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Why This Book Was Written
This book was written for two main reasons. First, themes associated with landscape change have become a major focus of conservation biology and landscape ecology (McGarigal and Cushman 2002; Fahrig 2003; Hobbs and Yates 2003). A review of all papers published in 2001 in the journals Conservation Biology, Biological Conservation, and Biodiversity and Conservation found that landscape change and habitat fragmentation were the two most frequently studied processes threatening species persistence (Fazey et al. 2005a). Similarly, a database search of journal articles prior to writing this book produced over 2000 published papers where the abstract or keywords contained the words "habitat loss" or "habitat fragmentation." Because the literature on landscape change and habitat fragmentation is so large and complex (and becoming increasingly so), we believe there is an important role for a book that provides an overview of the varied and interrelated topics encompassed by it.
The second reason we wrote this book was because the term "habitat fragmentation" has become vague and ambiguous due to its imprecise use, thereby limiting its practical value for conservation managers (Haila 2002; Regan et al. 2002). The term is losing its meaning because it is frequently used as an umbrella term for a wide range of interacting processes, including habitat loss, the subdivision of remaining habitat, an increase in edge effects, and altered species interactions or ecological processes. In essence, there are multiple processes grouped under the term "fragmentation," and there are multiple consequences of those processes; thus it is difficult to determine which consequences relate to which process. As a result, the "blizzard of ecological details" characteristic of the fragmentation literature, coupled with the way the term "habitat fragmentation" often means "all things to all people," has made the study of fragmentation a "panchreston" (Bunnell 1999a; see discussion in the next section). Given this, it is perhaps not surprising that even some major reviews of fragmentation (e.g., McGarigal and Cushman 2002) have failed to identify "clear insights into system dynamics" (Bissonette and Storch 2002). The "panchreston problem" is clearly hampering progress in the study of landscape change and ways to mitigate its negative impacts on species and ecosystems.
A possible way to gain a better understanding of the field is to disentangle the subcomponent parts and themes that have been collected under the umbrella of "fragmentation" research. We have tried to do this in our book by summarizing and identifying links between what we believe are key topics associated with landscape change, habitat loss, and habitat fragmentation. We emphasize the word "summarizing" here because we have attempted to provide a brief overview and synthesis of key topics, rather than an exhaustive review of each and every topic related to fragmentation.
"Disentangling" Habitat Fragmentation
Bunnell (1999a) argued that the generic term "habitat fragmentation" is often used loosely to encompass a myriad of processes and changes that accompany landscape alteration—making it a panchreston; which he defined 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.
The Random House Dictionary of the English Language [undated] cited in Bunnell (1999a).
Bunnell (1999a) argued that a solution to the problems created by the fragmentation panchreston was to acknowledge that multiple processes take place and to work in ways to separate them so that consequences can be better assigned to particular processes (see also Figure 1.1). Unfortunately, the fragmentation literature rarely does this. Instead, typically what occurs is that one pattern (the spatial distribution of a species or suite of species) is correlated with another pattern (the spatial distribution of patches of native vegetation), with little emphasis on the ecological processes that link the two. In this book, we argue that tackling the panchreston will help: (1) key questions to be better defined—a major issue because problem-definition is something that ecologists often do poorly (Peters 1991); (2) ensure better and more focused studies that contribute an improved understanding of the effects of landscape change on biota; and (3) more rapidly evolve improved conservation and resource management strategies (Box 1.1). For example, although habitat loss and habitat subdivision often go together (see Chapter 4), the distinction between them is important. This is because dealing with habitat loss will require formulating quite different kinds of hypotheses, quite different types of studies, and, ultimately, different kinds of mitigation strategies than if the focus is on habitat subdivision.
While we argue that an important way forward is to focus on particular subcomponents of landscape modification, we also acknowledge the importance of recognizing the complexity that is typically associated with multiple biotic responses to multiple interacting processes of landscape change (Crome 1994). This is why we dedicate an entire section of this book (Part V) to approaches and general principles for mitigating the multiple threatening processes and negative effects on species and assemblages that often accompany landscape change.
The Scope of This Book: Definitions and Key Themes
Our overarching goal is to tackle the habitat fragmentation panchreston. To achieve this goal, we provide a conceptual framework for the study and management of modified landscapes. We also provide many empirical examples from around the world to give tangible support to this conceptual framework. An important part of tackling the fragmentation panchreston is to carefully define appropriate terms such as "habitat" and "fragmentation." This is because the imprecise use of terminology can lead to the inappropriate use of concepts and theories in management (Murphy 1989) or inappropriate testing of theory (Fazey 2005). Given this, we have attempted to be as precise as possible in our use of terms throughout this book—although errors have undoubtedly crept in.
Key terms that we use frequently throughout the text and that have a specific meaning in this book (sometimes different from standard usage in the literature) are outlined in Table 1.1. From an ecological perspective, what constitutes a landscape will usually be a function of the scales over which a given species moves and how it perceives its surrounds (Wiens 1997; Manning et al. 2004a). However, for the practical purposes of this book, we define the term "landscape" from a human perspective and consider it as an area that covers hundreds to thousands of hectares. When landscapes are changed by vegetation clearing or other kinds of anthropogenic modification, we use the interchangeable terms "landscape change," "landscape alteration," or "landscape modification." We use the term "habitat" to mean the environments suitable for a particular species. Following this definition, habitat therefore is a species-specific entity (see Table 1.1). "Habitat loss" refers to the loss of suitable habitat for a given species such that the particular species no longer occurs in that area. We do not consider the term "habitat loss" to be synonymous with the loss of native vegetation. This is because a landscape extensively altered by humans where vegetation loss has been substantial may effectively experience no loss of suitable habitat for some species. Conversely, a landscape supporting a complete cover of native vegetation may contain no suitable habitat for some species (e.g., because of a lack of naturally suitable environmental conditions) (Table 1.2). Given this, much of this book labors the point about the importance of understanding what constitutes suitable habitat as a prelude to mitigating habitat loss. Habitat subdivision is the process of subdividing a single large area of habitat into several smaller areas, a practice that is also referred to as habitat fragmentation. We follow the logic of Fahrig (2003) in recognizing that the spatial process of habitat subdivision (habitat fragmentation) is distinctly different from the process of habitat loss. Moreover, we rarely use the term "habitat fragmentation" in the remainder of this book because of the confusion that its loose application can create.
Finally, the term "biodiversity," which is a contraction of the words "biological diversity," is used occasionally throughout this book. There are many definitions of "biodiversity" (Bunnell 1998). For the purposes of this book, the term encompasses genes, individuals, demes, populations, metapopulations, species, communities, ecosystems, and the interactions between these entities (Box 1.2). This definition stresses both the numbers of entities (genes, species, etc.) and the differences within and between those entities (see Gaston and Spicer 1998, 2004). Bunnell (1998) reviewed approximately 90 interpretations of the biodiversity concept. Many of these definitions were very abstract, making it difficult to use them in management applications. For instance, definitions that include the maintenance of genetic diversity and the maintenance of ecosystem processes or functions are difficult to apply. On this basis, Bunnell et al. (2003) argued that the best surrogate for sustaining the diversity of biological entities is the maintenance of species richness (sensu Whittaker et al. 2001; Gaston and Spicer 2004), although there are some important caveats with this approach and it cannot be applied uncritically to all assemblages in all landscapes and under all circumstances (see Box 1.2).
The Importance of Spatial Scale
Humans can modify environments at several spatial scales (Angelstam 1996). For example, at a regional scale, massive changes can be caused by deforestation or urban expansion. Second, within landscapes completely covered by native vegetation, formerly continuous areas of distinct vegetation types or successional stages (e.g., old-growth forest stands) can be lost or become subdivided. Finally, within given areas of particular kinds of vegetation, structural and floristic elements can be lost (e.g., large fallen logs; Angelstam 1996). The appropriate scale of an investigation or explanation is also related to the species of interest (Wiens 1989; Box 1.3). Importantly, all species are affected by ecological phenomena at multiple spatial scales (Forman 1964; Diamond 1973; Mackey and Lindenmayer 2001). This means there is no single correct scale at which to study landscape change, or at which to mitigate its effects on ecosystems. Throughout the book, we have attempted to explicitly highlight the importance of considering multiple scales.
Putting Landscape Change into Context
A key problem in studies of modified landscapes is that landscape change has been regarded by some workers to be the single dominant reason for why species and populations come to be where they are in a landscape, and the single dominant reason for the loss or decline of species. However, we believe that landscape change needs to be put into context as only one of several factors which influence ecosystem processes, species richness, and the distribution of particular species in a landscape (see Table 1.2). In some cases, factors not directly related to habitat loss, habitat degradation, or habitat subdivision will be the most important ones. For example, the ecological literature is replete with examples of relationships between high latitude or high elevation and low levels of species richness in largely natural landscapes (reviewed by Gaston and Spicer 2004; Figure 1.2). In another example, in parts of Central America, many areas of forest remain relatively intact, yet many species of vertebrates have been lost because of intensive hunting pressure by humans (Redford 1992). Hunting effects have also had major negative impacts on a wide range of North American species, including North American elk (Cervus elaphus), pronghorn (Antilocapra americana), mountain goat (Oreamnos americanus), big-horned sheep (Ovis canadensis), gray wolf (Canis lupus), and grizzly bear (Ursus horribilis) (Mech 1970; Noss and Cooperrider 1994; Cole et al. 1997). Similarly, in some Australian landscapes where patterns of vegetation cover have not undergone major alteration, species loss and population decline have been substantial as a result of the impacts of introduced species such as the cane toad (Bufo marinus; Bennett 1997) and red fox (Vulpes vulpes; Kinnear et al. 2002). The decline of forest birds on the island of Guam is another classic example where landscape change has not been the primary cause of species loss (see Box 1.4).
Single Species or Multiple Species?
This book provides an overview of the wide range of topics associated with habitat loss, changes in landscape patterns, and other changes that typically accompany human landscape modification. A recurring theme throughout this text is that there is an inherent tension between approaches that explore the effects of landscape change on aggregate measures of (multi-) species occurrence, such as species richness or species composition (e.g., island biogeography theory [MacArthur and Wilson 1963, 1967] or nested subset theory [Patterson and Atmar 1986]), and other approaches that focus on the responses of individual species to landscape change (e.g., Lamberson et al. 1994; Ferreras 2001). While the former approach of focusing on aggregate measures of species occurrence groups several species together, single-species investigations often highlight that each individual species responds uniquely to landscape change, habitat loss, and habitat subdivision (e.g., Robinson et al. 1992; Lindenmayer et al. 2002a). Such findings emerge naturally from niche and habitat theory (Hutchinson 1958), which recognize that what constitutes suitable habitat (and hence what constitutes habitat loss) and what constitutes habitat subdivision will be species-specific. However, a detailed focus on the response of an individual species may tell us little about the overall pattern of change in larger assemblages of species, and the management reality is that it is rarely possible to consider more than a handful of individual species in any given area. Conversely, the identification of general patterns involving many species is often particularly useful from a management perspective. A potential problem is that management tools that attempt to predict general species distribution patterns from a few individual species rely on a range of assumptions about the nature of species co-occurrence patterns (e.g., indicator species [Landres et al. 1988; Lindenmayer et al. 2000; Rolstad et al. 2002], focal species [Lambeck 1997; Lindenmayer and Fischer 2003], or umbrella species [Caro 2001, 2004; Sergio et al. 2005]). In all cases there is an inherent tension between finding generalities that apply broadly across many landscapes and many species on the one hand, and accurately predicting the effects of landscape change for single species on the other hand (Harper 1982; Figure 1.3).
Excerpted from Habitat Fragmentation and Landscape Change by David B. Lindenmayer, Joern Fischer. Copyright © 2006 David B. Lindenmayer and Joern Fischer. Excerpted by permission of ISLAND PRESS.
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