Direct and indirect ecological consequences of human activities in urban and native ecosystems

Though cities occupy only a small percentage of Earth's terrestrial surface, humans concentrated in urban areas impact ecosystems at local, regional and global scales. I examined the direct and indirect ecological outcomes of human activities on both managed landscapes and protected native ecosystems in and around cities. First, I used highly managed residential yards, which compose nearly half of the heterogeneous urban land area, as a model system to examine the ecological effects of people's management choices and the social drivers of those decisions. I found that a complex set of individual and institutional social characteristics drives people's decisions, which in turn affect ecological structure and function across scales from yards to cities. This work demonstrates the link between individuals' decision-making and ecosystem service provisioning in highly managed urban ecosystems.

Second, I examined the distribution of urban-generated air pollutants and their complex ecological outcomes in protected native ecosystems. Atmospheric carbon dioxide (CO₂), reactive nitrogen (N), and ozone (O₃) are elevated near human activities and act as both resources and stressors to primary producers, but little is known about their co-occurring distribution or combined impacts on ecosystems. I investigated the urban "ecological airshed," including the spatial and temporal extent of N deposition, as well as CO₂ and O₃ concentrations in native preserves in Phoenix, Arizona and the outlying Sonoran Desert. I found elevated concentrations of ecologically relevant pollutants co-occur in both urban and remote native lands at levels that are likely to affect ecosystem structure and function. Finally, I tested the combined effects of CO₂, N, and O₃ on the dominant native and non-native herbaceous desert species in a multi-factor dose-response greenhouse experiment. Under current and predicted future air quality conditions, the non-native species (Schismus arabicus) had net positive growth despite physiological stress under high O₃ concentrations. In contrast, the native species (Pectocarya recurvata) was more sensitive to O₃ and, unlike the non-native species, did not benefit from the protective role of CO₂. These results highlight the vulnerability of native ecosystems to current and future air pollution over the long term. Together, my research provides empirical evidence for future policies addressing multiple stressors in urban managed and native landscapes.