P-27 Predator-Prey Dynamics of Eagles and Gulls at Protection Island, Washington

Authors: Shandelle M. Henson, Robert A. Desharnais, Eric T. Funasaki, Joseph G. Galusha, James W. Watson, and James L. Hayward. Abstract: Bald Eagle populations in North America recovered in the latter part of the twentieth century in response to the outlawing of pesticides such as DDT and increased protection. An unexpected consequence of this recovery, however, was a negative impact on seabirds which serve as prey items for Bald Eagles. During the 1980s, few Bald Eagles visited a large Glaucous‐winged Gull colony on Protection Island, Salish Sea, Washington, and breeding gull numbers in this colony rose rapidly during the late 1980s and early 1990s. A noticeable increase in Bald Eagle activity ensued in the 1990s, followed by a large decline in numbers of nesting gulls. We set out to determine if trends in the gull colony could be explained by eagle activity. To accomplish this, we fit a Lotka–Volterra‐type predator–prey model to gull nest count data and Washington State eagle territory data collected during most years between 1980 and 2016. Our model fit the data with a generalized R2 = 0.82. This result supports the hypothesis that gull dynamics and eagle population dynamics are linked. Although point estimates of the model parameters indicated a high possibility of stable coexistence, within the 95% confidence intervals for the parameters, 11.0% of bootstrapped parameter vectors predicted gull colony extinction. These results suggest that the impact of Bald Eagle activity on the dynamics of this gull colony could be explained by a predator–prey relationship that included the possibility of coexistence, but also included the possibility of gull colony extinction. Key words: Bald Eagles, Glaucous-winged Gulls, predator-prey dynamics, Lotka-Volterra mode