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Diverse taxa have undergone phenological shifts in response to anthropogenic climate change. While such shifts generally follow predicted patterns, they are not uniform, and interspecific variation may have important ecological consequences. We evaluated relationships among species' phenological shifts (mean flight date, duration of flight period), ecological traits (larval trophic specialization, larval diet composition, voltinism), and population trends in a butterfly community in Pennsylvania, USA, where the summer growing season has become warmer, wetter, and longer. Data were collected over 7-19 years from 18 species or species groups, including the extremely rare eastern regal fritillary Speyeria idalia idalia. Both the direction and magnitude of phenological change over time was linked to species traits. Polyphagous species advanced and prolonged the duration of their flight period while oligophagous species delayed and shortened theirs. Herb feeders advanced their flight periods while woody feeders delayed theirs. Multivoltine species consistently prolonged flight periods in response to warmer temperatures, while univoltine species were less consistent. Butterflies that shifted to longer flight durations, and those that had polyphagous diets and multivoltine reproductive strategies tended to decline in population. Our results suggest species' traits shape butterfly phenological responses to climate change, and are linked to important community impacts.

Models of feeding and digestion predict that increased body size should result in longer gut passage time and improved assimilation efficiency. We examined the implications of digestion theory for size-structured interactions in a generalist zooplankton herbivore by studying the relationships between body size, ingestion rate, gut passage time (GPT), assimilation efficiency (AE), and growth rate in a clone of Daphnia pulex feeding on seven taxa of green algae that differed in digestibility. We also tested the effect of varying food concentration on GPT and AE while keeping body size constant. Food quality varied markedly among algal taxa, with mean juvenile growth rates at high food concentrations (1-2 mg/L) ranging from 0.10 to 0.61 d(-1). Juvenile growth rate for high food concentrations was highly correlated with juvenile AE (r2 = 0.96), verifying the importance of digestibility for food quality. AE, measured with 14C-labeled algae, increased with increasing age and body size for each of four digestion-resistant taxa but did not vary with age and body size for three readily digested algae. GPT decreased with decreasing body size, supporting the hypothesis that shorter GPT in juveniles leads to lower AE for digestion-resistant resources. Lower food concentrations led to increased GPT and improved AE for juveniles feeding on two digestion-resistant algae, providing further support for a role of longer gut retention in overcoming digestion defenses. The results suggest that increased abundance of digestion-resistant food will lead to growth and recruitment bottlenecks for juvenile herbivores, but that the effectiveness of digestion defenses will be decreased when large-bodied grazers predominate and when low food concentrations result in longer gut passage times. Gut processing constraints may favor either high concentrations of slow-growing, digestion-resistant resources or low concentrations of fast-growing, undefended resources.