There can't be a more remarkable sight than a mass migration of animals, be it across the plains of Africa, on a cloud-covered skyline, or along the wave-ridden ocean coasts. Even as you read this article, animals are charging forward, moving from one region or climate zone to another in search of new feeding and breeding grounds.
Songbirds may be finishing up their yearly round-trip migration from North America to Brazil and back; great herds of grazing wildebeests, zebras and Thomson's gazelles may be creating spectacular hoof-beaten paths across the Serengeti-Mara ecosystem, or Pacific salmon could be nearing the end of their lifelong migration to return to their birthplace to spawn. Any one of these events would be breathtaking to witness, but you may want to hurry up. Some scientists are saying mass animal migrations are in trouble, and perhaps humans are to blame.
According to an essay by David S. Wilcove and Martin Wikelski from Princeton University's Department of Ecology and Evolutionary Biology, published last year in the peer-reviewed journal PLoS Biology, "the threats to migrants fall into four nonexclusive categories: habitat destruction, the creation of obstacles and barriers such as dams and fences, overexploitation, and climate change."
Climate change, or more precisely global warming, may be negatively impacting one of the lengthiest migrations executed by any mammal, that of the Gray whale. These animals travel 10,000 to 12,000 miles round-trip each year, leaving behind the bays and warm winter waters of Baja California, Mexico, to summer in the cooler seas of the Arctic. Gray whales are bottom feeders, mostly sustained by small crustaceans called amphipods, which cover the sea floor like a carpet; rolling over on his side, the whale sucks up the amphipod-rich bottom, filtering out the sediment and saltwater.
Over the last several decades, however, oceanographers studying the Arctic and monitoring climate change have seen a slow warming of the sea surface temperatures and alterations in currents that move water about the Arctic. The Chukchi sea floor, having once been teeming with amphipods where Gray whales typically fed, is now vastly devoid of the small creatures, due to the effects of the warming waters.
Fewer Gray whales are being observed in their typical summer congregation areas and are moving further north in search of food. Scientists studying the mammals are seeing significant numbers traveling through the Bering straits into the Arctic Ocean and venturing further out to areas where their characteristic food source can still be found. Some whales have also been observed feeding on alternative prey, such as shrimp, pelagic red crabs and small fish.
Another effect the rising water temperature seems to be having on the migration of Gray whales is that some calves are being born further north. Whales born off the coasts of Alaska, Canada, Washington and Oregon, instead of the typical calving areas closer to Mexico, have to make a longer migration, which can jeopardize the likelihood of their survival.
Great animal migrations not only serve the purposes of those migrants, but also the predators that prey on those animals and the ecological benefits the migration itself creates. For instance, Wilcove and Wikelski write, “Several studies have shown that birds reduce insect populations in temperate forests, thus raising the question of whether ongoing declines in migratory birds pose a threat to the health of our forests and farmlands.”
Unfortunately, birds are all too often subject to overexploitation, as is the case in Cyprus. The island lies on an essential section of the migratory path for birds flying from Europe to warmer climates, where illegal bird trapping has been routine throughout the years. In 2008 alone, over one million songbirds were trapped, killed, and likely served up as a “culinary delicacies,” according to the ornithological conservation group BirdLife Cyprus. Approximately 90 percent of the birds following this migration route are protected species, some of which are even considered to be threatened.
Over the past several decades, breeding populations of eastern North American and European migratory songbirds have declined. Although situations such as the one in Cyprus beg to be labeled as the culprits, there are probably several other factors for these vanishing journeys.
Wilcove and Wikelski theorize that the decline of these migratory birds is the result of “a function of the loss of breeding habitat, the loss of winter habitat, heightened mortality during migration (due to habitat destruction, pesticides, communication towers, and other factors), or some combination of the three.” In their essay, they conclude that an answer cannot be known until full migratory cycles of individual birds can be tracked by satellite transmitters, which was an effort thought to be years away.
Last summer, however, researchers were able to retrieve data from seven geo-locator backpacks attached to two species of songbirds. The findings amazed the research scientist with the speed of the songbirds and the distance they covered on their migratory route. Fourteen wood thrushes and 20 purple martins were fitted with the tiny backpacks and sent forth from Pennsylvania a year prior; the birds would cover approximately 311 miles per day and traveled from Brazil to Pennsylvania in just shy of one month.
Mass animal migrations have constantly occurred over the millennia. The continuation of these phenomena is essential and will certainly be influenced by our actions. A collaborative effort to preserve their habitats, defend against overexploitation, find alternatives to closing off migration routes with barriers, and work toward solving the issues of climatic threats is needed to make their sustainability possible. With the unending fever of conservation, environmental and animal welfare groups, as well as the efforts of some individuals and governments, future generations may have a chance to enjoy the spectacle of great journeys.