 |
|
Decline in Alaskan sea otters affects bald eagles' diet
October 03, 2008Eagles shift their diet from aquatic marine animals to birds
Sea otters are known as a keystone species, filling such an important niche in ocean communities that without them, entire ecosystems can collapse. Scientists are finding, however, that sea otters can have even farther-reaching effects that extend to terrestrial communities and alter the behavior of another top predator: the bald eagle.
In nearshore marine communities, towering kelp can reach heights of 250 feet and function much like trees in a forest, providing food, homes and protection for fish and invertebrates. The most important enemies of these giant algae are tiny sea urchins, only inches in diameter, which live on the kelp's holdfasts and eat its tissue. When urchin populations become too large, they can defoliate entire kelp forests, leaving only barren remains.
Enter the sea otter. Otters can eat the spiky urchins whole, making them the major urchin predator. The otters' presence keeps urchin populations in check and maintains the balance of the ecosystem.
Scientists have known about these kelp forest community interactions since the 1970s. But in the October issue of the journal Ecology, Robert Anthony and colleagues report that the presence or absence of otters can also affect the diet of bald eagles, a neighboring terrestrial predator. Anthony is an ecologist with the Oregon Cooperative Fish & Wildlife Research Unit of the U.S. Geological Survey and Oregon State University.
Bald eagles live in high densities along the Aleutian archipelago off the coast of Alaska and place their nests on islets, coastal cliffs and shoreline sea stacks. Historically, more than 90 percent of the eagles' food comes from the ocean. Sea otters once also occupied a large range of coastal marine environments near these islands, but in recent years, otter populations have declined in response to their own main predator.
"All of the available data point to increased numbers of killer whales as the direct cause of the sea otter decline in southwest Alaska," says coauthor Jim Estes of the U.S.G.S. and the University of California at Santa Cruz. "The otter decline has caused a phase shift in the coastal ecosystem from a kelp dominated phase state to a deforested phase state."
This shift means many fewer kelp forest fish for the eagles to eat. In response, the eagles have adjusted their foraging tactics. Anthony and his colleagues surveyed remains of bald eagle prey in their nests during 1993 and 1994, when otters were abundant and the kelp forests were healthy, and in 2000, 2001 and 2002, when otters were scarce and the kelp forests had collapsed. They found that when otters were abundant, eagle prey consisted of predominantly kelp-forest fish and sea otter pups. When the otters were rare, however, the proportion of marine birds in the eagles' diet was much higher.
Anthony explains that because the eagles defend territories in dense patches along the coastline and there are few terrestrial animals to eat, they must be flexible in what they hunt.
"These bald eagles are opportunistic foragers as a consequence of their evolutionary history," he says. "They've developed foraging territories they defend against members of the same species along these coastlines, and the terrestrial environment provides very little for them. So they forage over the open water."
Anthony and his colleagues also found that the eagles had more young on average during 2000-2002, a fact that Anthony believes might be a result of a high caloric content in the eagles' increasingly seabird-dominated diet.
"Across the range of this species, their diet can be quite varied, but here it appears as though the change in diet had either a neutral or positive effect," he says. The propensity of the eagles to adapt quickly to a changing environment may have allowed them to flourish, but Anthony also cautions that adapting to this scenario might be difficult for more specialized predators.
The results are the first to show that the presence or absence of otters influences a terrestrial animal, and that the complex food web linkages can reach as far as five different food chain levels: from sea otters to sea urchins, kelp, marine fish and finally bald eagles.
"Top-down linkages can be very distant from their origin," says Anthony. "The effects of top predators can ripple throughout the ecosystem in ways we're just beginning to understand."
Ecological Society of America
Enter the sea otter. Otters can eat the spiky urchins whole, making them the major urchin predator. The otters' presence keeps urchin populations in check and maintains the balance of the ecosystem.
Scientists have known about these kelp forest community interactions since the 1970s. But in the October issue of the journal Ecology, Robert Anthony and colleagues report that the presence or absence of otters can also affect the diet of bald eagles, a neighboring terrestrial predator. Anthony is an ecologist with the Oregon Cooperative Fish & Wildlife Research Unit of the U.S. Geological Survey and Oregon State University.
Bald eagles live in high densities along the Aleutian archipelago off the coast of Alaska and place their nests on islets, coastal cliffs and shoreline sea stacks. Historically, more than 90 percent of the eagles' food comes from the ocean. Sea otters once also occupied a large range of coastal marine environments near these islands, but in recent years, otter populations have declined in response to their own main predator.
"All of the available data point to increased numbers of killer whales as the direct cause of the sea otter decline in southwest Alaska," says coauthor Jim Estes of the U.S.G.S. and the University of California at Santa Cruz. "The otter decline has caused a phase shift in the coastal ecosystem from a kelp dominated phase state to a deforested phase state."
This shift means many fewer kelp forest fish for the eagles to eat. In response, the eagles have adjusted their foraging tactics. Anthony and his colleagues surveyed remains of bald eagle prey in their nests during 1993 and 1994, when otters were abundant and the kelp forests were healthy, and in 2000, 2001 and 2002, when otters were scarce and the kelp forests had collapsed. They found that when otters were abundant, eagle prey consisted of predominantly kelp-forest fish and sea otter pups. When the otters were rare, however, the proportion of marine birds in the eagles' diet was much higher.
Anthony explains that because the eagles defend territories in dense patches along the coastline and there are few terrestrial animals to eat, they must be flexible in what they hunt.
"These bald eagles are opportunistic foragers as a consequence of their evolutionary history," he says. "They've developed foraging territories they defend against members of the same species along these coastlines, and the terrestrial environment provides very little for them. So they forage over the open water."
Anthony and his colleagues also found that the eagles had more young on average during 2000-2002, a fact that Anthony believes might be a result of a high caloric content in the eagles' increasingly seabird-dominated diet.
"Across the range of this species, their diet can be quite varied, but here it appears as though the change in diet had either a neutral or positive effect," he says. The propensity of the eagles to adapt quickly to a changing environment may have allowed them to flourish, but Anthony also cautions that adapting to this scenario might be difficult for more specialized predators.
The results are the first to show that the presence or absence of otters influences a terrestrial animal, and that the complex food web linkages can reach as far as five different food chain levels: from sea otters to sea urchins, kelp, marine fish and finally bald eagles.
"Top-down linkages can be very distant from their origin," says Anthony. "The effects of top predators can ripple throughout the ecosystem in ways we're just beginning to understand."
Ecological Society of America
Shift in bald eagle diet linked to sea otter decline
A newly published study has found that the decline of sea otters along Alaska's Aleutian Islands has forced a change in the diet of a terrestrial predator - the bald eagle. The study demonstrates the extraordinary complexity of marine ecosystems and how far-ranging the impacts can be when there is a population shift in a keystone species like the sea otter.
Why conservation efforts often fail
Modern conservation techniques have brought us the resurgence of American bald eagles, sustainable forest harvests and the rescue of prized lobster fisheries.
Early California: A killing field
When explorers and pioneers visited California in the 1700s and early 1800s, they were astonished by the abundance of birds, elk, deer, marine mammals, and other wildlife they encountered.
Gray wolves feed the masses while hunters feed the few
Gray wolves (Canis lupus) and human hunters both provide resource subsidies to scavengers in Yellowstone National Park, USA, by provisioning them with the remains of their kills. Carrion from wolf versus hunter kills is much more dispersed in both time and space. In the November issue of Ecology Letters, scientists from Berkeley and Yellowstone estimated the total amount of carrion consumed over a four year period by scavengers at both wolf and hunter kills. Highly mobile individuals such as bald eagles and ravens dominated consumption at hunter kills while competitively dominant but less mobile coyotes dominated at wolf kills. More importantly, a great diversity of species were found at wo
More Bald Eagles Current Events and Bald Eagles News Articles
 | The Bald Eagle: Haunts and Habits of a Wilderness Monarch by Jon M. Gerrard, Gary R. Bortolotti Describes the morphology, behavior, flight patterns, hunting, migration, nesting, development, and growth of bald... |
 | Majestic Eagles: Compelling Facts and Images of the Bald Eagle by Stan Tekiela Take a closer look at the most powerful birds of the sky with this picture guide--complete with details about eagles' lives, behaviors and habitats. Award-winning nature photographer Stan Tekiela has captured the essence of what makes the eagle a perfect choice as our national... |
 | The Bald Eagle Returns by Dorothy Hinshaw Patent The inspiring story of the bald eagle"s resurgence is the good news in this follow-up to WHERE THE BALD EAGLES GATHER, published by Clarion in 1984. Full-color photographs and an updated text describe, in a handsome, newly designed format, the significant changes in the habitat and status of our national emblem. Young naturalists are sure to be encouraged by this account of a remarkably... |
 | The Illustrated Bald Eagle: The Ultimate Reference Guide for Bird Lovers, Woodcarvers, and Artists (The Denny Rogers Visual Reference series) by Denny Rogers Written with the artist or sculptor in mind, this painstakingly detailed reference to the bald eagle covers the anatomy of the bird down to a feather. Exact anatomical sketches—done in multiple dimensions, from various angles, and in many different postures—provide a visual study unmatched by other references. Some drawings focus specifically on one part of the bird, such as a single contour... |
 | The Bald Eagle (True Books) by Elaine Landau |
| Bald Eagle Poster (Posters) by Dover | |
 | The Bald Eagle (Welcome Books) by Lloyd G. Douglas Emergent readers will discover the history and heritage of American symbos, such as the U.S. flag and the bald eagle, in this patriotic series. The text introduces American symbols and explains why they represent... |
 | Just Eagles by Alan E. Hutchinson From early Native Americans to schoolchildren today, bald eagles hold a special place of reverence in our hearts and minds, spanning time and place. The eagle is a symbol of cultures, of a nation and of freedom. It is a symbol also of our changing view of natural resources and wild places: a change from a view of endless abundance with no limits to a growing understanding of the limits of ... |
 | Bald Eagles: Their Life and Behavior in North America by Art Wolfe Bald EaglesTheir life and behavior in North AmericaPhotographs by Art WolfeText by Donald F. BruningThe bald eagle is America's national bird. Protected by the Endangered Species Act of 1973, this bird had made a steady recovery from near extinction. It is fitting for our heritage that Art Wolfe has devoted his photographic talents to capturing bald eagles on film. His large color photographs... |
 | Bald Eagle by Gordon Morrison In this informative portrait, acclaimed nature artist Gordon Morrison takes us deep into the wilderness for a close up look at a family of American bald eagles. Detailed color illustrations and readable text follow the developmental stages of two baby eagles as they hatch and grow plumage, learn to Xy with the help of the adult eagles, and struggle to fish and hunt on their own. Accompanying... |
| © 2009 BrightSurf.com |