Managing individual nests promotes population recovery of a top predator

Abstract

Threatened species are managed using diverse conservation tactics implemented at multiple scales ranging from protecting individuals, to populations, to entire species. Individual protection strives to promote recovery at the population‐ or species‐level, although this is seldom evaluated. After decades of widespread declines, bald eagles, Haliaeetus leucocephalus, are recovering throughout their range due to legal protection and pesticide bans. However, like other raptors, their recovery remains threatened by human activities. Bald eagle nests are commonly managed using buffer zones to minimize human disturbance, but the benefits of this practice remain unquantified. Within Voyageurs National Park (VNP), Minnesota, USA, managers have monitored bald eagle populations for over 40 years, and since 1991, have protected at‐risk nests from human disturbance using buffer zones (200 and 400 m radius). We aimed to (1) quantify the recovery of bald eagles in VNP (1973–2016), and (2) provide a first‐ever evaluation of the individual‐ and population‐level effects of managing individual nests. To do so, we developed Bayesian Integrated Population Models combining observations of nest occupancy and reproductive output (metrics commonly collected for raptors) to estimate nest‐level probabilities of occupancy, nest success, and high productivity (producing ≥2 nestlings), as well as population‐level estimates of abundance and growth. The breeding population of bald eagles at VNP increased steadily from <10 pairs in the late 1970s to 48 pairs by 2016. At the nest‐level, management significantly improved occupancy and success. At the population‐level, management led to 8% and 13% increases in nest success and productivity rates, respectively, resulting in a 37% increase in breeding pair abundance. Synthesis and applications. There is a clear need to evaluate how management approaches at multiple scales assist in species recovery. Our study uses an Integrated Population Model to reveal the population‐level benefits of a widely used, individual‐based management action (protecting nests using buffer zones) on a recovering raptor.

Nest use dynamics of an undisturbed population of bald eagles

“…Our observed estimate of chick productivity <1 per used nest, and stable occupancy dynamics are in line with populations elsewhere in Alaska (e.g., Steidl, Kozie, & Anthony, 1997;Zwiefelhofer, 2007) where density dependence may play a larger role (Elliott, Elliott, Wilson, Jones, & Stenerson, 2011). Our results strongly suggest that some mandated productivity guidelines may be unrealistic for evaluating successful recovery (Cruz et al, 2018), especially when there is consistent evidence that fecundity is density dependent in eagles (Elliott et al, 2011;Ferrer & Donazar, 1996;Mougeot et al, 2013).…”

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

“…At Voyageurs National Park (hereafter Voyageurs) in northern Minnesota, USA, osprey, Pandion haliaetus, and great blue heron, Ardea herodias, (hereafter heron) populations increased in the early 1970s followed by increases in bald eagle, Haliaeetus leucocephalus, populations in the late 1980s ( Figure 1). Bald eagles (hereafter eagles) also commonly nest near areas of high human use, so their vulnerable nests are protected via buffer zones, which benefits their recovery to this day (Cruz et al, 2018). Bald eagles (hereafter eagles) also commonly nest near areas of high human use, so their vulnerable nests are protected via buffer zones, which benefits their recovery to this day (Cruz et al, 2018).…”

“…In response to pressure from bald eagles, ospreys may, in turn, directly compete with herons for limited nesting habitat, causing heronry abandonment (Parker, 1980). Bald eagles are a flagship species for conservation, and at Voyageurs, they are the only species to benefit from nest site protection via buffer zones meant to minimize human disturbance and augment their reproductive success and population recovery (Cruz et al, 2018). We hypothesized that bottom-up and top-down factors influenced recovery dynamics through their effects on nest colonization, persistence (i.e., nest reuse) and success for ospreys, and occurrence and size of heronries.…”

“…The rise of all three species in this national park mirrored trends across North America and was hailed as the beginning of successful, multi-species recovery. Bald eagles (hereafter eagles) also commonly nest near areas of high human use, so their vulnerable nests are protected via buffer zones, which benefits their recovery to this day (Cruz et al, 2018). The early success of osprey and herons, however, was short-lived at Voyageurs, and both species declined again in the early 1990s ( Figure 1).…”

“…Specifically, we predicted that adverse weather including later ice-out dates, colder temperatures and increased rainfall, reduced quality of nesting habitats from ageing beaver ponds, declining fish stock, and smaller foraging habitats, all limited nesting ospreys and herons ( Figure 2). Bald eagles are a flagship species for conservation, and at Voyageurs, they are the only species to benefit from nest site protection via buffer zones meant to minimize human disturbance and augment their reproductive success and population recovery (Cruz et al, 2018). Consequently, we predicted that rising eagle numbers, higher local densities of nesting eagles, closer proximity to eagle nests and greater overlap with their foraging habitats hindered the recovery of ospreys and herons through intensified intraguild predation and interference competition for food and nesting sites ( Figure 2).…”