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.
I view this as a loss. I realize some of my readers see the expansion of humanity as some sort of Manifest Destiny which is a value that trumps all other values (really). But I do not see why the expansion of the human race up to 7, 8, 9+ billion people enriches my life. Seems quite the opposite is the case.
I think migrating geese honking at high altitudes are really cool. If I could go back in a time machine I would go back and (among other things) watch the massive carrier pigeon migration before hunters wiped them out entirely Non-migratory species aren't shrinking as much as migratory species.
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 . 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 .
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.
Migrant species that are not in immediate danger of extinction will come under greater pressure as more billions of humans populate the Earth and each human uses a larger ecological footprint. Do you care?
Salmon are a dim shadow of their former numbers.
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.  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.
Migratory bird numbers might undergo a big shrinking. Wetlands destruction probably will contribute to that. Biomass energy crops will reduce available habitats as well.
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).
|Share |||Randall Parker, 2008 August 06 05:16 PM Trends Habitat Loss|