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Global Ecology and Biogeography, (Global Ecol. Biogeogr.) (2006) 15, 8 – 20
Blackwell Publishing, Ltd.
RESEARCH
REVIEW
Not seeing the ocean for the islands: the
mediating influence of matrix-based
processes on forest fragmentation effects
John A. Kupfer1,*, George P. Malanson2 and Scott B. Franklin3
1
Department of Geography and Regional
Development, University of Arizona, Tucson,
AZ 85721, USA (Present address: Department
of Geography, University of South Carolina,
Columbia, SC 29208, USA), 2Department of
Geography, University of Iowa, Iowa City,
IA 52242, USA and 3Department of Biology,
University of Memphis, Memphis, TN 38152,
USA
*Correspondence: Dr John Kupfer,
Department of Geography, 709 Bull Street,
Room 127, Columbia, South Carolina 29208,
USA. E-mail: [email protected]
ABSTRACT
The pervasive influence of island biogeography theory on forest fragmentation
research has often led to a misleading conceptualization of landscapes as areas of
forest/habitat and ‘non-forest/non-habitat’ and an overriding focus on processes
within forest remnants at the expense of research in the human-modified matrix.
The matrix, however, may be neither uniformly unsuitable as habitat nor serve as
a fully–absorbing barrier to the dispersal of forest taxa. In this paper, we present
a conceptual model that addresses how forest habitat loss and fragmentation affect
biodiversity through reduction of the resource base, subdivision of populations,
alterations of species interactions and disturbance regimes, modifications of microclimate and increases in the presence of invasive species and human pressures on
remnants. While we acknowledge the importance of changes associated with the
forest remnants themselves (e.g. decreased forest area and increased isolation of
forest patches), we stress that the extent, intensity and permanence of alterations to the
matrix will have an overriding influence on area and isolation effects and emphasize
the potential roles of the matrix as not only a barrier but also as habitat, source and
conduit. Our intention is to argue for shifting the examination of forest fragmentation
effects away from a patch-based perspective focused on factors such as patch area
and distance metrics to a landscape mosaic perspective that recognizes the importance
of gradients in habitat conditions.
Keywords
Dispersal, forest fragmentation, habitat quality, heterogeneity, island biogeography,
matrix, spatial landscape ecology.
Many long-standing theories concerning the ecological effects of
forest fragmentation stem from conceptualizations of landscapes
in which forested ecosystems are viewed as islands of habitat
embedded in an uninhabitable matrix of non-forested uses
(Haila, 2002). While intuitively appealing, such a depiction 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 & Franklin, 2002). The ‘remnants as islands’ analogue 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 (e.g. Koelle & Vandermeer,
2005). Often, the most important aspects of non-forested
habitats, from the viewpoint of ecologists concerned with
biodiversity in forest remnants, are whether they provide
adequate habitat for forest species and whether species can disperse
through the nonforested habitats to other forest habitats.
The purpose of this paper is to present a conceptual model of
forest fragmentation effects that addresses not only aspects of the
forest remnants themselves (recently reviewed and critiqued by
Fahrig, 2003), but how the effects of changing forest area and isolation are mediated by a range of matrix effects identified by recent
studies. Throughout this paper, we use the terms ‘remnants’ for forest
areas not affected by specific human-caused disturbances and
‘matrix’ for the disturbed areas, which may include recently logged
areas, agricultural fields, pastures and other areas of human disturbance (Fig. 1). Although this usage of matrix does not necessarily
agree with other formal definitions of this term (e.g. the matrix
defined as the most extensive or dominant area; Forman, 1995),
it is intuitive and matches the usage of Lindenmayer & Franklin
(2002), who have theorized it most effectively. This definition also
recognizes that the matrix can take on a variety of forms in a given
landscape and can contain a range of varying habitat quality.
8 DOI: 10.1111/j.1466-822x.2006.00204.x
© 2006 Blackwell Publishing Ltd www.blackwellpublishing.com/geb
INTRODUCTION
Matrix-based processes and forest fragmentation effects
Figure 1 Examples of modified matrixes. Top left: row cropping of Phaseolus near Hill Bank, Belize (photograph by John Kupfer); top right:
active slash and burn milpa (field) near Indian Church, Belize (photograph by John Kupfer); middle left: recent selection cut of floodplain forest
along the White River, Arkansas, USA (photograph by John Kupfer); middle right: aspen (Populus tremuloides) regrowth on a former clear cut in
Pseudotsuga menziesii forest in Valles Caldera National Preserve, New Mexico, USA (photograph by Jeff Balmat); bottom left: urbanized area in
Tela, Honduras (photograph by John Kupfer); bottom right: fern regeneration on a boreal forest clearcut and subsequent burn, southwestern
Newfoundland, Canada (photograph by Scott Franklin).
It is also important to clarify that forest fragmentation can
refer to either the broad process of forest loss and isolation or
more specifically to changes in the spatial configuration of forest
remnants that are a result of deforestation. Bunnell (1999),
Fahrig (1997, 1999, 2003) and Haila (1999), among others, argue
that the ecological consequences of habitat loss (deforestation)
and isolation (fragmentation, in a narrow sense) need to be
distinguished. While we agree that this distinction is important
from a scientific standpoint and believe that it may be possible to
separate such effects in simulations and some controlled experiments (Caley et al., 2001; Davies et al., 2001), it is difficult or
impossible to do so in studies of natural ecosystems. Further, we
contend that all effects should be considered collectively in an
attempt to understand forest fragmentation in the broader,
Global Ecology and Biogeography, 15, 8– 20 © 2006 Blackwell Publishing Ltd
9
J. A. Kupfer et al.
Figure 2 Conceptual model of forest
fragmentation effects, modified from
Zuidema et al. (1996) and Lindenmayer &
Franklin (2002) to incorporate matrix effects.
process-based sense of the term, and rather than trying to isolate
each, we include their interaction across a range of effects. Finally,
although the term ‘biodiversity’ has many meanings, we focus
primarily on aspects at the population and community levels
(e.g. issues of species persistence and extinction, changes in species richness) because these are the levels at which most land
stewards make their decisions.
A CONCEPTUAL MODEL OF FRAGMENTATION
EFFECTS
Based in part on principles from island biogeography theory,
metapopulation models and source–sink dynamics, Zuidema
et al. (1996) developed a conceptual model that identified four
effects of forest fragmentation leading to the loss of biodiversity.
These effects focused primarily on the effects of pattern on processes in the remnants. Here, we build on their scheme by rethinking the effects of fragmentation and incorporating additional
matrix heterogeneity concepts (Fig. 2). We chose this starting
point because it includes an explicit pattern-process connection
and because it is at a landscape scale appropriate for management decisions.
Forest fragmentation: effects of forest remnant area,
isolation and edge
The process of forest fragmentation results in three distinct
changes in forest ecosystem pattern: reduced forest area, increased
isolation of resulting remnants, and the creation of edges where
remnant forest abuts modified ecosystems. Each of these
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influences a range of population, community and ecosystem
processes that may affect biodiversity. In this section, we assume
an inhospitable matrix, focusing attention solely on characteristics of the forest remnants.
Reductions in forest area
Island biogeography (MacArthur & Wilson, 1967) provides a
starting point in evaluating the effects of forest loss associated
with deforestation. Habitat loss has two consequences: (1) a
reduction in the range of habitat types results in fewer species;
and (2) a reduction in resources for species leads to smaller
populations that are more vulnerable to extinction. The latter is
related to niche packing and the inability of species to subdivide
a habitat into extremely narrow niches if the total resources are
limited. We represent these effects as changes in the resource base
in Fig. 2.
Changes in the resource base are linked inextricably to the
subdivision of populations. Smaller populations lead to higher
extinction rates (although such effects may not be immediately
manifested; Tilman et al., 1994) and changes in population
genetics (e.g. Templeton et al., 1990; Ricklefs & Miller, 2000).
Changes in trophic relations (Zanette et al., 2000), competition
(Huxel & Hastings, 1998), dispersal vectors (Cordeiro & Howe,
2001) and other species interactions can also lead to population
declines. Abundances of many species have thus been linked to
forest habitat loss, although local extinction following fragmentation is species-specific as some taxa are more vulnerable than
others (e.g. Newmark, 1991; Kattan et al., 1994; Marsden, 1998;
Henle et al., 2004).
Global Ecology and Biogeography, 15, 8 –20 © 2006 Blackwell Publishing Ltd
Matrix-based processes and forest fragmentation effects
The amount of forest removed and the resulting patch sizes
may constrain the types, extents, frequencies and even intensities
of disturbances. Some types of disturbances may depend directly
on area; for example, small forest remnants in non-forested
landscapes may be more subject to wind destruction than comparably sized areas in continuous forest (Esseen, 1994). Specific
disturbance characteristics such as fireline intensity may be
influenced by the amount of area over which the disturbance
develops, and the area of a disturbance per se can be part of a disturbance regime. The combination of these aspects of disturbance
regime will affect the dynamics of species and the resulting
biodiversity (e.g. Malanson, 1984).
The degree to which area-related changes occur is a function
of not only the total amount of forest loss, but also (1) the size of
remaining forest remnants, and (2) where in a landscape the
destruction occurs. These latter two effects are correlated with,
but not typically the same as, the total area of habitat destruction,
although all are components of reduced forest area. Even in a
continuously forested landscape, spatial structure (e.g. pattern of
heterogeneity or interspersion of forest types) is present. Rarely
are patterns of deforestation across a landscape random (Thiollay
& Meyburg, 1988; Kupfer & Franklin, 2000), so the spatial selectivity of forest clearing affects the degree to which forest fragments represent the original forest. Further, populations in the
remnants are merely a sample of the original population and may
not be representative of the precut populations, not only in terms
of numbers but also in terms of a range of other characteristics
(e.g. genetic diversity and demographic structure) (Terborgh
et al., 1997).
Increased isolation of forest remnants
Landscape structure has been shown to affect plant and animal
dispersal for colonization, movement patterns and gene flow.
Changes in population dispersal and dynamics ultimately affect
community assembly and interactions. Processes occurring at
the population and community level drive landscape-level processes (e.g. metapopulation dynamics) while landscape pattern
(e.g. distance to seed pool) constrains population (e.g. extinction) and community (e.g. succession) processes. The spatial
adjacency of existing patches, especially source patches, is an
important factor affecting animal movement so if landscape
fragmentation is severe, the adjacency of existing habitat patches
is crucial for maintaining populations. Isolation through
fragmentation is also important because it forces more crosslandscape movement (Forman, 1995), which may be energetically costly or increase risks of predation (one factor that makes
matrix habitat a barrier to dispersal).
Given an uninhabitable matrix, the immigration rate to an
isolated forest remnant will generally be lower than an equal area
surrounded by contiguous forest habitat. Fragmentation can
thus lead to the subdivision of a single population into multiple
disjunct but interacting subpopulations (e.g. a metapopulation)
or, if isolation is extreme enough, individual non-interacting
populations, which has attendant ramifications associated with
small population sizes. Further, because many disturbances are
spatially contagious processes, isolation and the resulting subdivision of populations may alter disturbance frequency and thus
threats from environmental fluctuations and catastrophes
(Akcakaya & Baur, 1996; Perkins & Matlack, 2002).
Isolation effects may feedback to affect species interactions.
Species that could interact because they were in a contiguous
area may no longer be able to do so because of isolation, and species interactions can be altered to the extent that local extinctions
may occur if some species are missing and cannot be rescued by
immigration. Among a group of interacting species, a particular
spatial configuration will mean different degrees of isolation
depending on the mobility of the species, and individuals may be
in a remnant without a primary predator or competitor. Such
effects may be interrelated to changes in area. For example, if top
carnivores are left on small remnants they are likely to become
locally extinct should the remnants be too isolated for interpatch
movement. Competition may also be affected if species are
released from competitors by local extinctions (Hanson et al.,
1990; Huxel & Hastings, 1998), and mutualisms or commensalisms can be lost if one interacting species becomes locally extinct
and cannot be rescued (e.g. Anstett et al., 1997).
Edge effects
Forest fragmentation results in the increased susceptibility of
forest remnants to edge effects: changes in microclimate, forest
structure, biotic composition and ecological function that occur
along forest edges exposed to non-forested habitats. Edges are
distinguished primarily by changes in microclimate, including
incoming solar radiation, temperature, wind speed and evapotranspiration (Ranney et al., 1981). A wide range of studies
have documented the depth to which various microclimatic edge
effects may be observed (e.g. Baker & Dillon, 2000), but it is
important to recognize that edges represent gradients of multiple
physical factors (e.g. Cadenasso et al., 1997) rather than discrete
forest communities. It is also important to note that edge effects,
while related to the effects of reduced forest area through the ratio
of edge:interior habitat, are a function of spatial configuration
independent of habitat amount.
Changes in abiotic conditions along edges lead typically to
biotic responses. Edge species are those adapted to edge microclimates and are often species found in the early stages of forest
succession in a given region (cf. Kupfer & Malanson, 1993). The
responses of forest interior species to conditions that develop
along the new edge vary, but some species are unable to survive
in the newly created conditions. For these species, the amount of
habitat lost to fragmentation is greater than that simply converted.
Invasions by exotic species may also be enhanced by habitat
destruction (Medley, 1997), as many invasive plant species are
prolific seed producers that thrive in higher light conditions and
have widely dispersed seeds. These traits make them more likely
to establish and thrive along edges, after which they may be able
to play an increasing role in vegetation dynamics within remnant
interiors (Burke & Nol, 1998).
Finally, edge effects alter species interactions by increasing the
degree of interaction among edge and interior species (e.g. brood
Global Ecology and Biogeography, 15, 8– 20 © 2006 Blackwell Publishing Ltd
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J. A. Kupfer et al.
parasitism and nest predation of bird populations; Gustafson
et al., 2002). Fagan et al. (1999) synthesized recent empirical
results concerning the effects of habitat edges on population
dynamics with contemporary theoretical developments to outline the ways in which species interactions, and the dynamics of
the communities in which they are embedded, can be changed.
Interactions among area, isolation and edge
The effects of changes in forest extent and spatial configuration
are not independent. Isolation, for example, tends to be a
function of area in that the two change simultaneously (albeit
non-linearly) for a given landscape (i.e. there is no isolation in real
landscapes without a simultaneous loss of area; Andrén, 1999;
Fahrig, 2003). When isolation is low, the remnants function as a
larger whole. Further, tests of island biogeography theory and
metapopulation models have demonstrated that patch area affects
their role as target and thus the rate of immigration because
larger patches are easier to locate (e.g. Lomolino, 1990). Finally,
there is arithmetic and geometric dependence between the proportionate importance of edge effects in a patch and its area and
shape. Managers are often interested in maintaining the species
that existed in an area prior to its becoming a remnant; because
of edge effects, these species and their functions may be confined
to a core. Edge–interior relationships for a forest fragment are
thus often quantified using metrics such as core-area models,
which provide an approximation of total edge and core habitat
based on a measure of edge penetration depths and forest
geometry (e.g. Laurance, 1991).
Forest fragmentation: the role of the matrix
The consequences of altering forest area, isolation and edge have
generally been conceptualized using a binary division of the
landscape into forested vs. non-forested habitats. Fragmentation
effects have thus often been interpreted with an explicit focus on
changing forest patterns, as we have discussed in the previous
section. In this section, we consider how the effect of pattern on
process is moderated by matrix characteristics. Specifically, as we
discuss the effects of a diminished resource base, population subdivision, altered disturbance regimes, modified microclimate,
and increased presence of invasive species and human pressures
on remnants, we do so with the understanding that the extent to
which each of these affects diversity in forest remnants is dependent on processes in the matrix.
Effects of the matrix on the resource base
When the matrix is uninhabitable, changes in the resource base
associated with forest loss and fragmentation are most closely
tied to reductions in forest area. However, the degree to which
the matrix differs from the remnants alters the resource base
differently for different species (Brotons et al., 2003), and the
definition of patch size (equated typically with forest patch size)
becomes less obvious if species are able to use the matrix as
feeding or nesting habitat (Sisk et al., 1997). Species differ in the
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degree to which they are habitat specialists or generalists, and
some may experience little difference between a remnant and the
matrix even if the matrix differs substantially in structure, while
others respond to even minor differences (Krauss et al., 2003;
Wethered & Lawes, 2003). Thus the functional notion of remnant
and matrix is dependent on the scale of operation for specific
species and may not always be clear.
Several studies have found that populations in forest remnants are
influenced strongly by the surrounding habitat (e.g. Hinsley et al.,
1995; Stouffer & Bierregaard, 1995; Metzger, 2000; Lindenmayer
et al., 2001), although the effects have sometimes been attributed
to an increased resource base in terms of additional foraging or
breeding area (discussed here), increased immigration through
the matrix (discussed in the next section) or some combination
of the two. Studies of avian populations, in particular, have cited
the importance of an expanded resource base in the matrix as a
factor determining the presence of specific species in remnants
(e.g. Jokimaki & Huhta, 1996; Sisk et al., 1997), and some species
may even be able to compensate for a loss of their natural forest
habitat by moving into other habitat types (e.g. plantations or
other modified habitats) (Norton et al., 2000; Cook et al., 2002).
Gascon et al. (1999) provided one of the most comprehensive
assessments of matrix effects on the resource base in their study
of birds, frogs, small mammals and ants in fragmented central
Amazonian landscapes. Their results showed that for all four
groups, a high proportion of nominally primary-forest species
were detected in matrix habitats, with 8–25% of species in each
group found exclusively in the matrix. The three vertebrate
groups (birds, small mammals, frogs) exhibited positive and
significant correlations between matrix abundance and vulnerability to fragmentation, suggesting that species that avoid the
matrix tend to decline or disappear in fragments, while those
that tolerate or exploit the matrix often remain stable or increase.
Effects of the matrix on population subdivision
Population subdivision is typically associated with increased isolation of forest remnants during the process of forest fragmentation; therefore, fragmentation effects should be affected by the
characteristics of the adjacent habitat as it influences interremnant movement (Lord & Norton, 1990; Wiens, 1994). Plant
and animal movement across fragmented landscapes depends
not only on the species’ dispersal ability and the isolation and
characteristics of suitable patches but also on species’ responses
to potential movement barriers. Specifically, the isolation of forest
habitat is based not only on a species’ means of dispersal but also
its tolerance to matrix habitat and edges, which can vary in their
effectiveness as landscape barriers. When species are readily able
to cross the matrix, the amount of available habitat decreases
largely as a function of total remaining forest, whereas available
habitat within a matrix more restrictive to movement decreases
at a disproportionately greater rate than the rate of forest clearing
(Dale et al., 1994). These results match other modelled expectations based on life-history trade-offs between competitive ability
and dispersal ability (Tilman et al., 1997; Malanson, 2002). The
matrix can thus reduce the effect of isolation on population
Global Ecology and Biogeography, 15, 8 –20 © 2006 Blackwell Publishing Ltd
Matrix-based processes and forest fragmentation effects
subdivision if it is not impassible, as often seems to be the case in
terrestrial landscapes.
As with the effects of the matrix on the resource base, a number
of studies have focused on the influence of matrix quality on
interpatch isolation. Renjifo (2001), for example, found an
increased connectivity among forest remnants surrounded by
exotic-tree plantations compared to pastures and observed that
structurally complex anthropogenic matrices have potential as
management tools for bird conservation by complementing habitat protection and restoration. Although the distinct contribution of matrix permeability to reducing isolation (as opposed to
increasing resource base) cannot always be demonstrated clearly
(Cooper et al., 2002), the well-documented existence of speciesspecific responses to changes in habitat quality and arrangement
necessitates recognition of the varying resistance of the matrix to
species’ movements rather than a simple measure of isolation
such as interpatch distance (Tischendorf et al., 2003; Verbeylen
et al., 2003). Metrics based on life history strategies (r-selected,
K-selected species), behaviour (social vs. solitary) and ecological
tolerance (generalist vs. specialist) may help predict the effects of
fragmentation on communities (e.g. Wolff, 1999).
The extent to which fragmentation results in the subdivision
of populations revolves around the level of isolation actually
caused by changes in the matrix from the pre-disturbance conditions, which raises an important distinction between structural
and functional connectivity. The former refers to the degree of
habitat connectedness and so is clearly altered by fragmentation.
Functional connectivity, while related to structural connectivity,
refers to the ease with which organisms move across a landscape
and also includes the effects of matrix quality, stepping stones,
width and quality of corridors, gap-crossing willingness of the
species and the ways in which species move along or across edges
(Tischendorf & Wissel, 1997; Malanson & Cramer, 1999; Haddad,
2000; Sondgerath & Schroder, 2001; Bélisle & Desrochers, 2002;
Bowman & Fahrig, 2002; Bakker & Van Vuren, 2004). At low
levels of forest loss, structural and functional connectivity both
remain high. Structural connectivity changes most rapidly
around some threshold value (e.g. 60% forest cover in random
landscapes; lower values in real landscapes; Andrén, 1994), but
functional connectivity may remain higher until much lower
levels of remaining habitat (e.g. Andrén, 1999). Measures of
structural connectivity are thus important only to the extent that
they capture the underlying functional connectivity, unless they
can somehow account for the heterogeneity of the matrix as well.
Effects of the matrix on disturbance regimes
While patch area and isolation affect the size, frequency and
intensity of disturbances in forest remnants (Baker, 1989), alterations of a remnant’s disturbance regime may be determined to a
similar or even greater degree by the nature of the matrix, which
may serve as a source of disturbance or modify the degree of
isolation. The probability of both fire ignition and rate of fire
spread, for example, have been shown to differ across different
types of human-modified habitat (Cochrane et al., 1999; RománCuesta et al., 2003). As with population subdivision, the degree
to which a matrix operates as a barrier, filter or conduit will vary
with the type of disturbance. It is even possible that the matrix
can act as a super-conduit for disturbance, increasing the rates of
disturbance transmission over what would have occurred in a
contiguous forest, as was probably true for the extensive fires in
Wisconsin in 1871, following logging that left a matrix of fine
dead wood. Research in western and northern forests has
similarly linked the amount of cutover area and forest susceptibility to bark beetle and fungal attacks (Franklin & Forman,
1987).
Effects of the matrix on microclimate
Forest fragmentation significantly alters microclimate across the
landscape (e.g. Olejnik et al., 2002; Laurance, 2004), which in
turn can feed back to influence disturbance regime (Holdsworth
& Uhl, 1997). The degree to which the matrix contrasts with the
remnant in structure, roughness, albedo and evapotranspiration
will modify microclimatic (and thus biotic) edge effects (e.g.
Matlack, 1994; Cadenasso et al., 2003). Didham & Lawton (1999),
for example, found that edge penetration distances for most
microclimate and vegetation structure variables were as much as
two to five times greater at open, fire-encroached forest edges
than at closed, non-fire-encroached edges in central Amazonian
forest remnants. The matrix can thus reduce the effect of small
size and edge on microclimate, if the matrix’s structure, albedo
and roughness are similar to that of the remnant forest.
Effects of the matrix on invasive species
With (2002) noted that landscape structure (and by association,
fragmentation) may affect the spread of invasive species and the
invasibility of communities by enhancing spread above some
threshold level of landscape disturbance directly or indirectly
(e.g. through landscape effects on dispersal vectors), interacting
with the distribution of invasive species to facilitate spread, promoting or altering species, interactions in ways that enhance the
invasibility of communities, compromising the adaptive potential of native species to resist invasion or enhancing the adaptive
response of invasive species and interacting with the dynamics of
the disturbance architecture to create spatiotemporal fluctuations in resource availability, which enhance system invasibility.
Many of these processes are directly affected by the nature of the
intervening matrix, and as with native species the matrix can
serve as a habitat, conduit, barrier or filter for invasive species. In
fact, as a habitat or conduit the matrix may function better, from
the perspective of the invasive species, than would a contiguous
forest.
Previous studies have proposed that forest fragments, often
disturbed by winds and other factors from the surrounding
matrix, may be prone to invasions of species adapted to recurring
disturbance (Janzen, 1983; Laurance, 1997). Indeed, studies of
exotic species mobility in the landscape corroborate this relationship (National Research Council, 2002). Many highly invasive exotics have similar life histories to ‘fragmentation-positive’
taxa, including generalist habitat preferences and adaptations for
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J. A. Kupfer et al.
disturbances, such as high dispersal ability (Bazzaz, 1986; Usher,
1988; Rejmánek & Richardson, 1996). The invasion of exotic
species whose primary use is aesthetic may be ignited by their
intensive use in urban centres (e.g. privet, Ligustrum sinense and
L. vulgare, Rudis and Jacobs, in review). The matrix may also
serve as habitat for generalist predator species that cause mortality in patch-resident prey species (Cantrell et al., 2001). Thus
disturbance, in general, tends to result in increased numbers of
exotics (Pyle, 1995).
Effects of the matrix on human pressure
The matrix is fundamentally associated with human pressure,
but the nature of human land use in the matrix will affect
fragmentation impacts. Where forest clearing is for forestry or
agriculture, increased human pressures on remnants may come
from hunting or fuelwood gathering (An et al., 2001; Bousquet
et al., 2001). Unless the remnants are small, edges will generally
have more activity than the interior. Cochrane (2001), for example, demonstrated that fire occurrence in two areas in the eastern
Brazilian Amazon was concentrated disproportionately near
forest edges, suggesting a synergism between forest fragmentation and fire that has been seen elsewhere. Where cutting is due
to land use such as housing, human pressures in terms of light,
noise, pets and local recreation will also have a strong distance
decay (Theobald et al., 1997).
The role of matrix extent, contrast and recovery
The importance of matrix-based processes in different landscapes will vary, but we believe that it is possible to generalize
their importance by coupling three key features: the extent and
pattern of modification in a given landscape, the contrast of the
matrix and remnant habitats, and the permanence of the matrix
disturbance.
Matrix extent, pattern and quality
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 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 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). As the matrix
becomes more extensive in a landscape its potential importance
14
as habitat grows, and the importance of matrix permeability (e.g.
as it affects interpatch movements) increases. Thus, the value of
agricultural fields as habitat or dispersal avenues may be of less
concern in cultivated areas where clearings represent a small percentage of the landscape than in areas where agricultural extensification leaves only scattered forest remnants in a primarily
agricultural landscape.
Forman’s stages relate to changes in the spatial structure of the
landscape associated with fragmentation but not necessarily to
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 & Hobbs (1999)
provide an alternative approach in which landscapes are represented as intact, variegated, fragmented or 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. The effects of
the matrix on the remnants and thus landscape dynamics as a
whole depend particularly on how similar it is to the remnant
and how well it supports fluxes among remnants (e.g. Ricketts,
2001), which are not necessarily directly related.
Matrix contrast
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. Åberg et al., 1995; Marzluff &
Ewing, 2001), including contrast of the boundary between the
two (Collinge & Palmer, 2002). We can consider the varied roles
of matrix functions 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 individuals), 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) or a non-permeable barrier, blocking all movement. Many of these functions are determined by structural
contrast between the matrix and remnant areas, and previous
research has even shown that the type(s) of disturbance within a
landscape may influence community composition more than
disturbance extent through its effect on habitat contrast (Rodewald
& Yahner, 2001). In cases with an extensive matrix that offers
little for forest species, the forest remnants do essentially become
habitat islands (Driscoll, 2004), but in many or most cases the
matrix will serve as a filter, allowing selective passage of certain
species while restricting others.
Matrix recovery
In most studies of fragmentation, the matrix has been treated not
only as uninhabitable but also static. This may effectively be true
in some cases (e.g. a densely urbanized matrix) but not in others
(e.g. selectively logged or cultivated areas). In the first situation,
Global Ecology and Biogeography, 15, 8 –20 © 2006 Blackwell Publishing Ltd
Matrix-based processes and forest fragmentation effects
the modified habitat may be affected indefinitely 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. In general, changes in forest structure
and composition are relatively rapid following disturbances that
impact primarily forest canopies but considerably slower following disturbances that heavily impact all aboveground vegetation
or, even more so, soils, such as bulldozing, heavy or long-term
grazing and severe fires (Chazdon, 2003).
Recovery of the matrix will be through the process of plant
succession, which is driven by processes of mortality, colonization and the abiotic characteristics of the area, all of which may
be altered significantly by forest fragmentation. Much has been
written about mechanisms of plant succession (e.g. Glenn-Lewin
et al., 1992), and we have no intention of summarizing this literature. Instead, we want to emphasize that the role of the matrix
in shaping fragmentation effects will change through time so
that, for example, the effects of changes in spatial pattern on the
resource base or the subdivision of populations vary with the
recovery of the matrix.
The state of the matrix following disturbance as well as its
subsequent recovery are a function of the amount of vegetation
removed, the presence of regenerating roots, stems and seeds
(e.g. advanced regeneration), and the availability of propagules
from offsite sources. The first two factors are largely a function of
the severity of the disturbance, while the latter factor, which is a
key determinant of colonization (Hewitt & Kellman, 2002), is
influenced by the spatial arrangement of seed sources in the
mosaic of remnant and disturbed areas and the response of dispersal agents to landscape structure (e.g. Darley-Hill & Johnson,
1981; McClanahan & Wolfe, 1987; Hughes & Fahey, 1988). Seed
density generally decreases with distance from a seed source, and
patterns of arrival will vary by species because of differences in
seed morphometry. Patterns of seedling establishment and, as a
result, plant recruitment and colonization, during recovery will
vary spatially because seeds will not be dispersed uniformly into
the adjacent matrix from the remnants (McClanahan, 1986; Silva
et al., 1996; Kupfer et al., 2004). In addition, predation of seeds
and seedlings will vary spatially (Ostfeld et al., 1999).
There are a number of direct feedbacks between recovery
processes in the matrix and population processes in the remnants. Dispersal of seeds from the matrix may subsequently
affect recruitment in remnant forests (e.g. Cadenasso & Pickett,
2000), and edges may influence seed banks, recruitment and
population dynamics within forest interiors, at least temporarily,
by acting as seed sources for shade-intolerant plant species in
forest remnants (Ranney et al., 1981; Matlack, 1993; Kupfer &
Runkle, 1996, 2003; Kupfer et al., 1997; Laurance, 1997; Goldblum
& Beatty, 1999; Landenberger & McGraw, 2004).
Overall, the dynamics of the matrix affect remnants by
increasing the area of remnants as matrix adjacent to remnant
edge approaches full recovery, by reducing isolation as forest
recovery increases the permeability of the intervening matrix,
and by changing the edge effect as edge becomes interior. These
effects are interactive and non-trivial; for example, the contrast
between the matrix and remnants and how quickly the matrix
recovers following habitat destruction may govern the extinction
debt (sensu Tilman et al., 1994), because extinctions can potentially be prevented if recovery occurs quickly enough.
A MATRIX-INCLUSIVE APPROACH TO
FRAGMENTATION
Despite the widespread recognition that forest fragmentation is a
pressing environmental issue, there continues to be an active
debate concerning the most appropriate areas for future study of
fragmentation effects and the manners by which theory can be
applied to real-world situations (Villard, 2002). Whether the
explicit focus has been on specific species or more generally on
biodiversity, research on the effects of deforestation and forest
fragmentation has concentrated typically on pattern and process
within habitat remnants. Understanding the effects of forest loss
on biota in the remnants is critical to assessing impacts on the
diversity of species and community composition, and a considerable amount of work on forest fragmentation has stressed how
declining forest area and/or increasing isolation of forest remnants influences both richness and species survival. The underlying theory (stemming from applications of island biogeography
and metapopulation theory) is that as forests are fragmented,
populations decline in size and become increasingly isolated
from one another, leading to higher extinction rates, lower
immigration rates and lower species richness in remaining
forests.
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 how the matrix affects
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 & Carvajal, 2001). The matrix may be neither
uniformly unsuitable nor serve as a fully absorbing barrier to the
dispersal of forest taxa, while remaining forest areas represent
typically a gradient of conditions and habitat quality. Studies of
mosaic landscapes containing old-growth forest, successional
habitats and agriculture (e.g. Fox et al., 2000; Perfecto & Vandermeer, 2002) exemplify how landscapes can represent a range of
conditions from deforestation to varying degrees of forest degradation in otherwise ‘intact’ forest. 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 number of papers have: (1) addressed the effects
of spatial variation on the propagation of ecological entities (e.g.
individuals, disturbances) across heterogeneous landscapes
(Reiners & Driese, 2001), (2) highlighted the diverse roles of the
matrix in fragmented landscapes (e.g. Cantrell et al., 1998;
Lomolino & Perault, 2001; Rodewald, 2003), and (3) argued that
management in fragmented landscapes needs to focus as much
or more on processes in the intervening matrix (Crome, 1997;
Franklin et al., 1997; Hobbs, 2001; Lindenmayer & Franklin,
2002; Lomolino & Smith, 2003).
Patch-orientated measures of fragmentation such as patch
area or interpatch distance have become popular because they
Global Ecology and Biogeography, 15, 8– 20 © 2006 Blackwell Publishing Ltd
15
J. A. Kupfer et al.
are perceived to capture the effects of a diminished resource base
or population subdivision and because they are easy to implement with modern geospatial tools (e.g. geographical information systems). However, 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 focus on how processes in the matrix contribute to patterns of species persistence
across modified landscapes as a whole. In fact, we posit that the
degree to which ‘typical’ fragmentation effects (i.e. those attributed
to area and isolation) are exhibited in any given case is as much a
function of characteristics in the matrix as those in the remnants.
In particular the extent, degree and permanence of alterations to
the matrix have a tremendous and under-appreciated potential
to mediate the negative effects of changes in forest area, isolation
and edge. Such a view argues for greater emphasis on the spatial
and temporal dynamics of deforestation, including not only
patterns of forest loss and fragmentation, but also spatial heterogeneity of recovery (e.g. Messina & Walsh, 2001). Recognizing
the variability of conditions in the matrix and its potential role as
habitat, source, conduit and filter (rather than as a simple barrier)
also helps to shift the examination of forest fragmentation
effects away from a patch-based perspective (focused on factors
such as patch area and distance metrics) to a landscape mosaic
perspective that recognizes the importance of gradients in
habitat conditions (e.g. Murphy & Lovett-Doust, 2004).
ACKNOWLEDGEMENTS
This research was supported by a grant to the authors from the
National Commission on Science for Sustainable Forestry and
made possible by a fellowship to JAK from the Udall Center for
Studies in Public Policy at the University of Arizona. We appreciate valuable comments by John Vandermeer and an anonymous
reviewer.
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BIOSKETCH
John Kupfer is an Associate Professor of Geography at
the University of South Carolina. His research focuses on
landscape-level pattern-process relationships in forest and
rangeland ecosystems, with a particular emphasis on
forest fragmentation effects. George Malanson is
Coleman-Miller Professor of Geography at the University
of Iowa. He is a biogeographer whose research focuses on
biodiversity through modeling landscapes undergoing
change due to direct or indirect human activity. Scott
Franklin, an Associate Professor in the Biology
Department at the University of Memphis, is a plant
community ecologist focusing on disturbance and
vegetation dynamics. His research is applied, integrating
management questions into studies of exotic species,
stream channelization, and slash and burn agriculture.
Current research focuses on restoring canebrakes
(Arundinaria gigantea) in southeastern US and bamboo
regeneration in China's Qinling Mountains.
Editor: David Currie
Global Ecology and Biogeography, 15, 8 –20 © 2006 Blackwell Publishing Ltd