Temperature can show strong spatial gradients leading to the widespread evolution of thermal performance curves (TPCs). The spatial differentiation of TPCs may basically take two not mutually exclusive adaptive patterns: 1) The “hotter-colder” model, where TPCs shift horizontally, and 2) the “faster-slower” model, where TPCs shift vertically (i.e., countergradient variation). Although mainly documented at the macrogeographic scale, strong thermal gradients can also occur locally and drive the evolution of microgeographic variation in TPCs. One particularly strong microgeographic thermal gradient exists between rural and urban areas, with urban areas having considerably higher mean temperatures (“urban heat islands”). Given the steadily increasing urbanisation, and because urbanisation gradients have been recently promoted as suitable study systems for predicting biological responses to global warming, there is a need for a better understanding of the differentiation in TPCs between rural and urban populations. In our study, we focused on differentiation in TPCs for growth rate between replicated urban and rural populations. For this, we conducted a common garden rearing experiment with a gradient of temperatures. As study species, we chose the damselfly Coenagrion puella, occurring in both rural and urban areas. A total of 900 larvae were reared from the egg stage at five temperatures.
Results/Conclusions
Rural and urban larvae did not differ in early growth rates (days 0-30). In accordance with countergradient variation, the cold-adapted rural larvae grew faster than the warm-adapted urban larvae across temperatures later on (days 30 – 50). In line with costs of rapid growth stabilizing the countergradient pattern, the faster growing rural larvae showed consistently lower survival than urban larvae across temperatures, suggesting survival costs of the fast growth in the cold-adapted genotypes. Countergradient variation in growth rate is likely a widespread pattern along the urbanisation gradient. We hypothesize that the lower larval degree-days before the onset of winter in rural populations compared to the populations in the urban heat island impose stronger time constraints to reach a certain developmental stage before winter, thereby selecting for faster growth rates. Our results contribute to the limited evidence for microgeographic countergradient variation and support the recent view that urbanisation may drive life history evolution. Moreover, our results are in accordance with the current view that urbanisation gradients are suitable study systems for predicting biological responses to global warming.