Animal Migration: An Endangered Phenomenon?

DAVID S. WILCOVE

Animal Migration An Endangered Phenomenon?

Timely international action can sustain this inspiring natural process before it becomes a crisis.

Animal migrations are among the world’s most visible and inspiring natural phenomena. Whether it’s a farmer in Nebraska who stops his tractor on a cold March morning to watch a flock of sandhill cranes passing overhead or a Maasai pastoralist who climbs a hill in southern Kenya and gazes down on a quarter million wildebeest marching across the savanna, migration touches the lives of most people in one form or another. Although animal migration may be a ubiquitous phenomenon, it is also an increasingly endangered one. In virtually every corner of the globe, migratory animals face a growing array of threats, including habitat destruction, overexploitation, disease, and global climate change. Saving the great migrations will be one of the most difficult conservation challenges of the 21st century. But if we fail to do so, we will pay a heavy price—aesthetically, ecologically, and even economically.

The decline of migratory species is by no means a new problem. North America’s two greatest migratory phenomena—the flocks of passenger pigeons that literally darkened the skies during their spring and fall journeys in the East and the herds of bison that once stretched from horizon to horizon on the Great Plains—were snuffed out well over a century ago. (The passenger pigeon vanished completely in 1914; bison held on only because of last-minute conservation efforts.) Even as far back as the American Revolution, colonial leaders were alarmed enough about declines in Atlantic salmon to push legislation banning the practice of placing nets across the complete span of a river in order to catch every salmon heading upstream to spawn.

Yet the rate at which migratory species are declining seems to have accelerated in recent years. Ornithologists using radar to monitor the spring migration of songbirds across the Gulf of Mexico report that the number of nightly flights dropped by nearly 50% between 1963 and 1989. University of Montana ecologist Joel Berger has estimated that 58% of the elk migratory routes and 78% of the pronghorn routes in the Greater Yellowstone Ecosystem have been lost due to development. The American Fisheries Society has tallied more than 100 stocks of salmon in the Pacific North-west that have been driven to extinction because of dam construction, logging, water diversion, and other human activities. Meanwhile, in Michoacán, Mexico, illegal loggers are destroying the high-elevation fir forests where virtually all of eastern North America’s monarch butterflies spend the winter. These diminishing forests serve as a blanket for the overwintering monarchs, protecting them from cold weather, rain, and even snow.

North America is hardly the only place where migratory animals are in trouble. European scientists are deeply concerned that overgrazing and desertification in Africa’s Sahel are harming populations of songbirds that breed in Europe and winter in northern Africa. (These same birds are also shot and trapped by the tens of millions as they pass through the Mediterranean region during their spring and fall migrations.) In East Africa, the spread of agriculture is severing the migratory routes of many populations of zebra, wilde-beest, elephants, and other large mammals. In Finland, wild Atlantic salmon have disappeared from more than 90% of the rivers where they spawned historically; in France, they have vanished from nearly a third of their historic spawning rivers and are endangered in the remaining two-thirds.

To be fair, most of these species are in little danger of disappearing altogether. Few if any scientists are predicting the extinction of the wildebeest, Atlantic salmon, or monarch butterfly. But what is at stake is the continued abundance of these animals as they make their long-distance journeys through an increasingly human-dominated landscape.

Special vulnerabilities

The threats facing migratory species are not qualitatively different from those confronting nonmigratory species. But migratory animals seem especially vulnerable by virtue of the long distances they travel. Their populations can be harmed not only by the loss of breeding habitat but also by changes in their wintering grounds and stopover sites. The cerulean warbler, for example, nests in deciduous forests across a wide swath of eastern North America, from southern New England and southern Ontario west to Minnesota and south to Arkansas and Mississippi. It winters primarily in forests in the foothills of the eastern slope of the Andes, from Venezuela to Peru. By some estimates, the breeding population of cerulean warblers in North America has declined by as much as 80% during the past 40 years. This decline, evident to birdwatchers in the United States and Canada, probably reflects habitat destruction at both ends of the
warbler’s migratory route. Mountaintop-removal mining, an extraordinarily destructive practice in which the tops of mountains are scraped away to expose coal seems, has already destroyed hundreds of thousands of acres of breeding habitat in the Appalachians. Meanwhile, much of the warbler’s wintering habitat has been converted to cattle pastures, coffee and coca plantations, and other agricultural uses.

Moreover, many migratory animals aggregate at key places during certain times of the year, a habit that makes them vulnerable to overexploitation. Gray whales in the eastern Pacific largely escaped persecution until the mid-1800s, when whalers stumbled on the shallow lagoons in Baja California where most of the animals gather in the winter to mate and give birth. Within two decades, whaling operations had driven the gray whale close to extinction, although they subsequently rebounded because of protection. All of the world’s sea turtles are imperiled in part because adult females return year after year to the same beaches to lay their eggs; the slow-moving and defenseless turtles and their eggs are easily harvested at their nesting beaches.

Climate change, too, has the potential to disrupt the migratory patterns of a wide range of animals. Rising sea levels could submerge the nesting beaches of sea turtles and shore-birds. Songbirds breeding in the temperate forests of Eurasia and North America depend on a summer flush of insects, particularly caterpillars, to feed themselves and their off-spring. In some places, these caterpillars are emerging earlier and earlier in response to rising temperatures. In theory, the songbirds could simply push up their departure from their winter quarters in Central America, the Caribbean, or Africa to catch the earlier flush of insect prey. If, however, the birds are relying on a fixed cue such as increasing day length to decide when to head north, they may be unable to adjust the timing of their migration. Precisely this disruption in the timing of bird migration relative to the emergence of insect prey has been identified as the cause of a decline of 90% in populations of pied flycatchers in the Netherlands. In East Africa, where the movements of wildebeest, zebras, and other grazers are timed to the seasonal rains, any change in rainfall patterns due to global warming will probably produce concurrent changes in migratory routes. As land outside Africa’s existing game reserves is converted to villages and farm fields, it may be difficult or impossible for the mammals to adjust their migratory routes in response to the changes in rainfall. It’s possible, of course, that warblers and wildebeest will find ways to cope with the twin dangers of habitat destruction and climate change. But the opposite could also be true, with declines occurring even faster and deeper than we anticipate.

The decline of the world’s great animal migrations is clearly a major aesthetic loss. But it is also a major environmental and economic problem, given the important ecosystem services these species provide. Consider the case of salmon in the Pacific Northwest. They head for the ocean when they are young and small, taking advantage of the productivity of the seas to grow to full size. They then return to their natal streams where they spawn, die, and decompose. They are, in essence, self-propelled bags of fertilizer, gathering important nutrients such as nitrogen and phosphorus from the ocean and delivering them to the streams, where these same nutrients can then taken up by other aquatic species or carried onshore by scavenging eagles, bears, and other animals. As salmon runs across the Northwest have declined because of dams, overfishing, and habitat degradation, so too has the free delivery of nutrients. In the Columbia River, for example, annual salmon runs have dropped from roughly 9.6 to 16.3 million fish before the arrival of white settlers to about 0.5 million today. According to one estimate, the weight of carcasses in the Columbia has dropped from nearly 50,000 tons per year to 3,700 tons. Perhaps some of the nutrient deficits caused by the lack of salmon have been erased by fertilizer runoff or other human-created sources. But even if our overuse of fertilizers (with its attendant runoff) has somehow lessened the impact of the salmon shortfall, it has not helped the Northwest’s beleaguered fishing industry, which has lost jobs as a result of the drop in salmon populations. From 1990 to 2005, unemployment rates in British Columbia’s commercial fisheries averaged 17.2%, twice the rate for the province’s economy as a whole.

Migratory songbirds perform their own important ecosystem service by consuming vast numbers of caterpillars that would otherwise eat the foliage of trees and shrubs. As numbers of songbirds drop, one might predict an increase in insect damage to forests or, alternatively, an increase in pesticide use to counteract any increase in defoliation.

Twin challenges

Given the strong aesthetic, environmental, and economic reasons for protecting animal migrations, the question naturally arises: Why have we been so unsuccessful at conserving them? The answer may lie in the fact that conserving migratory animals poses two unique challenges. First, it demands coordinated planning across borders and boundaries that mean a great deal to us but nothing to the animals. A single Swainson’s thrush winging its way from Canada to Brazil may pass through 10 or more countries. Each of these nations must provide safe nesting, wintering, or refueling stops in order for the thrush to complete its journey. Bison in Yellowstone National Park face harassment or even death if they cross an invisible line separating the park from adjacent land managed by the U.S. Forest Service and the state of Montana. The bison need access to lower-elevation rangelands outside the park during harsh winters, when the snowpack prevents them from finding sufficient forage inside the park. However, ranchers in Montana fear that the bison will spread brucellosis, a disease that causes some cattle to abort their fetuses, to their livestock, and they have used their political leverage to force the federal government and the state to curtail the bison migration.

The second key challenge associated with conserving migrations is convincing agencies, institutions, and individuals to agree to protect these animals while they are still abundant. The United States and many other countries have a long tradition of protecting endangered species, usually when the plant or animal in question is teetering on the brink of extinction. But for the reasons cited above, this type of 11th-hour intervention is wholly unsuited to the task of saving migrations, where the goal should be to protect the species while they are still plentiful.

Fortunately, there are a number of examples of successful efforts to conserve migratory animals, and we can look to them for guidance on addressing these problems. By the early 1940s, commercial whaling operations had dramatically reduced populations of the great whales, many of which undertake lengthy migrations through international waters where no one nation has sovereignty. In response to these declines, the major whaling nations signed the International Convention for the Regulation of Whaling in 1946. This treaty created a scientific and administrative body, the International Whaling Commission (IWC), with the power to curtail commercial whaling operations. After many years of stalling, the IWC finally halted commercial whaling in 1982, resulting in increased whale populations. (Japan, Norway, and Iceland continue to hunt several species of whales by exploiting loopholes in the treaty, but at levels well below what prevailed in the heyday of whaling).

The success of the International Convention for the Regulation of Whaling is due in large part to the fact that it created an administrative body with regulatory teeth. In contrast, an even more ambitious treaty, the 1979 Convention on the Conservation of Migratory Species (also known as the Bonn Convention) was designed to protect migratory animals of all kinds, but it lacks a powerful administrative body. Instead, it creates a mechanism whereby groups of nations can come together to address problems facing particular migratory species. The treaty does not specify what conservation measures must be taken, leaving that task to the nations involved in the agreements. Because the Bonn Convention lacks a strong administrative body, it has had relatively few successes thus far.

In a promising new development within the United States, the Western Governors’ Association approved a policy resolution in February 2007 aimed at protecting “wildlife migration corridors.” Alarmed by losses of migratory routes for elk, deer, bighorn sheep, and other animals caused by energy development and sprawl, the governors of the western states have pledged to identify and protect migratory routes in a more aggressive, coordinated manner. They recognize that the administrative barriers among states or among agencies within a state can undermine conservation programs for migratory species.

To address the second big challenge associated with conserving migratory species—protecting these species while they are still common—the institutions charged with managing natural resources will need to embrace the idea that migration is fundamentally a phenomenon of abundance and must be protected as such. To that end, it would be useful to have a standardized early-warning system to identify migrations at risk. One approach would be to develop a threat-ranking scheme for migrations akin to the one now used by the World Conservation Union for endangered species. Under that approach, species are listed as critically endangered, endangered, or vulnerable based on quantitative criteria related to factors such as population size, amount of habitat, trends in population size, and trends in habitat. Similar criteria, emphasizing trends in numbers, could be developed for discrete populations of migratory species, such as runs of salmon, populations of pronghorn, and monarchs wintering in Michoacán, Mexico. A migration that declined by more than a certain percentage over a fixed period of time could be classified as endangered; a slightly lower rate of decline might place it in the less serious category of threatened. Even if the designation did not carry any immediate legal consequences in terms of habitat protection, restrictions on harvest, and so forth, it would nonetheless bring welcome attention to the issue. To some degree, consumers can also play a useful role in protecting migrations by virtue of what they buy or don’t buy. Places where coffee is grown under a canopy of native tropical trees, typically marketed as shade-grown coffee, provide suitable winter habitat for a variety of North American songbirds; places where sun-tolerant coffee is grown in sterile monocultures do not.

Finally, for any conservation program to succeed, it must be adequately funded. The North American Waterfowl Management Plan, a joint agreement between Canada, the United States, and Mexico to regulate hunting and protect the habitats of ducks, geese, and swans, has protected or restored millions of acres of wetlands. This accomplishment was made possible by a stable, secure funding source: a tax on the sale of guns and ammunition in the United States, plus mandatory purchase of an annual permit to hunt waterfowl. Hunters want waterfowl to remain abundant in order to enjoy longer hunting seasons and bigger bag limits, and they are willing to pay for that goal via the tax and permit. One would hope that the nation’s birdwatchers could be inspired to support a similar tax on binoculars, birdseed, and other tools of their trade, with the revenues going to support habitat protection and restoration programs. However, an attempt to enact such a tax in the 1990s floundered in the face of strong resistance from the affected industries, an antitax (and anticonservation) sentiment in Congress, and too little support from birdwatchers.

If we are successful at saving the world’s great animal migrations, we will have protected natural phenomena that provide us with inspiration, sustenance, recreation, and numerous ecosystem benefits. We also will have learned to take timely, cooperative action to solve a complex environmental problem. It is even possible that efforts to protect migratory animals will inform our efforts to address other environmental and social ills that similarly transcend artificial borders and boundaries. At the very least, we will have ensured that future generations can enjoy some of the same flocks of birds, schools of fish, and herds of mammals that have inspired and sustained us for thousands of years.


David S. Wilcove () is professor of ecology, evolutionary biology, and public affairs at Princeton University and the author of No Way Home: The Decline of the World’s Great Animal Migrations (Island Press, 2008).