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1 June 2008 The Wonder and Decline of Animal Migrations
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Conservation strategies have struggled for decades to match the geographic scope of large-scale ecosystem processes, on which the survival of so many species depend. More than 20 years ago, growing awareness of habitat fragmentation sparked what is today an ambitious global agenda that aims to conserve ecosystem-level processes across vast landscapes. David Wilcove, a professor of ecology, evolutionary biology, and public affairs at Princeton University, has written a compelling book that brings our attention to a special suite of species for which even these large-scale conservation ambitions are not nearly large enough.

In separate sections on air, land, and water, Wilcove describes a diverse selection of bird, mammal, fish, and insect species that have two traits in common: they migrate vast distances, and they do so in colossal numbers. Migration is distinguished from other types of animal movement by back-and-forth passage, often performed on a seasonal basis, between sites that may be half a world apart. Migration has evolved separately in many species as an opportunistic survival strategy in which the rewards accruing to individuals from long-distance movement (say, abundant but ephemeral food) outweigh the penalties incurred (say, from energy expenditure). Wilcove's main theme is that, for many migratory species, humans have upset this fragile advantage—with disastrous consequences—by reducing the rewards and amplifying the penalties.

The trials and challenges of migration are detailed in case after case in No Way Home, and one marvels at how nature overcomes them. Tens of thousands of red knots (a small shorebird) migrate from the tip of South America to northern Canada each year, stopping along the way to refuel at a few key places. Delaware Bay is one of these, where resource-depleted birds descend at the end of May to gorge for a few weeks on the eggs of horseshoe crabs. The timing of their arrival is critical, because this feast is itself the product of a stunning annual migration by millions of horseshoe crabs, which move up from the continental shelf to the shores of the bay to spawn. Another migration involves millions of songbirds that arrive in the North American woods in spring, having overwintered in central and southern regions of the hemisphere. How such delicate creatures achieve this arduous passage is fairly astonishing—the intricate vignettes describing these migrations amount to fascinating short stories.

Descriptions of the research that led to the discovery and understanding of each migration are effortlessly woven into the narrative. The importance of research is underscored by the dearth of information we have about migrations that vanished before research could be done. Tens of millions of bison once roamed the Great Plains, but only sketchy impressions of where, why, and how these migrations operated can be gathered from the few brief anecdotes that exist.

Piecing together the details of ecological processes that span continents is not trivial. We learn that it took almost 50 years of perseverance to track down the monarch butterfly migration in North America by recording the flight directions of marked individuals at different stages. It turns out that monarchs reach as far north as the US-Canadian border, but then turn south and westward, eventually funneling into a few sites on the forested slopes of a volcano in south-central Mexico, where they spend the winter attached to trees in gigantic clusters. It also turns out that individuals do not make the entire journey; rather, it takes several generations of butterflies to complete the round trip each year. Exactly how this intergenerational feat is accomplished has yet to be discovered. Today, technological miracles allow tracking transmitters to be affixed to migrating birds and even to dragonflies, and Wilcove takes us to the cutting edge of this research.

His case studies are carefully selected to portray not only the wonder and diversity of migration strategies in nature but also the threats they face from modern humanity, and the full range of fates that have ensued: extinction (Rocky Mountain locust and bison in the American West), decline (new world songbirds, monarch butterflies, North Atlantic right whales), and recovery (gray whales). Declining numbers of migrating songbirds are partly due to loss and modification of habitats along the way, and increased exposure to nest predators and brood parasites. The over-harvesting of horseshoe crabs to be used as bait for the fishing industry has drastically reduced numbers of knots. Monarch butterflies are losing habitat to illegal logging in the Mexican forests where they overwinter. A barbed-wire fence may close off a migratory route that pronghorn antelopes have followed for millennia. At least 14 major dams impede salmon migration on the Columbia River. That any of these great migrations persist at all is testimony to the resilience of the species undertaking them.

Wilcove is as well-versed in conservation as in research, and he deftly explains the measures, policies, and tactics that have been crafted to halt the decline of migrating species. “Turtle-excluding devices” on fishing nets have helped some sea turtle populations. Whale watching and monarch ogling provide incentives for local communities to actively conserve these species. Securing sufficient staging habitats for birds along their entire migration routes entails complex collaborations and negotiations across local, state, and international boundaries, involving independent agencies bound by different policies and languages. This is possible—as attested by the successful conservation of migrating ducks and geese—but most migrations remain critically threatened.

Migration is a theme that would be well served by spectacular photography (which presumably was a victim of production costs in No Way Home). But Wilcove's descriptions succeed in conveying the sense of awe evoked by witnessing a mass migration by any species, for example, by tens of thousands of sandhill cranes taking off in unison from the Platte River lowlands in Nebraska. The emotions evoked by such spectacles are qualitatively different from those that arise from observing a few dozen or even a few hundred individuals, as this poignant comparison makes clear: 20 years after Wilcove had visited Delaware Bay, he returned with a group of students to observe the shorebird phenomenon. This was the students' first exposure to the congregation, and they were dazzled. But Wilcove was disappointed by the apparent reduction in species' numbers. This extraordinary book helps us to guard against such creeping intergenerational loss of awareness and experiences that enrich the human spirit.

With no conscious effort, one emerges with the impression of having become expansively informed about the natural history of migrations, the research that has defined them, and the solutions necessary to conserve them. Wilcove manages to describe the extreme perils that threaten these migrations, and still impart confidence that they can be conserved. Putting the book down, one ponders how best to make that happen.

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Nicholas Georgiadis "The Wonder and Decline of Animal Migrations," BioScience 58(6), 557-558, (1 June 2008). https://doi.org/10.1641/B580614
Published: 1 June 2008
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