Top‐down effects of repatriating bald eagles hinder jointly recovering competitors

Abstract

The recovery of piscivorous birds around the world is touted as one of the great conservation successes of the 21st century, but for some species, this success was short‐lived. Bald eagles, ospreys and great blue herons began repatriating Voyageurs National Park, USA, in the mid‐20th century. However, after 1990, only eagles continued their recovery, while osprey and heron recovery failed for unknown reasons. We aimed to evaluate whether top‐down effects of bald eagles and bottom‐up effects of inclement weather, habitat quality and fish resources contributed to the failed recovery of ospreys and herons in a protected area. We quantified the relative influence of top‐down and bottom‐up factors on nest colonization, persistence (i.e., nest reuse) and success for ospreys, and occurrence and size of heronries using 26 years (1986–2012) of spatially explicit monitoring data coupled with multi‐response hierarchical models and Bayesian variable selection approaches. Bald eagles were previously shown to recover faster due to intensive nest protection and management. Increased numbers of eagles were associated with a reduction in the numbers of osprey nests, their nesting success and heronry size, while higher local densities of nesting eagles deterred heronries nearby. We found little evidence of bottom‐up limitations on the failed recovery of herons and ospreys. We present a conservation conundrum: bald eagles are top predators and a flagship species of conservation that have benefited from intensive protection, but this likely hindered the recovery of ospreys and herons. Returning top predators, or rewilding, is widely promoted as a conservation strategy for top‐down ecosystem recovery, but managing top predators in isolation of jointly recovering species can halt or reverse ecosystem recovery. Previous studies warn of the potential consequences of ignoring biotic interactions amongst recovering species, but we go further by quantifying how these interactions contributed to failed recoveries via impacts on the nesting demography of jointly recovering species. Multi‐species management is paramount to realizing the ecosystem benefits of top predator recovery.

Transient dynamics during kelp forest recovery from fishing across multiple trophic levels

“…For example, a growing population of bald eagles hindered population growth of ospreys and herons via reduced nesting success (Cruz et al. 2019), and protected white sharks may be contributing to slowed recovery of sea otters due to naïve juvenile sharks mis‐targeting sea otters as a prey item (Moxley et al. 2019).…”

“…Nevertheless, examples of jointly recovering species offer relatively few generalities for managers beyond the need to recognize trade-offs as predator populations recover. For example, a growing population of bald eagles hindered population growth of ospreys and herons via reduced nesting success (Cruz et al 2019), and protected white sharks may be contributing to slowed recovery of sea otters due to naı ¨ve juvenile sharks mis-targeting sea otters as a prey item (Moxley et al 2019). Similarly, accounting for species interactions is integral to an ecosystem-based approach to fisheries management (EBFM; Larkin 1996).…”

Similar species, different fates: Abundance dynamics in spatially structured populations of common and threatened frogs

“…The sharp differences between the two species suggest that the substitute species approach can be misleading in some cases. The use of multispecies models to assess community‐level responses to main ecological drivers is growing in recent years (Cruz et al, 2019; Ferreira & Beja, 2013; McColl‐Gausden et al, 2021; van Strien et al, 2019). These models are extremely powerful to identify the factors that determine the distribution and dynamics of the majority of species, sometimes assuming correlated responses between them.…”

“…The sharp differences between the two species suggest that the substitute species approach can be misleading in some cases. The use of multispecies models to assess community-level responses to main ecological drivers is growing in recent years (Cruz et al, 2019;Ferreira & Beja, 2013;McColl-Gausden et al, 2021;van Strien et al, 2019).…”

Shared functional traits explain synchronous changes in long‐term count trends of migratory raptors

“…These conflicting trend estimates might suggest that inland osprey population levels are regulated by environmental factors that we did not measure (Farmer et al, 2008). In fact, there is good evidence that growing populations of bald eagles can limit recovery of osprey populations (Cruz et al., 2019). In a similar manner, predation by Cooper's hawk might contribute to the decreasing trends in counts of American kestrels (Farmer et al, 2008).…”