Urban areas are hotspots of anthropogenic activity, integrating different selection gradients as a result of changes in hydrology, atmospheric chemistry, microclimate, nutrients, land use and vegetation cover. They are characterized by higher ambient temperatures, eutrophication, noise and light pollution and a low overall habitat quality and connectivity. A striking difference between rural and urban areas is the change in thermal profiles. Higher average ambient temperatures are observed in core city centers as a result of urban geometry and thermal properties of materials, with a gradual temperature decline towards the rural surroundings. This phenomenon, known as the Urban Heat Island effect, is a clear example of anthropogenic climate modification, impacting atmospherical, biological, and socio-economical systems. We here test the hypothesis that natural populations of the water flea Daphnia magna show evolutionary adaptation in thermal tolerance to urbanisation. A total of 72 D. magna clones originating from 12 ponds along urban-rural gradients were cultured at both 20°C and 24°C and scored for their critical thermal maximum (CTMAX) at the stage of first adult instar. CTMAX, defined as the thermal endpoint at which locomotory functions are compromised and the organism faints, is a common metric used as a proxy for thermal tolerance.
Results/Conclusions
A plastic response in thermal tolerance was observed for both urban and rural populations, with higher CTMAX values when exposed to 24°C compared to 20°C (b = 1.09; p < 0.001). In addition, we found genetic adaptation in thermal tolerance with populations showing a higher CTMAX with increasing levels of urbanisation (b = 0.06; p < 0.001). A marginally significant negative correlation of CTMAX with body size was observed (b = -0.68; p = 0.05181), but no interaction with urbanisation was found (p = 0.95). In concordance with the Urban Heat Island theory, our results show adaptive evolution in thermal tolerance along an urban-rural gradient in D. magna, and thus confirm evolutionary responses to urbanisation in this aquatic organism. These adaptive responses likely impact ecological food web dynamics, thereby influencing both top down control of algae and water quality related ecosystem functions, and the socio-ecological value of urban ponds. In the light of current climate change scenarios, urban ecosystems provide a powerful operational framework to study eco-evolutionary responses to increased temperatures and human driven environmental changes.