mountain, San Mateo County, California (1982–2000) and implications for habitat management

“…In the northern hemisphere, north facing slopes are typically cooler and wetter than south facing slopes, with significant differences in vegetation composition as a result. The distribution of larvae of the Bay checkerspot changes substantially from year to year across slopes, reflecting spatial patterns of prediapause survivorship (Weiss et al 1988). Larvae from egg masses laid on cooler slopes were found to nearly always have a better chance to reach diapause than those laid concurrently on warmer slopes, because of the later onset of plant senescence on cooler slopes.…”

“…The importance of maintaining habitat on north facing exposures has been illustrated for the Bay checkerspot butterfly (Euphydryas editha bayensis), a threatened species also found in the San Francisco Bay area that utilizes grassland habitat (Weiss et al 1988). In the northern hemisphere, north facing slopes are typically cooler and wetter than south facing slopes, with significant differences in vegetation composition as a result.…”

“…Larvae from egg masses laid on cooler slopes were found to nearly always have a better chance to reach diapause than those laid concurrently on warmer slopes, because of the later onset of plant senescence on cooler slopes. Cool slopes are high quality habitat for prediapause larvae, and become better relative to warmer slopes as the flight season progresses (Weiss et al 1988).…”