Montane grasshopper fitness constraints in changing environments

The ability to predict how populations and species will respond to climate change is a central goal in the modern era of biology, and requires empirical data on fitness responses to climate means, variability, and extremes. Given that fitness is the product of fecundity and survival, it is critical to consider how/whether these two fitness components separately shift in response to the environment. To this end, we performed a two-year reciprocal transplant experiment on two montane grasshopper species that differ in their dispersal propensity and phenology. We hypothesized that fecundity constraints would be strongest at high elevation due to lower temperatures and a shorter season, while survival constraints would dominate at low elevation, where the incidence of extreme temperatures is higher. We transplanted grasshoppers across elevations, measuring development time, growth, fecundity, and survival. In the first year, we found that survival to adulthood was lowest at low elevation, while time to reproduction was longest at high elevation, consistent with our hypothesis. Size clines were opposite to our expectation from wild grasshoppers, although this may have been driven by food limitation at low elevation. Transplant elevation, not elevation of origin, was the strongest factor influencing phenotypic outcomes, providing little evidence for local adaptation. Overall, we found evidence supporting different responses of fitness components to environmental conditions.