Direct and indirect interactions between owls, mice and nocturnal seabirds: integrating marine and terrestrial food webs

Climate variability in semi-arid ecosystems can influence species interactions from the bottom-up, and through these perturbations we can gain insight into both direct and indirect interactions in food webs. In this thesis, I studied the effects of ENSO-driven rainfall pulses and drought on the interactions between a top predator, the Barn Owl (Tyto alba), a mesopredator, an island endemic deer mouse (Peromyscus maniculatus elusus), and a threatened nocturnal seabird, the Scripps’s Murrelet (Synthliboramphus scrippsi). On Santa Barbara Island in the Channel Islands National Park in California, adult breeding murrelets are killed by owls, but their eggs are eaten by mice, which is the main cause of reduced murrelet nest success. First, I assessed how owl predation on murrelets varies with the availability of mice, the primary prey of owls. I found that heavy rainfall years drive the irruptions in the mouse population that precede peaks in owl abundance, which results in high murrelet predation by owls when the mouse population subsequently crashes. Next, I examined evidence for positive indirect effects of owls on murrelets through their influence on mouse foraging behavior. I found that mouse foraging was strongly suppressed as the abundance of owls increased, and survival of murrelet eggs was also positively related to owl abundance. I also examined how both the terrestrial and marine environments influenced overall murrelet nest success over a span of 21 years. I found that the severity of drought was the most important variable determining nest success, which suggests that during severe droughts, mice consume substantially more eggs when there are fewer terrestrial resources and also less risk from predation. Climate-driven indirect interactions with predators therefore influences both survival and nest success of murrelets on this island. Finally, I developed a mathematical model of island community dynamics to assess whether owl management might benefit murrelets given projected changes to rainfall patterns in this region. I found no evidence that managing the owl population would enhance murrelet abundance, demonstrating the importance of considering both direct and indirect effects of predators when evaluating potential conservation strategies.