Forest Fragmentation

A Point to Be Made

Many long-standing theories concerning forest fragmentation effects stem from island biogeographic conceptualizations of landscapes in which forested ecosystems are viewed as areas of habitat embedded in an uninhabitable matrix of non-forested uses. While intuitively appealing, such a depiction of terrestrial landscapes has severe limitations for a range of reasons (Margules et al. 1982; Kupfer 1995), including its inability to account for variability in habitat quality in both the forested and non-forested habitats (Lindenmayer and Franklin 2002). The “remnants as islands” analog is also conceptually flawed in that it focuses attention almost solely on population (or metapopulation) dynamics within forested habitats while ignoring dynamic linkages and feedbacks with the non-forested habitat. Often, the most important aspects of non-forested habitats, from the viewpoint of ecologists concerned with biodiversity in forest remnants, are whether they provide some form of suitable habitat for forest species and whether species can disperse through the non-forested habitats to other forest habitats.

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A MATRIX-INCLUSIVE APPROACH TO FRAGMENTATION

After a long period in which comparatively little attention had been paid to how processes in the matrix affect longer-term responses of forest remnants to fragmentation, a large number of papers have begun to address the effects of spatial variation on the propagation of ecological entities (e.g., individuals, disturbances) across heterogeneous landscapes (Reiners and Driese 2001) and the role of the matrix as habitat, source, conduit and barrier (e.g., Cantrell et al. 1998; Lomolino and Perault 2001; Lindenmayer and Franklin 2002; Rodewald 2003). The application of island biogeography theory to forest fragmentation rests on the definition of forest remnant area and the distance from other patches irrespective of the role of the matrix in affecting habitat area and patch isolation, and even few applications of metapopulation theory, island biogeography's more recent conceptual cousin, have addressed the importance of matrix effects (e.g., Vandermeer and Carvajal 2001, Murphy and Lovett-Doust 2004 ). The matrix, however, may be neither uniformly unsuitable nor serve as a fully-absorbing barrier to the dispersal of certain forest taxa while the remaining forest areas typically represent a gradient of conditions and habitat suitability. Studies of mosaic landscapes containing a range of old-growth forest, successional habitats, and agriculture (e.g., shifting cultivation systems: Fox et al. 2000; Kupfer et al. 2004) exemplify how landscapes can represent a range of conditions from deforestation to varying degrees of forest degradation in otherwise “intact” forest.

We argue in this paper that ecologists need to move beyond solely documenting fragmentation effects from a remnant-based perspective (e.g., determining whether patch area or isolation have unique effects) to also focus on how processes in the matrix contribute to patterns of species persistence across modified landscapes as a whole. As Wiens (1989) noted,

“(A) focus exclusively on fragmentation of habitats misses the point that it is often the structure of the entire landscape mosaic rather than the size or shape of individual patches (that matters)....”

In fact, we posit that the degree to which “typical” fragmentation effects (i.e., those attributed to area and isolation effects) are exhibited in any given case is as much a function of characteristics of the matrix as those in the remnants. An awareness of the importance of matrix habitats is demonstrated in recent trends in timber harvesting techniques such as structural retention, in which elements such as large diameter trees, snags, and downed coarse woody debris are left during logging to help maintain the quality of the disturbed site, assist the movement of organisms through the harvested area, and facilitate its rapid recovery (Franklin et al. 1997). The importance of matrix-based processes in different landscapes will vary, however, and we believe that it is possible to address their importance by coupling three key features: the extent of modification in a given landscape, the contrast of the matrix and remnant habitats, and the permanence of the matrix disturbance.

The extent and pattern of human-caused changes in a forested landscape (and thus the extent and pattern of the matrix) can be defined by comparing pre- and post-disturbance landscapes (e.g., the amount of canopy area affected by selective logging of canopy trees; Anderson et al. 2000). Forman (1995) described a loosely-ordered set of stages in the modification of landscapes, ranging from perforation and dissection (the initial creation of holes or openings in a forested landscape by patches or linear features, respectively) through fragmentation (in which pieces of forest become isolated from one another) to shrinkage and attrition (where remnants decrease in size and are eventually lost). These stages also correspond to a gradient of potential importance in matrix processes. That is, processes in the matrix are likely to be less important when disturbed areas represent a small percentage of the landscape and remnant forests are widespread and well connected (e.g., a forested landscape with a few small patch cuts vs. a formerly forested landscape dominated by agriculture).

Forman's stages relate to changes in spatial structure of the landscape associated with fragmentation but not necessarily changes in habitat quality in either the matrix (i.e., the clearing may not reduce habitat quality to zero) or the remnants (e.g., increased susceptibility to invasion). McIntyre and Hobbs (1999) provide an alternative approach in which landscapes are represented as being intact, variegated, fragmented, and relictual as the amount of cutting increases; each portion could also be unmodified, modified, highly modified and destroyed. Although they show modification only on edges, one can consider impacts across both the remnants as well as variability in the matrix (e.g., Malanson et al., in review). The effects of the matrix on the remnants and thus landscape dynamics as a whole particularly depend on how similar it is to the remnant and how well it supports fluxes among remnants (e.g., Ricketts 2001). The two are not necessarily directly related.

A number of studies have shown the importance of contrast (often structural contrast of the plant community) between the remnants and the matrix (e.g., Aberg et al. 1995; Marzluff and Ewing 2001), including contrast of the boundary between the two (Ambrose 1987; Collinge and Palmer 2002). We can consider how a matrix functions in these ways with a modified version of Forman's (1995) typology for edge-related processes. One function of the matrix is habitat; the degree to which some forest species can live in or utilize the matrix as alternative habitat will affect landscape functioning. In terms of flux, we can think of the matrix as a conduit (allowing or facilitating the movement of species), a filter (a variably permeable membrane allowing selective movement of species), a source (serving as an origin of individuals that move into the remnants), a sink (accepting individuals from the remnants but not allowing them to leave; e.g., as a result of predation) or a nonpermeable barrier blocking all movement. Many of these functions are determined as much or more by structural contrast between the matrix and remnant areas, and previous research has even shown that type of disturbance within landscapes may influence community composition and relative abundance more than extent of disturbance through its effect on the habitat contrast (Rodewald and Yahner 2001) . In cases with an extensive matrix that offers little for forest species, the forest remnants do essentially become habitat islands (e.g., Driscoll 2004). In many cases, however, the matrix will serve more as a filter or conduit than a barrier.

Finally, although the primary focus in fragmentation research tends to be on the pattern of fragmentation and the resulting effects on remnant forest ecosystems, non-forested ecosystems in fragmented landscapes are not static and may, through plant succession, increasingly come to resemble the forested remnants (e.g., Moran and Brondizio 1998). The permanence of the disturbance to the matrix depends primarily on the nature of the disturbance itself, including its initial impact and any continuing effects. This is important because the dynamics of a forest fragmented by agriculture or urban development may be vastly different from those associated with the mosaic of mature and regenerating stands resulting from timber harvesting. The first situation represents cases where the modified habitat may be indefinitely affected and have little or no habitat value for forest species whereas the latter situation may involve only a temporary reduction in habitat for species that rely on mature forests. Changes in forest structure and composition are relatively rapid following disturbances that primarily impact forest canopies but considerably slower following disturbances that heavily impact soils as well as aboveground vegetation, such as bulldozing, heavy or long-term grazing, and severe fires (Chazdon 2003).