UCLA

Anthropogenic change alters environments and introduces novel stressors to multiple species and communities. Understanding how populations shift and cope in response to these stressors is essential to test evolution in real-time and predict our impacts on natural populations. In urban landscapes, populations that survive undergo rapid evolutionary and behavioral changes in response to strong selective pressures (Chapter 1). I am interested in how these populations respond to multiple stressors and in the patterns and processes underlying adaptive response to strong anthropogenic change. I led a reanalysis of population response data to multiple stressors across species and habitats from the last 25 years of studies by applying a new and generalizable framework—Rescaled Bliss Independence (RBI)—to evaluate stressor interactions (Chapter 2). We found that antagonism and additivity, rather than synergy, are the most frequent interaction types and that our novel method is likely to re-classify previously synergistic interactions as non-synergistic. I applied the RBI to three stressor combinations and found that antagonism was the dominant net interaction type, while synergy was the dominant emergent interaction type (Chapter 3). Next, I delved into one species to assess how populations respond to strong anthropogenic change in the wild. I tested four independent urban colonist populations of Dark-eyed Juncos (Junco hyemalis) to test if the birds have adapted morphological traits similarly in comparison to three non-urban populations or if similar environmental differences can result in different adaptations (Chapter 4). I found that there are complex patterns where some traits converge in some cities, while others are associated with urbanization generally. I quantified behavioral plasticity in response to the change in human activity due to COVID-19 restrictions (“the anthropause”) in one of the urban populations (Chapters 5 and 6). I found that fear response surprisingly decreased following campus re-opening, but didn’t change during the anthropause, posing new hypotheses to the development of a trait essential for urban living. I also found that novel urban nesting did not change during the anthropause, suggesting that novel nesting behavior is driven by landscape or urban predator effects, not human disturbance. This dissertation in sum investigates the patterns and processes underlying population response to novel stressors, with an emphasis on determining the predictability of how organisms respond to the growth of urbanization, a present threat to biodiversity.