Simple Concepts, Elusive Clarity


Rochelle, J. A., L. A. Lehmann, J. Wisniewski Forest Fragmentation: Wildlife and Management Implications. , editors. 1999 . Brill Academic Publishers , Leiden, The Netherlands . 301 pp. $95.00. ISBN 90-04-11388-6 .

As a student of ecology in Wisconsin, I was introduced to the concept of habitat fragmentation through reproductions of the gradual cutting of forests in Cadiz Township. John Curtis's 1956 reconstruction of how the once contiguous pre-European settlement forests of Green County, Wisconsin, had been gradually cleared and isolated between 1831 and 1950 was the proverbial “picture worth a thousand words.” It is one thing, however, to provide a superb illustration of the process and product of fragmentation and quite another to elucidate the ecological processes that result from and characterize fragmented habitat. That a landscape is fragmented—so that habitats spatially separated from one another—can be conveyed quickly with a single map or air photo. A challenge that continues to face conservation biologists is building a generalizable conceptual framework that can be applied to predicting the effects of fragmenting habitat. What, then, are conservation biologists to do with a concept that is both simple and useful in the abstract, yet which often fails to lend clarity when applied to real landscapes? This is the central question that Forest Fragmentation: Wildlife and Management Implications seeks to address.

This 15-chapter volume stems from a 1998 Portland, Oregon, conference organized with the goal of providing “a synthesis of the current state of knowledge related to fragmentation in managed forests of the Pacific Northwest.” The editors assembled regional authorities on habitat fragmentation in the western United States and Canada with the following objectives: (1) “to bring more clarity to the terms describing phenomena at the forest level (phenomena such as fragmentation, connectivity, and edge effects)” and (2) “to collate and present what evidence could be found of the expected outcomes of habitat fragmentation in western forests” (p. vii).

Only in the last 20 years has fragmentation received widespread attention from ecologists. The result of two decades of research into fragmentation (primarily in forested habitats) is a body of knowledge characterized by substantial taxonomic and geographic biases. Moreover, there appears to be a growing consensus that habitat fragmentation, although a useful conceptual tool, may have limited value as a generalizable phenomenon. In the foreword of this edited volume, Bunnell makes precisely this point, defining the little-used term panchreston as “a proposed explanation intended to address a complex problem by trying to account for all possible contingencies but typically proving to be too broadly conceived and therefore oversimplified to be of any practical use” ( p. vii). The editors are left with the self-appointed task of nailing down what forest fragmentation means in western North America.

The language is largely jargon-free and accompanied throughout by useful graphics (color and black and white). Chapters range from landscape design to vertebrate genetics. In the first chapter, Bunnell follows the thread laid down in the foreword with a discussion of why habitat fragmentation has defied easy translation from concept to application; he then critically evaluates the usefulness of applying island biogeography theory to heterogeneous western forests. Additional chapters review past research on edge effects and the effects of forest fragmentation on avian nest predation rates and on the distribution and composition of bird communities in several western forest types. Birds feature prominently in the book—four chapters are devoted to birds and bird studies are heavily cited in several others—an unavoidable artifact of an avian bias in the fragmentation literature.

Resource management professionals in agencies and universities will find this book an accessible overview of current thinking on forest fragmentation in the western United States. It could also serve as a text for upper-level or graduate seminars with a regional focus on Pacific Northwest forests. Most chapters are structured around reviews of the literature, with some new analyses of published data, although substantial new data are presented in chapters by Schmiegelow and Hannon, McGarigal, and Bunnell et al. McComb concludes with a valuable synthesis of the book and the 1998 conference.

One conclusion that emerges is that much of the early work on habitat fragmentation occurred in places where edges were crisp and habitat patches could be defined clearly, such as England and eastern North America. Such studies translate poorly to most forests in western North America. Issues of natural landscape heterogeneity, edge permeability, and the relative importance of absolute habitat loss versus fragmentation make it difficult to identify wildlife distribution patterns that are attributable to fragmentation alone. Several chapters make the convincing argument that habitat fragmentation should usually be viewed as less important to most vertebrate populations than simple loss of habitat. At the landscape level “… it is the sheer amount of habitat of [sic] a particular species present in the landscape that is the most significant associate of animal abundance and only secondarily does the pattern influence abundance” (p. 296). A related point made throughout the book is that scientists and managers must pay attention to scale when dealing with fragmentation. Several of the book's authors (especially Fahrig) claim that, too often, results from patch-level studies are presumed to apply equally well to landscapes. This point is well supported.

One weakness of the book is its focus on individual abundance rather than on the potential effects of landscape pattern on the demographic characteristics of populations, such as productivity and mortality rates. To some degree, this reflects a lack of available data on how demographic traits are influenced by landscape pattern. Several recent studies that address the relationship between vertebrate demography and landscape pattern are not included in the book, perhaps because they have come from eastern and central North America. The book's regional focus is its strength, however, and such efforts may prove to be the most useful way of bridging the current gap between the abstractions and applications of fragmentation science.