Pleistocene to historic shifts in bald eagle diets on the Channel Islands, California

Abstract

Studies of current interactions among species, their prey, and environmental factors are essential for mitigating immediate threats to population viability, but the true range of behavioral and ecological flexibility can be determined only through research on deeper timescales. Ecological data spanning centuries to millennia provide important contextual information for long-term management strategies, especially for species that now are living in relict populations. Here we use a variety of methods to reconstruct bald eagle diets and local abundance of their potential prey on the Channel Islands from the late Pleistocene to the time when the last breeding pairs disappeared from the islands in the mid-20th century. Faunal and isotopic analysis of bald eagles shows that seabirds were important prey for immature/adult eagles for millennia before the eagles' local extirpation. In historic times (A.D. 1850-1950), however, isotopic and faunal data show that breeding bald eagles provisioned their chicks with introduced ungulates (e.g., sheep), which were locally present in high densities. Today, bald eagles are the focus of an extensive conservation program designed to restore a stable breeding population to the Channel Islands, but native and nonnative prey sources that were important for bald eagles in the past are either diminished (e.g., seabirds) or have been eradicated (e.g., introduced ungulates). In the absence of sufficient resources, a growing bald eagle population on the Channel Islands could expand its prey base to include carrion from local pinniped colonies, exert predation pressure on a recovering seabird population, and possibly prey on endangered island foxes.Haliaeetus leucocephalus | historic ecology | stable isotopes

Reply to Hudgens et al.: Bald eagles, no-analog ecological scenarios, and conservation strategies on the Channel Islands

“…Movement data of reintroduced bald eagles show evidence of dispersal from the CI to the mainland, and in some cases, individuals return to the islands after using inland habitats across western North America (3). Such movements likely took place in the past and may have been more common when CI bald eagle densities were significantly higher than today (2). The dispersal capabilities of bald eagles, higher eagle densities in the past, and greater diversity and abundance of megafauna on the mainland support our explanation.…”

“…First, they provide an alternative explanation for the terrestrial isotopic signatures of Pleistocene bald eagles from the CI, suggesting that they regularly fed on pygmy mammoth carrion (1). Although this explanation is plausible, it is not more parsimonious than the one that we provided (2). Movement data of reintroduced bald eagles show evidence of dispersal from the CI to the mainland, and in some cases, individuals return to the islands after using inland habitats across western North America (3).…”

Evolution in coyotes (Canis latrans) in response to the megafaunal extinctions

“…It is important to understand how ecosystems function and change through time, and species interactions are an integral part of any ecosystem (61)(62)(63)(64). Some of the carnivore species that were present at RLB are still extant today (e.g., coyotes, gray wolves, mountain lions, etc.).…”

Dedication. Acknowledgments

“…These methods are not limited to modern systems, and are used to elucidate both large-and small-scale patterns of interaction in mammalian communities with temporal spans ranging from the recent to the Plio-Pleistocene. Reconstructing species interactions in past ecosystems has a rich history [85], and recent investigations have demonstrated that our understanding of the past can facilitate knowledge of modern ecosystems [39,86,126]. Given the scale over which interactions occur, particularly for systems with long-lived organisms, historical and paleontological approaches o↵er windows into nature unavailable to modern ecology.…”