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.