Abstract:

The urban homogenization hypothesis has suggested that human development causes urban ecosystems to become more like other geographically distinct urban areas in comparison to rural ecosystems in the local landscapes. This effect has been observed in lotic ecosystems as described under urban stream syndrome. However, evidence of homogenization has not yet been so well observed in lentic ecosystems.  Ponds are vital for both providing freshwater habitat and maintaining landscape connectivity, especially in more developed landscapes. This thesis aims to find evidence of urban homogenization of ponds by looking at salt, one of the most common pollutants in urban freshwater systems, and its impacts on amphibians at multiple scales. Firstly, we are looking for landscape level patterns of homogenization across space and time in amphibian communities linked to water pollution and urban development in the USA using large open governmental datasets. Secondly, we are using a system of experimental open mesocosm ponds to investigate how salt affects habitat choice and larval development, allowing us to test behaviour when isolation is not a constraint. Thirdly, we plan to use a second set of mesocosms to more thoroughly investigate the mechanism of a potential ecological trap by comparing the impact of timing of salinity spikes on American toad larval mortality and development. To date, we have seen significant impacts to breeding behaviour, with pond dwelling frogs (Lithobates) avoiding breeding in salted ponds and American toads (Anaxyrus americanus) showing a preference for elevated salinity. Elevated salinity has also correlated with elevated larval mortality for American toads, indicating the potential for polluted freshwater ponds to act as an ecological trap for that species.