Habitat effects on local adaptation and plasticity of thermal tolerance across life stages in tropical Bicyclus butterflies

Abstract

Climate change impacts on ectotherms will be a consequence of an interplay between species-specific evolutionary effects, population-level local adaptation, and developmental or plastic effects in individuals. While variation in thermal tolerance resulting from species physiological differences and local adaptation are well researched, how variation in plasticity across habitats might impact vulnerability to climate change remains poorly understood. We studied microhabitat (understory vs. open) distributions and the plasticity in thermal tolerance of four Bicyclus butterfly species across forest and ecotone (savanna-forest transition zone) habitats in Cameroon. For each species, we performed common garden experiments at two stable temperature regimes (20 and 30 °C) and quantified larval and adult thermal tolerance. We found clear differences in distributions across species such that two species were more associated with open microhabitats ( B. dorothea and B. vulgaris ) while two others were more understory associated ( B. sanaos and B. sandace ), with variation across seasons and habitats (forest vs. ecotone). Three species exhibited higher plasticity in critical thermal maximum (CT max ) in the ecotone relative to the forest indicating the importance of the interaction between habitat and developmental temperatures in influencing thermal tolerance. Microhabitat distributions were also consistent with trends in thermal tolerance; the most understory-associated species had both the lowest average CT max and lowest plasticity in CT max in the ecotone. Our findings suggest that microclimate and thermal adaptation shape plastic responses to thermal tolerance, and these factors will likely result in heterogenous responses to climatic change for tropical insects.

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