Most approaches for assessing species vulnerability to climate change, including extreme events like drought, have focused on direct impacts via abiotic changes and less so on species interactions. In this study, we examine the consequences of rainfall variability that subsequently impacts the breeding success of a small nocturnal seabird in a cross-ecosystem cascade on a semi-arid island in the Channel Islands National Park. Egg predation by deer mice is the main cause of reduced nest success for the Scripps’s Murrelet (Synthliboramphus scrippsi), but it varies substantially between years. Nest success of birds in a variety of environments can be strongly influenced by the availability of other prey types for predators as a result of predators that consume prey in accordance with the alternative prey hypothesis. Therefore, given the impacts of drought on terrestrial productivity in semi-arid environments, we expected that murrelet nest success would decline during drought years. We examined murrelet nest success over a span of 21 years (1993-2013) along with several indicators of terrestrial and marine productivity. In addition, we determined how changes in nest success, associated with key differences in environmental conditions, influenced population growth rates with a matrix population model.
The best-ranked model explaining variation in murrelet nest success included a drought severity index, marine PC1 scores, and log mouse density. Of these variables, the severity of drought had the strongest impact on murrelet nest success. Mean nest productivity during drought years produced a strongly declining population growth rate when this value was input as the fecundity rate in the matrix population model. During closer to normal rainfall conditions or during wet spells, murrelet nest success was much higher, but how much higher depended on whether marine conditions were highly productive or not. Southern California is part of a region that has been identified as a climate change ‘hotspot’, where some models project the risk of future unprecedented drought conditions. Therefore, our results suggest that the effect of increasing drought on this key vital rate is potentially substantial enough to trigger population declines in a rare nocturnal seabird. These results also have wider implications about the profound impact of drought on species interactions in semi-arid island environments and we suggest that other species affected by rodent predation of nests could be similarly affected.