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- W2048347789 abstract "Animal migration surely ranks as one of nature's most visible and widespread phenomena. Every minute of every day, somewhere, some place, animals are on the move. The migrants span the animal kingdom, from whales and warblers to dragonflies and salamanders. But is migration an endangered phenomenon? Around the world, many of the most spectacular migrations have either disappeared due to human activities or are in steep decline. Those of us living in eastern North America can no longer experience the flocks of millions of passenger pigeons that temporarily obscured the sun as they migrated to and from their breeding grounds. Nor can residents of the Great Plains climb to the top of a hill and gaze down up hundreds of thousands of bison trekking across the prairies, as was possible less than two centuries ago.Even the less iconic migrations show signs of trouble. 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. Indeed, a recent continent-wide analysis of European breeding birds concluded that long-distance migrants (i.e., those species that breed in Europe but winter in sub-Saharan Africa) have suffered sustained and often severe population declines, more so than related nonmigratory species [1]. In central Asia, the number of saiga, a peculiar migratory antelope of the dry steppe grasslands and semi-desert, has dropped by over 95% in the past two decades, from over one million to fewer than 50,000 [2].The causes of all these declines vary depending on the species and the locale, but in general, 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. Most of the migrants are in little immediate danger of extinction; rather, they are becoming less and less common. Thus, birdwatchers can still see all of the species of migratory songbirds they seek each spring; they simply have to work harder to do so. Bison still roam national parks and private ranches in the American West, but today's herds number in the hundreds or low thousands, rather than the hundreds of thousands or millions. And there are still lots of salmon to catch off the coast of Norway or British Columbia—just not as many as there used to be.The question thus arises: Given the panoply of environmental problems we now face, is the fading glory of migration really a significant issue? We would argue that it is. Protecting the abundance of migrants is the key to protecting the ecological importance of migration. As the number of migrants declines, so too do many of the most important ecological properties and services associated with them.Consider the case of salmon in the Pacific Northwest. The seven species of salmon and seagoing trout in this region share a similar life history strategy: as young fish (smolt), they leave their natal rivers and head to the sea where, aided by the productivity of the ocean, they increase tremendously in size and weight. After a year or two at sea, they return to their natal rivers to spawn, whereupon they die. By migrating upstream, spawning, and dying, they 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. Thus, salmon fry are literally sustained by their parents.Prior to European settlement, 160–226 million kilograms of salmon migrated each year up the rivers of Washington, Idaho, Oregon, and California. Today, after decades of dam construction, overfishing, water withdrawals for irrigation, logging, and streamside grazing by livestock, salmon populations have plummeted. The total biomass of spawning salmon in the Pacific Northwest is now estimated to be only 12–14 million kilograms. Gresh et al. [3] have calculated that the rivers of the Northwest receive just 6%–7% of the marine-derived nitrogen and phosphorus they once received from the abundant salmon population. How this shortfall may be affecting the ecology of the region's rivers or adjacent farmlands is largely unknown.We can imagine an analogous situation developing with respect to migratory birds. Each spring, more than 30,000 tons of migratory songbirds migrate from their wintering grounds in Latin America and the Caribbean to their breeding grounds in the United States and Canada. (This biomass value is derived by combining breeding population totals from the North American Landbird Conservation Plan with species-specific weights from various sources.) If we assume these birds consume 10%–35% of their body weight per day in insects (roughly matching the requirements of a 100-gram bird and a 10-gram bird, respectively), then they are eating anywhere from 3,000–10,500 tons of insects per day. (During the breeding season, when the birds are feeding offspring, these figures would be much higher.) 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.Similarly, one wonders how the ecology of the Serengeti would change if its migratory population of wildebeest (exceeding 1 million individuals) were to collapse, given the major role these animals surely play in terms of consuming herbaceous vegetation and redistributing nutrients via their urine and dung (Figure 1).Figure 1Migratory Wildebeest in the Serengeti" @default.
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- W2048347789 title "Going, Going, Gone: Is Animal Migration Disappearing" @default.
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- W2048347789 doi "https://doi.org/10.1371/journal.pbio.0060188" @default.
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