Stable isotopes reveal strong marine and El Nino effects on island food webs

Stable isotope analysis is a powerful tool for unravelling the complex structure of food webs(1-3). This technique is particularly well suited for studies at ecosystem boundaries, where physical processes and mobile consumers link the dynamics of seemingly disparate systems(4-6). In coastal and insular environments, seabirds play a crucial role in transporting marine-based energy and nutrients to islands(7-9). Here we show using stable isotopes that nutrients from the ocean drive the dynamics of terrestrial food webs on small islands. The indirect effects of seabird-derived nutrients on plant productivity are particularly prominent during wet El Nino Southern Oscillation years on our Gulf of California study sites. During dry years that characterize the region, many terrestrial consumers are subsidized by carrion and prey from the ocean. Shifts in trophic structure related to El Nino Southern Oscillation could only be elucidated because of the distinct nitrogen isotope ratios associated with seabird islands. The contributions of seabirds and other marine sources are reflected in the isotope signatures of terrestrial consumers in ways that challenge conventional interpretations of stable isotope results in studies of food webs.