Long-term avian responses to drought and urbanization in Northern California

Birds support agricultural systems, promote ecotourism, and engender empathy for nature. However, North America has lost three billion birds since 1970 due to many anthropogenic threats, including climate and land-use change. Nationwide annual surveys often lack the resolution to accurately assess many local population trends or determine the effect of global change across the annual cycle for targeted conservation action. This thesis leverages long-term community science data, previously published literature, and novel field surveys to assess changes to Northern California bird communities over the last three decades. First, I analyzed 30 years of bird transect data recorded along representative trails of Stanford University’s Jasper Ridge Biological Preserve to determine the population trends for a sample of 67 species. Of these, 21 species demonstrated declines, including some West Coast and California endemic species. Second, I used the same transect dataset to investigate how climate change-induced drought over different time scales affected bird occurrence. Of 98 species evaluated, 50% of them were impacted by drought either positively or negatively over 1, 2, 3, 4, or 5 years, indicating that climate change may cause rapid bird community turnover. Furthermore, this research uncovered that avian responses to climate change may additionally vary depending on local land use. Lastly, to determine how land use modulates long-term changes to local bird communities, I resurveyed 95 sites along an urbanization gradient that were originally surveyed in 1992. To my knowledge, this is the first study to examine long-term bird population changes along an urbanization gradient. This approach highlighted the interactive effects between land use and other global change drivers, including climate change. Out of 73 resurveyed species, 45% experienced different population trends under varying levels of urbanization. Urban areas experienced more extreme population trends than those observed at the Jasper Ridge Biological Preserve (the least urbanized site on the gradient). For example, some native songbirds became more abundant in urban areas. At the same time, other species, like the Mourning Dove, declined across the study area, but experienced slower declines within Jasper Ridge. While preserves now cradle the last populations of many species that have disappeared from suburbia, protected land only slows the continued decline of many species. Despite the multifaceted analysis presented here, it is difficult to ascertain whether population declines observed at Jasper Ridge are due to changes happening within the reserve (e.g., changes in climate or populations of exotic species) or changes happening in the broader landscape (e.g., habitat fragmentation, edge effects, or urbanization). For example, a species may be extirpated from a small, isolated nature preserve if its fluctuating population lacks sufficient connectivity to its metapopulation (the network of populations within a mosaic landscape). Since many species still prefer less-developed areas, bird conservation planning should implement further gardens and greenspaces to support birds’ continued incursion and likely adaptation to urban spaces. Finally, given that historically marginalized communities have been redlined into areas with limited green spaces, they have likely lost some bird species faster than privileged communities – an issue of environmental justice that requires serious consideration.

Keywords:  Department of Biology
Biology
Ecology
Birds
Bird populations
Birds > Conservation
Science
Climatic changes
Droughts
Urbanization
Land use, Urban
Land use
California
Biodiversity conservation
Nature conservation
Wildlife conservation
California > San Francisco Bay Area
California > Jasper Ridge Biological Preserve
Text
Thesis
Advisor: Mordecai Erin
McFadden Tyler
Dirzo Rodolfo