Predator–prey dynamics of bald eagles and glaucous‐winged gulls
An unintended consequence of the recovery of bald eagles (Haliaeetus leucocephalus) has been the impact on seabirds. The authors of a 2019 paper published in Ecology and Evolution suggest that the effects of bald eagle activity on a large glaucous-winged gull (Larus glaucescens) colony on Protection Island in the Strait of Juan de Fuca include the possibility of coexistence but also the possibility of gull colony extinction.
Summary
The recovery of bald eagle populations in North America during the latter part of the twentieth century constitutes one of the conservation movement’s great success stories. Nowhere has this recovery been more successful than in the Pacific Northwest. An unintended consequence of the recovery of bald eagle populations, however, has been the impact on populations of seabirds. Protection Island at the east end of the Strait of Juan de Fuca hosts the largest seabird breeding colony of Washington State’s inland waters. The seabirds, including glaucous-winged gulls, serve as favored prey items of bald eagles which both breed and feed on the island. In a predator-prey modeling study, we found that numbers of bald eagle territories in Washington State exhibited a Lotka-Volterra-type relationship with numbers of glaucous-winged gull nests on the island, with a model-to-data fit of R2 = 0.82. Our results suggest that bald eagle activity functions as a primary determinant of the glaucous-winged gull breeding population on the island. Model predictions suggest a reasonable likelihood that bald eagles and glaucous-winged gulls will continue to coexist on the island, but also the possibility that the breeding gull population will be extirpated as a result of bald eagle predation. Managers should monitor numbers of nests in seabird colonies as well as eagle activity within colonies to document trends that may lead to colony extirpations.
