UC Santa Cruz

Generalist predators can alter or limit the survival of individuals and the dynamics of populations, especially for imperiled prey. The effects of predation can be further exacerbated if anthropogenic subsidies provide novel food sources to predators that decouple predator-prey dynamics and contribute to extinction risk of native prey. However, quantifying predation rates on rare prey is challenging; as is identifying shifts in predator diet and resource use over time. For my research, I developed new approaches and integrated existing methods to quantify predation and evaluate the impact of predation risk of a generalist avian predator (Western Gull, Larus occidentalis) on threatened populations of steelhead (Oncorhynchus mykiss) in central California (USA), and document temporal shifts in anthropogenic resource use by gulls and the effect on predation risk for steelhead.

This dissertation focuses on three key components to disentangle the relationship between generalist gull predators, threatened steelhead prey, and the role of anthropogenic subsidies in impacting predator-prey dynamics. In Chapter 1, I quantified predation by Western Gulls on imperiled steelhead populations through a novel Bayesian integration of fish tag recoveries, an experiment to quantify tag transportation rates, and monitoring to quantify tag loss. Together, these analyses provide the most robust estimates of avian predation on these imperiled fishes to date, and I found that predation rates are extremely high (median probability of per capita predation ≥ 0.306 per year). In Chapter 2, I compared individual traits of juvenile steelhead that influence survival to maturity and predation risk by generalist gulls through analyses of over nine years of PIT (passive integrated transponder) tag recapture data. I discovered that large and wild-origin juvenile steelhead were more than 20 times more likely to survive the marine ecosystem than small hatchery individuals, and gull predators preferably consumed wild-origin steelhead, regardless of size class. Based on these data, I estimated that gull predation provides an additive source of mortality and reduces the total population of returning adults by approximately 24%. In Chapter 3, I used stable isotope analysis of δ¹³C and δ¹⁵N to conduct a 100-year retrospective analysis to identify changes in resource use by Western Gulls and implications for threatened steelhead populations. Results from this study suggest gulls have become increasingly reliant on anthropogenic food sources which likely contribute to wider gull niche width and increased predation risk to native steelhead.

Together, these dissertation chapters demonstrate that even opportunistic predation from a generalist predator (i.e., Western Gulls) can provide a significant source of additive mortality to imperiled prey (i.e., juvenile steelhead). Furthermore, my research highlights the role of humans in contributing to the degradation of coastal ecosystems. By providing new food sources from landfills, humans indirectly enhance abundance of opportunistic generalist predators such as gulls and contribute to steelhead population decline.