Restoring North America's Birds: Lessons from Landscape Ecology. 2000 . Yale University Press , New Haven, CT . 320 pp. $35.00. ISBN 0-300-07967-2 .
The last decade has seen an intensified focus on monitoring North American bird populations and on determining causes and solutions for declines in abundance. The impetus for this explosion of research was the alarming indication in the late 1980s that many birds, not just species designated officially as endangered, were declining. Terborgh's 1989 book was a major factor in raising concerns about North American birds. Today, Askin's well-written book is a valuable update on the state of our knowledge, some of which counters indications that existed when Terborgh wrote his landmark account.
In the late 1980s, concern was focused on Neotropical migrants, especially those nesting in eastern U.S. forests. We now know that these species are not undergoing an overall decline, despite early data to the contrary. These early data were based largely on monitoring studies that showed declines and even local extirpations in particular patches of forest over many decades. But work in the 1990s on the pervasive negative effects of forest fragmentation provided an appropriate perspective on these long-term data sets. Because of high levels of nest predation and cowbird parasitism, many bird populations in small forest fragments have such low breeding success that they are population sinks. Furthermore, many forest species will not even nest in forest patches below a certain species-specific size. Sites typically monitored for long periods were small forest preserves conveniently located near urban and suburban centers. Initially, these preserves were embedded in larger forest fragments that were whittled away by development, so the long-term studies reflected the effects of forest fragmentation rather than global declines of forest species. Although a few species of Neotropical forest migrants are genuinely in trouble, the most threatened birds in North America are grassland species, most of which are declining throughout their geographic ranges.
The initially misleading results from long-term monitoring studies of forest birds epitomizes Askins's main message, namely that bird populations have to be considered from a landscape perspective. Population trends and potential solutions to declines must be assessed on much larger spatial scales than that of a single study plot, and complex and dynamic processes occur over large spatial and temporal scales. Another message throughout much of the book is that natural disturbances such as floods and fires that destroy part or all of some habitats are essential for generating the conditions many bird species require. These are far from novel ideas among current researchers, but Askins's book is intended more for land managers and conservationists who may place an undue focus on the parcels of land under their stewardship and who may find the enormous mass of recent literature difficult and confusing. Askins has done an admirable job of providing his intended audience with a most useful book. Moreover, even active researchers will find much of value in this book because of its in-depth yet engaging treatments of many diverse habitats and landscapes.
The book is, in fact, arranged along habitat-landscape lines, and each chapter details the status of birds in a particular landscape, focusing on actual or potential large-scale declines in abundance and on essential landscape or disturbance processes. Perhaps the most eye-opening chapter is the first one, on grassland birds of the eastern United States. Askins convincingly demolishes the notion that eastern North America was covered by unbroken forest before European colonization. Native Americans prevented forest growth over large tracts, a theme explored in other recent books (Kennedy 1994; Krech 1999), and natural processes created and maintained eastern grasslands before any people were on the scene. This historical perspective is important in light of recent drastic declines of grassland birds in the eastern United States. Because these declines are often attributed to reforestation due to the decline of agriculture, it is sometimes suggested that the presence of grassland species in eastern North America is an artifact of forest clearing for agriculture. But Askins shows that grassland species are native to the East and that reforestation is only part of the problem, because the natural processes that maintained eastern grasslands have been altered.
In later chapters, Askins focuses on additional habitats such as eastern thickets, the Great Plains, old-growth forests, southwestern riparian habitats, and others. Included also are discussions of endangered or extinct species such as Kirtland's Warbler and the Ivory-billed Woodpecker. In each case, we see that habitat destruction alone fails to explain declines of birds. Other key factors are interruptions of natural processes, such as fires, that once created or maintained habitats that used to be widespread, such as the open pine woodlands of the southeastern United States. Some of these later chapters deal with issues that may be unfamiliar even to many researchers, such as the decline of many birds in northern Scandinavian forests that essentially have become industrialized tree farms. Ominously, Askins points out that similar management practices may arise in the boreal forests of North America. A teaching tool that Askins uses to show the necessity of maintaining natural processes over huge spatial scales is his occasional focus on coevolved interactions between certain bird and plant species, such as phainopeplas and mistletoes and crossbills and conifers.
Although this book has much to recommend it, it has some faults. Little attention is paid to providing the reader with an overall plan or guide to the book. Despite my delight at learning more about naturally occurring grasslands in the East, I found it odd that a general book on bird conservation and landscape ecology would start off with an in-depth treatment of a fairly specialized topic. A general introduction to the book and an explanation of the order of its subjects would have helped. An introduction with basic concepts important to landscape ecology and the nature of spatial scales and of complex processes also would have been useful. Although the final chapter provides a useful synthesis of important lessons from the preceding nine habitat-oriented chapters, greater clarity regarding take-home messages is needed, such as a table or a series of bulleted key principles. The book could also make it clearer that there has been something of a debate over whether the major threat to Neotropical migrants is the loss and degradation of breeding habitat in North America or rather alterations to wintering habitat in the Neotropics. Although, Askins provides some brief discussions of the Neotropics, he obviously believes that the loss of habitat in North America is the primary threat. I suspect he is right, but more discussion of the Neotropics would have given readers a better perspective on this active area of research.
Despite these criticisms, Askins's book is a valuable resource. It should have a major effect on the ways in which land managers and conservationists view threats to North American birds and should make efforts to conserve birds considerably more effective.