Contrasting responses to changing ocean conditions: A tale of two seabirds

Background/Question/Methods

In the central California Current, annual productivity is primarily determined by the wind-driven upwelling of nutrients. In this bottom-up system, we expect parallel trends in productivity across trophic levels. For this study, we utilized 41 years of data from the Farallon Islands in California, to examine recent contrasting trends in productivity between two seabird species at different trophic levels: the piscivorous Brandt’s Cormorant (Phalacrocorax penicillatus), and the planktovorous Cassin’s Auklet (Ptychoramphus aleuticus). We often assume that a species’ relationship to the environment is stable through time. Here, we test the hypothesis that contrasting productivity trends for these two species are linked to changes in how each species responds to the environment. We used a sliding correlation analysis with a 10-year window to examine how ocean conditions (measured locally and basin-wide) influence productivity of each seabird species and how these relationships change over time.

Results/Conclusions

We found that both species productivity initially responded strongly to El Niño variability. In the last 15 years, the response of Cassin's Auklet productivity to El Niño weakened. At the same time, the correlation between Cassin’s Auklet productivity and the North Pacific Gyre Oscillation (NPGO) increased. This occurred at about the same time that productivity of the two species began to diverge. The amplitude of variability of NPGO has been increasing in recent decades. High variability and novel conditions may have facilitated a shift from a strictly El Niño dominated system to one where NPGO is an increasingly important driver of productivity. Our analysis has shown that the relationships between seabird productivity and physical forcing can change over time and the changes differ by trophic level. Understanding what influences these non-stationary relationships will be critical for population modeling and predicting the consequences of future climate change for marine systems.