Predator‐mediated effects of severe drought associated with poor reproductive success of a seabird in a cross‐ecosystem cascade

Despite the profound impacts of drought on terrestrial productivity in coastal arid ecosystems, only a few studies have addressed how drought can influence ecological cascades across ecosystem boundaries. In this study, we examine the consequences of rainfall pulses and drought that subsequently impact the breeding success of a threatened nocturnal seabird, the Scripps's Murrelet (Synthliboramphus scrippsi). On an island off the coast of southern California, the main cause of reduced nest success for one of their largest breeding colonies is egg predation by an endemic deer mouse (Peromyscus maniculatus elusus). Mice on the island have an opportunistic diet of primarily terrestrial sources, but drastic declines in terrestrial productivity from drought might be expected to increase their reliance on marine resources, including murrelet eggs. We compiled data on terrestrial and marine productivity between 1983 and 2013 to determine how conditions in these ecosystems influence murrelet nest success. We found that the severity of drought had the strongest negative impact on murrelet nest success. We calculated that the reduction in fecundity during drought years due to increased egg predation by mice was substantial enough to produce a declining population growth rate. Nest success was much higher under normal or high rainfall conditions, depending on whether oceanic conditions were favorable to murrelets. Therefore, the more frequent and severe drought that is projected for this region could lead to an increased risk of murrelet population decline on this island. Our study highlights the need for understanding how species interactions will change through the effects of increasing drought and altered rainfall regimes under global change. Seabirds are often heralded as sentinels of ocean climate variability and change. In this study, however, we found that Scripps's Murrelet nest success since the 1980s in southern California was mainly associated with a different hallmark of climate variability, the severity of drought. Specifically, our results support the hypothesis that climate‐driven changes in terrestrial productivity influence murrelet nest success via increased egg predation by an endemic deer mouse during severe drought conditions. Nest success was much higher under normal or high rainfall conditions and was highest when oceanic conditions were favorable to murrelets.