Terrestrial insects and climate change: adaptive responses in key traits

Understanding and predicting how adaptation will contribute to species' resilience to climate change will be paramount to successfully managing biodiversity for conservation, agriculture, and human health‐related purposes. Making predictions that capture how species will respond to climate change requires an understanding of how key traits and environmental drivers interact to shape fitness in a changing world. Current trait‐based models suggest that low‐ to mid‐latitude populations will be most at risk, although these models focus on upper thermal limits, which may not be the most important trait driving species' distributions and fitness under climate change. In this review, we discuss how different traits (stress, fitness and phenology) might contribute and interact to shape insect responses to climate change. We examine the potential for adaptive genetic and plastic responses in these key traits and show that, although there is evidence of range shifts and trait changes, explicit consideration of what underpins these changes, be that genetic or plastic responses, is largely missing. Despite little empirical evidence for adaptive shifts, incorporating adaptation into models of climate change resilience is essential for predicting how species will respond under climate change. We are making some headway, although more data are needed, especially from taxonomic groups outside of Drosophila, and across diverse geographical regions. Climate change responses are likely to be complex, and such complexity will be difficult to capture in laboratory experiments. Moving towards well designed field experiments would allow us to not only capture this complexity, but also study more diverse species.     

Variation in heat shock protein expression at the latitudinal range limits of a widely‐distributed species,the Glanville fritillary butterfly (Melitaea cinxia)

Studies of heat shock response show a correlation with local climate, although this is more often across altitudinal than latitudinal gradients. In the present study, differences in constitutive but not inducible components of heat shock response are detected among populations of the Glanville fritillary butterfly Melitaea cinxia L. that exist at the species' latitudinal range limits (Finland and Spain). The study demonstrates that macroclimatic differences between these sites should cause greater exposure of the Spanish population to higher temperatures. Thermal stress treatments are used to estimate differences in the expression of four genes potentially relevant for tolerating these temperatures. For the analysis, three heat‐shock proteins and glyceraldehyde‐3‐phosphate dehydrogenase (G3PDH), a glycolysis enzyme that also modulates cell growth based on metabolic state, are chosen. Two constitutive differences are found between the sites. First, insects from Spain have higher levels of Hsp 21.4 than those from Finland regardless of thermal stress treatment; this protein is not inducible. Second, insects from Finland have higher levels of G3PDH. The two remaining Hsps, Hsp20.4 and Hsp90, show dramatic up‐regulation at higher temperatures, although there are no significant differences between insects from the different populations in either constitutive levels or inducibility. In nature, differences between the study populations likely occur in the expression of all four genes that were studied, although these differences would be directly climate‐induced in Hsp20.4 and Hsp90 and constitutive in Hsp21.4 and G3PDH. Inducibility may mitigate the need for constitutive variation in traits that adapt insects to local climate.     

Variation in sexual size dimorphism among populations: testing the differential‐plasticity hypothesis

Sexual size dimorphism (SSD) is a common phenomenon in animals and varies widely among species and among populations within species. Much of this variation is likely due to variance in selection on females vs. males. However, environmental variables could have different effects on females vs. males, causing variation in dimorphism. In this study, we test the differential‐plasticity hypothesis, stating that sex‐differential plasticity to environmental variables generates among‐population variation in the degree of sexual dimorphism. We examined the effect of temperature (22, 25, 28, and 31 °C) on sexual dimorphism in four populations of the cockroach Eupolyphaga sinensis Walker (Blattaria: Polyphagidae), collected at various latitudes. We found that females were larger than males at all temperatures and the degree of this dimorphism was largest at the highest temperature (31 °C) and smallest at the lowest temperature (22 °C). There is variation in the degree of SSD among populations (sex*population interaction), but differences between the sexes in their plastic responses (sex*temperature interaction) were not observed for body size. Our results indicated that sex‐differential plasticity to temperature was not the cause of differences among populations in the degree of sexual dimorphism in body size.