Kristien I. Brans, Joost Vanoverbeke, Robby Stoks, Mieke Jansen, Nedim Tüzün, Luc De Meester
Worldwide, urbanization leads to tremendous anthropogenic environmental alterations, causing strong selection pressures on populations of animals and plants. Although a key feature of urban areas is their higher temperature (“urban heat islands”), adaptive thermal evolution in organisms inhabiting urban areas has rarely been studied. We tested for evolution of a higher heat tolerance (CTMAX) in urban populations of the water flea Daphnia magna, a keystone grazer in freshwater ecosystems, by carrying out a common garden experiment at two temperatures (20°C and 24°C) with genotypes of 13 natural populations ordered along a well-defined urbanization gradient. We also assessed body size and haemoglobin concentration to identify underlying physiological drivers of responses in CTMAX. We found a higher CTMAX in animals isolated from urban compared to rural habitats and in animals reared at higher temperatures. We also observed substantial genetic variation in thermal tolerance within populations. Overall, smaller animals were more heat tolerant. While urban animals mature at smaller size, the effect of urbanization on thermal tolerance is only in part caused by reductions in body size. Although urban Daphnia contained higher concentrations of haemoglobin, this did not contribute to their higher CTMAX. Our results provide evidence of adaptive thermal evolution to urbanization in the water flea Daphnia. In addition, our results show both evolutionary potential and adaptive plasticity in rural as well as urban Daphnia populations, facilitating responses to warming. Given the important ecological role of Daphnia in ponds and lakes, these adaptive responses likely impact food web dynamics, top-down control of algae, water quality, and the socio-economic value of urban ponds.
Urbanization causes habitat warming. The impact of “urban heat islands” on evolutionary processes in organisms is, however, rarely studied. A common garden experiment at two rearing temperatures (20°C and 24°C) revealed substantial evidence for thermal adaptation and adaptive plasticity in Daphnia in response to urbanization and rearing temperature. Urban Daphnia and animals reared at 24°C have a higher CTMAX and mature at a smaller size. While urban Daphnia are smaller, this only in part contributed to the evolutionary increase in CTMAX. The observed higher haemoglobin levels in urban animals did not induce a higher CTMAX. We additionally report substantial genetic variation for CTMAX within both rural and urban populations, facilitating responses to warming. Given that urban areas currently experience temperature increases expected to occur over the next 100 years due to climate change, these results, the first to be presented for aquatic organisms, provide the evidence that Daphnia are well equipped to cope with current and future anthropogenic warming.