The enemy of my enemy: Indirect interactions influence how marine predators become terrestrial prey on an oceanic island
Changes to predator-prey dynamics have been increasingly recognized for their potential role in contributing to species declines. Scripps’s Murrelets (Synthliboramphus scrippsi) are a state-listed Threatened seabird that nests only on islands in Southern California and Northern Mexico. On Santa Barbara Island, California, these small nocturnal seabirds face threats from native predators including the Barn Owl (Tyto alba) as well as an endemic deer mouse (Peromyscus maniculatus elusus) that consumes murrelet eggs. These deer mice undergo extreme variations in density and are also important prey for owls. Therefore, we investigated climate effects on the density of owls and deer mice and predator effects on murrelet adults and their eggs. Specifically, we evaluated evidence for both positive and negative effects on murrelets from sharing a predator with another enemy. First, we confirmed the links between an ENSO-driven rainfall pulse, terrestrial productivity, and the subsequent increase and sharp decline in mouse and owl numbers on the island from 2010 to 2013. Next, we evaluated evidence for prey switching in Barn Owls. Lastly, we also examined whether changes in owl abundance and moonlight influenced mouse foraging activity (by measuring giving up densities) and therefore egg predation rates via a behaviorally mediated cascade.
The number of murrelets consumed by owls each year varied substantially among years, and was consistent with the alternative prey hypothesis. When mice were super abundant in 2011, owl diet consisted of almost entirely deer mice, and owl abundance also increased dramatically in response. After mouse numbers crashed the following year, owls killed a substantially larger proportion of the estimated breeding population of murrelets on the island. We also found evidence of strong positive indirect effects of owls on murrelets. Mice were sensitive to predation risk and foraged less with increasing owl activity and less during the full moon compared to the new moon. Murrelet egg predation was consequently lower with increasing owl activity, and lower during the full moon compared to the new moon. We discuss the implications of a climate-driven impact on a seabird mediated through terrestrial predators, as well as the next steps for evaluating the overall impact of both positive and negative effects of shared predation on murrelets.