If we could turn back the clock 200 years, we could watch as millions of whales took to their migration routes. Around 150 years ago, we could witness bison filling the vast America prairie or a billion passenger pigeons blotting out the sky for days. Only a few decades back, and more than a million saiga antelope could be seen crossing the plains of Central Asia.

Fast-forward to today: the humpback whale (Megaptera novaeangliae) population is only 5 percent of its estimated historic population. Based on DNA data, the species has fallen from up to 1.5 million behemoths to perhaps 80,000. Around 30,000 American bison (Bison bison) are left out of a population that may have reached 100 million; the percentage remaining is not even a whole number. The saiga antelope (Saiga tatarica) has dropped 95 percent in 20 years, from a million individuals to 50,000. But the passenger pigeon (Ectopistes migratorius) proves the most drastic, going from one of the world’s most populous birds to extinct in a few decades.

But unlike en­dangered species, massive populations of the migrating species must be preserved to warrant success, while researchers often consider a few hundred healthy breeding pairs enough for the recovery of an endangered animal.

Such examples illustrate a common occur­rence: the phenomenon of mass migration going the way of the passenger pigeon. From whales to sea turtles and insects to songbirds, from hoofed mammals to the predators that track them, mas­sive migrations are declining worldwide, and in a number of cases simply vanishing altogether.

In a paper in PLoS Biology, David S. Wilcove and Martin Wikelski, both with the Depart­ment of Ecology and Evolutionary Biology at Princeton, discuss the ramifications of such losses in abundance and the importance of putting new conservation attention on beleaguered migrants.

Wilcove and Wikelski point to four major reasons why massive migrations are gravely threat­ened: human-created barriers like dams, fences and roads; habitat destruction; climate change; and overexploitation of a species, particularly important in the case of oceanic and freshwater migrants. All of these reasons are anthropogenic (human-related), but Wilcove and Wikelski believe that those who caused the demise of the great migrations could also save them, arguing that the world’s great migrations deserve suitably large-scale conservation initiatives. In fact, they state that mass migrations should be protected much like endangered species. But unlike en­dangered species, massive populations of the migrating species must be preserved to warrant success, while researchers often consider a few hundred healthy breeding pairs enough for the recovery of an endangered animal.

Although no one knows exactly how each migration affects its environment, the authors believe diminishing migrations drastically alter the productivity of an ecosystem, challenging its ability to provide essential services. For example, the authors illustrate that salmon “by migrating upstream, spawning and dying … transfer nu­trients from the ocean to the rivers. A portion of the nutrients is delivered in the form of feces, sperm and eggs from the living fish; much more comes from the decaying carcasses of the adults.

Phosphorus and nitrogen from salmon carcasses enhance the growth of phytoplankton and zoo­plankton in the rivers, which provide food for smaller fish, including young salmon.” However, the northwestern rivers of America receive only about 6–7 percent of the nutrients they once did due to a drastic decline in the migratory popula­tion of salmon. Fewer nutrients ultimately lead to fewer salmon in the next generation and less biomass altogether.

It is not just one-species migrations, such as salmon and saiga that suffer from decline.

“Birdwatchers in North America and Europe, for example, complain that fewer songbirds are returning each spring from their winter quarters in Latin America and Africa, respectively,” the authors write, citing a recent study of Europe’s birds, which show migratory birds have suffered greater declines in population than stationary spe­cies. Such drops in population are also bound to have drastic impact on ecosystems; for example, migratory birds help control insect populations. Fewer birds may mean a population explosion of insects, some of which could be detrimental to forests or nearby farmland.

A 2005 study of the passenger pigeon’s ex­tinction argued that the bird’s demise caused the current prevalence of Lyme disease. Deer ticks (Ixodes scapularis) only spread Lyme disease after feeding on an infected host, often mice. But mice, researchers theorized, are more abundant now since the extinction of the passenger pigeons. Why? Passenger pigeons used to compete with mice for the same food source, acorns. Thereby, the loss of passenger pigeons may have caused an incomprehensible rise in the deer tick population due to more mice.

Of course, when migratory species dimin­ish, predator numbers also decline as their food sources dry up. In addition, plant diversity and populations change when thousands of her­bivorous mammals fail to make their seasonal appearances.

Illustrating just how imperiled global migra­tions are, a 2009 study in Endangered Species Re­search surveyed two dozen large ungulate species (hoofed animals) known for their migratory pat­terns, including some well-known species, such as caribou (Rangifer tarandus), American bison, elk (Cervus elaphus), Burchell’s zebra (Equus burchellii), blue wildebeest (Connochaetes tau­rinus), Tibetan antelope (Pantholops hodgsonii) and saiga.

In Eurasia half of the migratory species have been largely ignored by science.

Shockingly, almost all 24 focal species lost migration routes and suffered population declines. Six of the focal species either no longer migrated at all or, in a couple of cases, no longer survived in the wild: the springbok (Antidorcas marsu­pialis) used to form some of the world’s largest migrations; the black wildebeest (Connochaetes gnou) was nearly exterminated and has relied on reintroduction efforts; the blesbok (Damaliscus dorcas) is not endangered but no longer migrates; the dwindling population of wild ass or kulan (Equus hemionus) of central Asia was cut in half in just 16 years; the scimitar horned oryx (Oryx dammah) is extinct in the wild, but there are plans for reintroduction; and the quagga (Equus quagga) from southern Africa is simply gone.

Part of the problem has simply been a lack of awareness: Researchers found that many of these migrations have been little studied. Although Af­rica includes the most large-scale migrations, the authors discovered that three migrating species had no publications on their population status at all. In Eurasia half of the migratory species have been largely ignored by science.

Preserving migrations, however, has proven even more difficult than identifying the causes in their decline.

“If we are going to conserve migrations and species, we need to identify what needs to be done: where migrations remain, how far animals move, their habitat needs and location, threats, and the knowledge gaps that needed to be filled,” says Joel Berger with the Wildlife Conservation Society and the University of Montana. “For some of these species, such as the wildebeest and eland in Botswana, threats were identified decades ago. We as a society have made little progress at figuring out how to save migrations.”

Grant Harris of the American Museum of Natural History says that “a large part of this is an awareness issue. People don’t realize what we have and are losing.”

But preserving thousands to millions of in­dividuals will be far from easy. Wilcove and Wikelski write that saving these migrations will pose “unique scientific and social challenges.” How does one approach preserving abundance, rather than settling for simple existence? The writers believe that protecting migrations will require action on the local, national and global level. Those in power will have to change their mind-set and protect a species before its popula­tion declines.

However, to date the importance of migra­tions has not penetrated the policy sphere. Even the world’s most well-known migration—the wil­debeest in the Serengeti—is facing an existential threat from a road that could potentially cut off the movement of wildebeest, essentially stalling one of the greatest natural spectacles on Earth. Warnings from the tourism sector, environmental groups and international governments have to date failed to stop the Tanzanian government from proceeding on the road.

“If we are successful,” Wilcove and Wikel­ski write, “it will be because governments and individuals have learned to act proactively and cooperatively to address environmental problems, and because we have created an international network of protected areas that is capable of sustaining much of the planet’s natural diversity.”

The authors believe it would be well worth the energy and sacrifices required, considering the ecological services provided by these massive movements, the scientific importance of studying the mechanisms behind such migrations, and the perfect wonder of such spectacles. Migrations are a kind of culmination of nature’s potential—once so prevalent across the world, now only surviving in a few aberrant places.

Some great migrations do remain. Although in decline, monarch butterflies still cross inter­national boundaries in astounding numbers. At least for now, some 2 million wildebeest, zebra and Thomson’s gazelles travel across the African plains, providing food for many of Africa’s large predators, from lions to hyenas to crocodiles. Caribou still migrate in the thousands across the Arctic tundra. And as recently as 2007 a previ­ously unknown migration was observed in the southern Sudan, with over a million antelopes, including the white-eared kob (Kobus kob), the tiang (Damaliscus lunatus) and the mongalla gazelle (Eudorcas albonotata).

Conservationist and adventurer Michael Fay said of the discovery: “This could represent the biggest migration of large mammals on Earth. I have never seen wildlife in such numbers, not even when flying over the mass migrations of the Serengeti.”

Although on the wane, great migrations still exist: The discovery of a new migration contain­ing a million individuals buoys that point. Now, with proactive attention, great energy and global cooperation, such migrations could not only survive, but thrive. In the future—as in the past—millions of whales, saiga antelopes and even bison could move along migratory routes, completing their ecological role.

Excerpted from Life is Good: Conservation in an Age of Mass Extinction, published in 2011 by Mongabay and available at amazon.com in paperback and Kindle versions. This article originally appeared in the Fall 2012 issue of Momentum magazine, Ensia’s predecessor, as “On the Move.”