Migration, interrupted

Book review by Nigel A. Clark published in Science


The Narrow Edge A Tiny Bird, an Ancient Crab, and an Epic Journey Deborah Cramer Yale University Press, 2015. 303 pp.


In order for the shorebirds known as red knots to survive their annual migration from the Southern Hemisphere to the species’ Arctic breeding grounds, they rely on a series of sites along the way to provide enough food for their stay and to fuel their onward flight. If just one of these sites is compromised, the species itself will fail.

Unfortunately for the knots, many of these vital sites are vast, intertidal flats that humans have come to see as areas ripe for development and exploitation. This is especially true for the main spring stopover sites that knot populations on each continent rely on, including areas in the Delaware Bay in the Americas, the Wadden Sea in Europe, and the Yellow Sea in East Asia. The marvel of their long migration and the beauty of their synchronously weaving flocks have inspired many to try to ensure that there is a place on the planet for the knots for years to come.

In The Narrow Edge, Deborah Cramer follows the migration route of the American rufa knot from the southern tip of South America to the high Arctic, where the ground thaws just long enough for them to raise their chicks before winter sets in again. The book is perhaps more about people than birds, as Cramer tries to understand how and why humans and birds have come into conflict. She explores what motivates developers, fishermen, and conservationists and explores the ways scientists are working to understand how we can live together on an increasingly crowded planet. Throughout the book, Cramer explores the history that has led to the tenuous existence of the knot, as well as the tangled ecological web that it, and we, are part of.

The book is written from a conservationist’s viewpoint, but Cramer also tries to understand the driving forces that, often inadvertently, make it more difficult for the knots to survive. For example, she recounts how, until recently, the salt marshlands in Argentina’s Río Gallegos estuary were being sold off to developers and used as a municipal waste dump. As the population of the nearby city expanded over the past half-century, the number of knots observed each year in the estuary dropped precipitously. Now, as a result of the efforts of scientists Silvia Ferrari and Carlos Albrieu, Río Gallegos has two protected areas where further development of the marsh is prohibited. The city has also made an asset of the marsh, creating a visitor center and bird-viewing areas to educate locals and tourists about the value of conservation efforts. This has come too late for the knots, which no longer stop there, but has proved vital for many other species of shorebirds. Perhaps, in time, the knots will return.

Many knots that migrate from the Americas make their last stop to refuel in the Delaware Bay before heading to the Arctic. Here, they gorge on horseshoe crab eggs that are washed out of the sand, where they are laid in May of each year. The crabs and, hence, the birds have declined over the past two decades. It would be easy to blame modern fishermen for overexploiting the crabs, but Cramer shows that the tendency toward aggressive harvesting is consistent with historical practices. During the midto late 1800s, horseshoe crabs were fed to pigs or ground into fertilizer in specially built factories. More than a million crabs were harvested from a single mile of beach in 1857, and over 4 million were taken from all of Delaware Bay in 1880. By the late 1800s, the horseshoe crab population in the Delaware Bay was almost nonexistent. It slowly recovered over the next century but crashed again when fishermen began collecting the crabs to use as bait for conch and eel fishing in the 1990s. With a history of exploitation, it is not surprising that the fishermen saw the crabs as a resource to be taken advantage of, rather than a population to be managed sustainably. Since 1998, there have been progressively more severe harvest restrictions on the crabs but, with a 10-year life cycle, we are just beginning to see signs of recovery.

If the horseshoe crab had gone extinct, the knots would not have been the only ones to suffer. In the early 1950s, the physician Frederick Bang discovered that the crab’s blue blood clots in the presence of Gram-negative bacteria. By the mid-1980s, limulus amebocyte lysate (LAL), made from the blood of horseshoe crabs, had become an invaluable aid in the fight against infection in humans.

By focusing on the plight of one flagship species and the people who have dedicated their lives to understanding how to protect it, this book shows how conservation efforts are critical to maintaining coastal biodiversity. It also offers important lessons and strategies that may be implemented for the protection and preservation of other species.