- Jeremy Hance writes about wildlife migrations becoming increasingly endangered.
- He argues that the conservation of migrations would preserve important ecosystem services.
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If you could turn back the clock about 200 years, you could watch as millions of whales swam along their migration routes. Around 150 years ago, you could witness bison filling the vast American prairie, or a billion passenger pigeons blotting out the sky for days. Only a few decades back, you could see a million saiga antelope crossing the plains of Asia.
Fast-forward to today and the humpback whale population is less than 2 percent of its estimated historic population, from 1.5 million behemoths to 20,000. Only 350,000 bison are left, less than 1 percent of a population that may have reached 100 million. The saiga antelope has dropped 95 percent in 20 years, from a million individuals to 50,000. But the passenger pigeon was the most drastic case, going from one of the world’s most populous bird to extinct in a few decades. These examples illustrate a common occurrence: the phenomenon of mass migration is going the way of the passenger pigeon. From whales and sea turtles to insects and songbirds, from hoofed mammals to the predators that track them, massive migrations are declining worldwide. In an essay in PLoS Biology, scientists David S. Wilcove and Martin Wikelski discuss the ramifications of these losses in abundance and the importance of new conservation attention to the beleaguered migrants.
The writers describe 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 causing the demise of the great migrations can also save them. The authors argue that the world’s great migrations deserve large-scale conservation initiatives. In fact, they say 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 a few healthy breeding pairs can be enough for the recovery of an endangered species.
Although no one knows exactly how each migration affects its ecosystem, the authors believe that the diminishing of such migrations drastically alters 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.” Today, however, the rivers of the American Northwest receive only about 6 to 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 antelope, that suffer from decline. “Even the less iconic migrations show signs of trouble,” the authors write. “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 cite 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. One example, provided by the authors, is migratory birds’ importance in controlling insect populations in forest. Fewer birds may mean a population explosion of insects, some of which could be detrimental to the forests or nearby rural land. In fact, a 2005 study of the passenger pigeon’s extinction argued that the bird’s demise caused the current prevalence of Lyme disease. Noting that passenger pigeons competed for the same food source — acorns — as the deer hosting the ticks that spread Lyme’s disease, the researchers concluded that the loss of passenger pigeons caused an incomprehensible rise in the deer tick population due to a steadfast acorn surplus.
Preserving thousands to millions of individuals will not be easy. The scientists 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 levels. Those in power will have to change their mind-set and protect a species before its population declines.
“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 will 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. Such 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. Butterflies still cross international boundaries in astounding numbers. Every May, more than a million wildebeests travel across the African plains, providing food for many of the continent’s large predators, from lions to hyenas to crocodiles. Caribou still migrate in the thousands across the Arctic tundra. And only a year ago, a previously unknown migration was observed in South Sudan, with over a million antelopes, including the white-eared kob, the tiang, and the Mongalla gazelle. 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, with 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 once again move along migratory routes, completing their ecological role.
- Wilcove DS, Wikelski M (2008) Going, going, gone: Is animal migration disappearing? PLoS Biol 6(7): e188. doi:10.1371/journal.pbio.0060188
This story originally appeared on Mongabay July 28, 2008 and later appeared in Jeremy Hance’s book, Life is Good. Photos were updated when this story was re-posted in November 2018.