Predator–prey dynamics of bald eagles and glaucous‐winged gulls at Protection Island, Washington, USA

Abstract

Bald eagle ( Haliaeetus leucocephalus ) populations in North America rebounded in the latter part of the twentieth century, the result of tightened protection and outlawing of pesticides such as DDT. An unintended consequence of recovery may be a negative impact on seabirds. During the 1980s, few bald eagles disturbed a large glaucous‐winged gull ( Larus glaucescens ) colony on Protection Island, Washington, USA, in the Salish Sea. Breeding gull numbers in this colony rose nearly 50% during the 1980s and early 1990s. Beginning in the 1990s, a dramatic increase in bald eagle activity ensued within the colony, after which began a significant decline in gull numbers. To examine whether trends in the gull colony could be explained by eagle activity, we fit a Lotka–Volterra‐type predator–prey model to gull nest count data and Washington State eagle territory data collected in most years between 1980 and 2016. Both species were assumed to grow logistically in the absence of the other. The model fits the data with generalized R 2 = 0.82, supporting the hypothesis that gull dynamics were due largely to eagle population dynamics. Point estimates of the model parameters indicated approach to stable coexistence. Within the 95% confidence intervals for the parameters, however, 11.0% of bootstrapped parameter vectors predicted gull colony extinction. Our results suggest that the effects of bald eagle activity on the dynamics of a large gull colony were explained by a predator–prey relationship that included the possibility of coexistence but also the possibility of gull colony extinction. This study serves as a cautionary exploration of the future, not only for gulls on Protection Island, but for other seabirds in the Salish Sea. Managers should monitor numbers of nests in seabird colonies as well as eagle activity within colonies to document trends that may lead to colony extinction.

COVID-19 lockdown reveals tourists as seabird guardians

“…The phasing out of the organochlorine contaminants and lead in shotgun ammunition, protection, and winter feeding have resulted in a recovery of sea eagle populations that can be regarded as one of the most remarkable conservation achievements in human history ( Stier et al, 2016 ). This recovery has in some areas surpassed previous population sizes, and the sea eagles have become a threat to other species, including several seabird populations ( Henson et al, 2019 ; Hipfner et al, 2012 ), adding to the ongoing discussion on new conservation challenges with predator recovery ( Cruz et al, 2019 ; Marshall et al, 2016 ; Stier et al, 2016 ).…”

Every-other-day clutch-initiation synchrony as an adaptive response to egg cannibalism in Glaucous-winged Gulls (Larus glaucescens)

“…At a superficial level this suggests that the trends observed for ENSO fluctuations during 2006-2014 might be extrapolated to account for the effects of long-term warming. While this may be true, it is important to also note that Bald Eagle numbers have increased in the region, and that they now frequently prey on seabird eggs, chicks, and adults, given that populations of their more usual prey species have experienced dramatic declines (White et al 2006, Cowles et al 2012, Hipfner et al 2012, Henson et al 2019. Thus, earlier Mandarte Island egg-loss numbers may be related in part to less eagle predation.…”

“…In the case of Protection Island gulls, an increase in the fitness of cannibalistic individuals could play a role in colony extirpation, although extirpation would most likely occur by some individuals shifting reproductive activities to other colonies. A possible decline and eventual extirpation of the Protection Island gull colony in response to egg cannibalism, however, would be compounded by the direct effect of eagle predation on gull eggs, chicks, and adults (Hayward et al 2010, Hipfner et al 2012, Henson et al 2019, and by a reduction in the availability of nest habitat (Cowles et al 2012).…”

Century‐long stomatal density record of the nitrophyte, Rubus spectabilis L., from the Pacific Northwest indicates no effect of changing atmospheric carbon dioxide but a strong response to nutrient subsidy

“…There are several possibilities, including factors, not considered here. For example, the recently increased presence of Bald Eagles Haliaeetus leucocephalus has had strong impacts on seabirds on Triangle Island and elsewhere (Henson et al, 2019 ; Hipfner et al, 2012 ) that could propagate to other trophic levels. Another possibility is that the loss of nearby Lanz and Cox Islands as suitable breeding colonies (due to the introduction more than a century ago of the American Mink Neovison vison and the Raccoon Procyon lotor as furbearers; Hipfner et al, 2010 ) has so increased the nesting density on Triangle Island that nutrient effects are now especially strong.…”

“…There are several possibilities, including factors, not considered here. For example, the recently increased presence of Bald Eagles Haliaeetus leucocephalus has had strong impacts on seabirds on Triangle Island and elsewhere (Henson et al, 2019;Hipfner et al, 2012)…”