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Abstract:

Human occupation on earth has had a detrimental effect on the environment on a global scale. Currently, more than half of the human population is living in cities and that number is expected only to increase in the following decades. Urbanization directly and indirectly impacts ecosystems by modifying biogeochemical and nutrient cycles, hydrology and energy flow. Urbanization has led to habitat fragmentation and degradation, pollution, and is recognized as one of the most important drivers of the ongoing biodiversity crisis. Moreover, urban development is one of the major causes of species extinctions worldwide and is a key driver of biotic homogenization. The ecological integrity of urban freshwater systems in particular is under great threat, as they are major receivers of nutrients, pollutants, and other anthropogenic runoff, and are often of artificial, manmade origin. Understanding the effects urbanization has on the environmental conditions and biodiversity of the ponds is crucial The main goal of this PhD dissertation was to examine the impact of urbanization on pond systems and to provide insights into the mechanisms by which urbanization affects water quality (Chapter 1), temperature (Chapter 2), and zooplankton communities in ponds (Chapter 3). Our results demonstrate that pond environment was mainly affected by urbanization- driven warming, while the effects of land use on other physico-chemical variables are minimal (Chapter 1 and 2). We also demonstrated that pond management and pond morphometry has a strong impact on local environmental conditions. Moreover, pond environment and biodiversity were mainly impacted on the local scale of urbanization, most likely through runoff and thermal pollution. This suggests there might be an “easy” solution for urban management to restore ecological conditions in these ponds. We recommend that conservation efforts should be directed to promote a more natural urban planning in the immediate surroundings of the ponds. The creation of a green corridor around blue urban space will buffer the adverse effects of urban heat islands and minimize runoff of anthropogenic substances to the ponds. Furthermore, we recommend using a variety of pond types in urban systems, maximize connectivity, and maintain well-developed aquatic vegetation to promote regional biodiversity. In a second part, we aimed to provide a mechanistic understanding to explain the observed community assembly pattern, by looking at species specific co-occurrence patterns, and functional traits. Our study revealed a body size shift of zooplankton communities of larger species being filtered out in urban environments (Chapter 3). Moreover, urban ponds appear to select for generalist species with widespread distributions, which suggests that biotic homogenization is happening in urban pond systems (Chapter 4). The generalist species that inhabit these urban ponds did not show a niche shift compared to natural ponds. Intraspecific body size variability did not seem to contribute much to among-community functional pattern. However, it does play an important role in maintaining local functional diversity within communities (Chapter 5). In general, traitbased metacommunity analyses provided more informative power on community assembly patterns than the analysis based on species identities alone. Moreover, functional traits directly provide information on the physiological response of a species to environmental change. It has been proven that high functional diversity can positively affect ecosystem processes. We therefore argue to incorporate functional traits as they may provide a good tool to understand community responses to environmental change and may offer insights into the potential drivers of these biodiversity patterns.