Boundaries and Corridors as a Continuum of Ecological Flow Control

Review
Boundaries and Corridors as a Continuum of
Ecological Flow Control: Lessons from
Rivers and Streams
LINDA M. PUTH* AND KAREN A. WILSON†
*Department of Botany, University of Wisconsin–Madison, 430 Lincoln Drive, Madison, WI 53706, U.S.A., email
lmputh@students.wisc.edu
†Center for Limnology, University of Wisconsin–Madison, 680 North Park Street, Madison, WI 53706, U.S.A.
Abstract: Landscape boundaries and corridors are areas of small spatial extent relative to their large effects
on ecological flows. The trend in ecological literature is to treat corridors and boundaries as separate phenomena
on the landscape. This approach, however, misses a fundamental aspect they have in common: their
strong influence on ecological flows. Corridors and boundaries exist at opposite ends of a permeability gradient,
differing in their effects on rates and direction of flow. The position of landscape structures along this permeability
gradient depends on attributes of both the flow and of the structure itself. We discuss boundaries
and corridors in terms of mover specificity, scale, and effects on different levels of ecological organization, using
rivers and streams to illustrate our points. We predict which structures will act as boundaries or corridors
and at what spatial and temporal scales they are likely to be relevant. Considering the function of landscape
structures across the boundary-corridor continuum will provide researchers and managers with a more complete,
holistic viewpoint and will allow better strategies to attain conservation goals.
Linderos y Corredores Como un Control de Flujo Ecológico Continuo: Lecciones de Ríos y Arroyos
Resumen: Los linderos y corredores de paisajes son áreas de extensión espacial pequeña con relación a la
gran magnitud de sus efectos en los flujos ecológicos. En la literatura ecológica existe la tendencia de tratar a
los corredores y a los linderos como un fenómeno separado en el paisaje. Sin embargo, esta estrategia pierde
la característica común fundamental entre estos dos elementos: una fuerte influencia en los flujos ecológicos.
Los corredores y los linderos existen a en extremos opuestos de un gradiente de permeabilidad, difieren en
sus efectos en cuanto a las tasas y la dirección del flujo. La posición de estructuras del paisaje a lo largo de
este gradiente de permeabilidad depende de los atributos tanto del flujo, como de la estructura en sí misma.
Nosotros discutimos los linderos y los corredores en términos de especificidad del transportador, la escala y
sus efectos en diferentes niveles de organización ecológica y utilizamos ríos y arroyos para ilustrar nuestros
puntos. Predecimos cuales estructuras actuarían como linderos o corredores y a que escala espacial y temporal
podrían ser relevantes. Considerando que la función de las estructuras del paisaje a lo largo de un continuo
lindero-corredor proporcionará a los investigadores y manejadores un punto de vista mas completo e
integral y dará cabida a mejores estrategias para alcanzar las metas de conservación.
Introduction
The spatial and temporal distribution of materials and
energy is a fundamental determinant of biological activity
Paper submitted December 8, 1999; revised manuscript accepted
August 9, 2000.
across the landscape. Materials and energy are rarely distributed
homogeneously across a landscape but are more
often heterogeneous, with concentration in patches
within a broader matrix (Forman 1995; Lidicker 1999).
With the realization that ecological systems are rarely
closed, the focus of much ecological research and management
has shifted from looking at patches as isolated
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Boundaries and Corridors Puth & Wilson
entities on the landscape to examining the connections
among such patches (Gosz 1991; Holland & Risser 1991;
Wiens 1992; Anderson & Danielson 1997; Polis et al.
1997). Materials and energy are often packaged as organisms
but may also flow across the landscape as organic
and inorganic matter (e.g., limiting nutrients). To understand
the flow of materials and energy among patches
on the landscape, it is necessary to consider the boundaries
that delimit ecological patches and the channelized
routes, or corridors, by which materials often move.
Here, we define a boundary as an area of sharp gradients
in ecological flows that slows or redirects flows of
organisms, matter, or energy between patches (Wiens et
al. 1985) and a corridor as a structure that channelizes
and directs the flow of organisms, materials, or energy
between patches. Corridor structures may be permanent
or temporary, stationary or mobile, but all limit the
possible paths a mover may take relative to those possible
in adjacent patches. Landscape boundaries and corridors
represent critical zones of interaction, mediating
fluxes between landscape patches by modifying or maintaining
ecological flows.
As interfaces between landscape patches, and therefore
places of strong biotic and abiotic interactions (di
Castri et al. 1988), boundaries and corridors have disproportionately
large influences on landscape functions
(e.g., population, community, and ecosystem processes)
beyond what their relative area on the landscape would
suggest. They are essential for ecosystem function and,
by virtue of their small but critical area, are vulnerable to
modification by humans (Naiman & Décamps 1997). With
accelerated human alteration of the landscape, boundaries
and corridors have been lost, modified, or created
in many ecosystems (Loney & Hobbs 1991; Forman
1995). These transformations have disrupted natural
flows that formerly occurred within ecosystems, communities,
and populations (Harris & Scheck 1991) and
have created new ones (Bennett 1991). Forests have
been subdivided into isolated woodlots set in a matrix of
agricultural land (Burgess & Sharpe 1981), rivers have
been blocked by hydroelectric dams (Magnuson 1978;
Holmquist et al. 1998; Jansson et al. 2000), and canal systems
link previously isolated waterways (Mills et al.
1993). Even in national parks and other protected areas,
high-use trails and roads connect the exterior and interior
portions of many remaining forests (Benninger-Truax et al.
1992; Tyser & Worley 1992; Parendes & Jones 2000;
Trombulak & Frissel 2000).
Although both boundaries and corridors regulate the
flow of matter, organisms, and energy between landscape
patches, researchers and managers treat boundaries and
corridors as separate and unrelated structures on the landscape.
This treatment misses the relationship of boundaries
to corridors as opposite ends of a gradient of permeability
to these flows. The position along this range of
permeability for a single structure is a function of char-
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Volume 15, No. 1, February 2001
acteristics of both the ecological flow and of the structure
itself. The function of a structure on the landscape
depends on its regional context (Lidicker 1999), the gradients
of ecological flows between patches, and the organisms,
materials, and energy that interact with the
landscape structure (Naiman & Décamps 1997; Lidicker
1999). Both boundaries and corridors modify ecological
flows, boundaries by stopping and redirecting flows and
corridors by channelizing them, but a single structure
can serve as both a corridor and a boundary, depending
on the flow. Because a given landscape structure can act
as both a boundary and a corridor for ecological flows,
managers and researchers need to account for a multiplicity
of landscape functions for any one structure. As
humans create new boundaries and corridors and modify
“natural” ones, it becomes increasingly important that
managers and researchers carefully consider the multiple
roles of these landscape structures in ecological flows
and ecosystem functions.
We review the differences and similarities between
corridors and boundaries, concentrating on their permeability
to ecological flows and their effects on the direction
of these flows. We use streams to illustrate many of
our points, with occasional examples from terrestrial or
other aquatic systems. Because we discuss flows including
but not limited to those of organisms, we use the
term mover to denote biotic or abiotic matter or energy
that moves across the landscape under its own power or
is carried by vectors. We define a vector as biotic or abiotic
matter that actively transports a mover (Forman &
Godron 1981). Vectors are distinguished by the ability to
move materials or energy against gradients (Wiens et al.
1985) or accelerate material and energy along gradients.
Traditional Views of Landscape Corridors
and Boundaries
Corridors
Scientists and managers historically have viewed ecological
corridors as structures that facilitate the movement
of game between forested remnants in agricultural landscapes
(Edminster 1938; Lewis 1964). The archetypal
corridor is linear (Saunders & Hobbs 1991; Hobbs 1992),
spatially continuous (Bennett 1990; Loney & Hobbs 1991),
terrestrial, and comprised of forest vegetation (Hobbs
1992). Thus, corridors have been seen primarily as facilitators
of vertebrate dispersal and gene exchange (Aars &
Ims 1999). Operating under these assumptions, scientists
have mainly studied the use of corridors by birds (Anderson
et al. 1977; Dmowski & Kozakiewicz 1990) and small
mammals (Farhig & Merriam 1985; Bennett 1990; Merriam
& Lanoue 1990; Hobbs 1992; Aars & Ims 1999), often
in agricultural matrices. The characteristics of this
type of corridor, however, are violated by other movers

Puth & Wilson Boundaries and Corridors
such as aquatic organisms (Fraser et al 1999; Micheli &
Peterson 1999), nutrients, or by migratory birds that
jump over unsuitable habitat (Date et al. 1991; Forman
1995). Thus, it is necessary to recognize the traditional
definition as a special case of a more general concept of
corridor that allows for various configurations but stresses
movement over form.
Boundaries
Historically, scientists and managers have recognized
landscape boundaries more for their structural distinctions
on the landscape than for their role in landscape
function. Botanists noticed boundaries between plant associations,
as described by the “tension zone” of Clements
(1905), and vegetation structure continues to be
the primary identifying attribute of landscape boundaries
(Risser 1995). Game managers such as Aldo Leopold
(1933) recognized the use of forest-field edges by
many game species, and edge creation was standard
wildlife management practice for many years (Alverson
et al. 1988). The convergence of two or more distinct
communities at landscape boundaries is believed to increase
the diversity of species. Boundaries may also augment
the number of edge-specialized species by providing
appropriate habitat (e.g., edge effects: Odum 1971;
Harris 1988; Reese & Ratti 1988; Yahner 1988; Lidicker
1999). With the exception of those directly interested in
edge species, however, most researchers have found homogeneous
patches on the landscape much easier to
study than the dynamic boundaries that separate them
and have focused primarily on what occurred within
boundaries, rather than on the effects of these structures
on their landscape matrix (Lidicker 1999).
Functional Views of Corridors and Boundaries
Corridors
Corridors function to channel and increase the rate of
flow of whatever is moving along them relative to the
diffuse flow of the same mover in the surrounding matrix
(Haddad 1999). They link patches but in structurally
diverse ways and at various scales, depending on the
mover (Forman 1995). The key components of the definition
of a corridor are channelization and movement.
For a structure to operate as a corridor, something must
travel along it (Lidicker 1999), but along a path more restricted
than those available in a patch or the matrix. For
an organism to perceive a stream as a corridor, it would
need to travel along it (rather than merely utilize the
stream as habitat) much more readily than through the
surrounding landscape. The movers may be organisms
but also will often be disturbances, genetic information,
nutrients, or water (Simberloff & Cox 1987; Hobbs 1992;
Simberloff et al. 1992; Hess 1994; Haddad 1999). In addition
to providing avenues for movement for a particular
mover, corridors for a particular mover may function as
barriers, habitat, sources, or sinks for others (Forman
1983, 1991, 1995; Fraser et al. 1999). Historically, researchers
have studied the use of corridors by mammals and
birds (Haddad 1999), although many cases of corridor
use by plants and invertebrates have been documented
in recent years (Benninger-Truax et al. 1992; Covich et
al. 1996; Johansson et al. 1996; Haddad 1999; Micheli &
Peterson 1999; Parendes & Jones 2000).
Linear, continuous corridors are easy to conceptualize
and are appropriate for the movement of some movers,
but they do not encompass all possible corridor configurations.
As defined above, corridors may take various
configurations (Forman 1991, 1995; Soulé & Gilpin 1991),
including those discontinuous in space or time for a particular
mover (Forman 1983, 1991, 1995; Date et al.
1991; Prevett 1991) and perform several ecological functions
(Forman 1983, 1991, 1995). Movers such as frugivorous
pigeons (Date et al. 1991) and koalas (Prevett 1991)
need only stepping stones to move across a landscape
and are not concerned with the intervening matrix.
Structures functioning as corridors may be temporally
discontinuous if the mover only requires them at certain
times or if they are replaced by other, newly occurring
corridors. A fire, for example, may create a temporary corridor
through a forest interior. A seasonal corridor might
be high-water conditions in intermittent streams that allow
the passage of fishes between deep, permanent pools
(Karr 1989; Covich et al 1996). As with other structural
characteristics, the degree of continuity of a corridor depends
on the requirements of the mover (Forman 1995;
Lidicker 1999). The causes of this spatial and temporal
discontinuity may be abiotic, as in the case of ice bridges
during the winter, or biotic, such as a newly fallen tree
forming a bridge over a stream. These diverse corridor
structural patterns and ecological functions match the
modes of transport and travel capabilities of the various
movers that travel along them (Henein & Merriam 1990;
Forman 1991; Harris & Scheck 1991).
Boundaries
Because boundaries are the points of interaction between
landscape patches, they regulate fluxes across the
landscape and are therefore important landscape features
(di Castri et al. 1988; Holland et al. 1991; Hansen &
di Castri 1992; Risser 1995; Lidicker 1999). Boundaries
are characterized by “the strength of the interactions between
ecological systems” [e.g., landscape patches] (Holland
1988; Risser 1995; Naiman & Décamps 1997) and
are locations where the rate or magnitude of ecological
flows (nutrients, organisms, matter, energy, or information)
change abruptly relative to those of the surrounding
patches (Wiens et al. 1985; Allen & Hoekstra 1992;
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Boundaries and Corridors Puth & Wilson
Naiman & Décamps 1997). By modifying ecological flows
as they move across the landscape, boundaries function
to delimit populations, communities, and ecosystems.
The effect of a boundary depends on characteristics of
the mover and of the structure (Naiman & Décamps
1997; Lidicker 1999). For example, because materials accumulate
at them (Forman & Moore 1992), boundaries
such as ocean fronts can be sinks for drifting plankton
and other organisms (Magnuson et al. 1981; Carr 1987).
Ecological boundaries include well-studied forest-field
interfaces (Gates & Gysel 1978; Burgess & Sharpe 1981)
and transitions between biomes (Gosz 1992), as well as
less recognized boundaries such as those between a decomposing
log and the underlying soil or between areas
of high and low predation risk.
Researchers have historically described boundaries in
terms of their structural form. Structural characteristics
of boundaries include size (length, width, and depth),
shape (sinuosity, structural complexity), physical composition,
and the degree to which they contrast with the
surrounding landscapes (Forman & Moore 1992; Forman
1995; Kolasa & Zalewski 1995; Risser 1995; Lidicker
1999). Although researchers have assumed that
boundaries of similar structure have the same landscape
function, the evidence for this has been inconclusive.
For instance, researchers studying avian nest predation
across forest-field boundaries found that nest-predation
risk differed between freshly cut edge with no underbrush
and old edge with extensive shrubs and small
trees (Ratti & Reese 1988). Other researchers working
with similar forest-field boundaries, however, found no
such relationships between the form of an edge and predation
risk (Yahner et al. 1989). Thus, like corridors,
boundaries are best defined by observed changes in fluxes
and processes they influence, rather than by form alone.
The Boundary-Corridor Continuum
Boundaries and corridors are linked by their strong effects
on ecological flows, but researchers and managers
treat boundaries and corridors as separate landscape
components. We argue that their charactistics and effects
are similar, including species specificity, scale, and
interactions with levels of ecological organizations. Consequently,
their influence on the flow of ecological materials
across the landscape is also similar, differentiated
only by their effects on the directionality and rates of
ecological flows.
Boundaries and corridors constrain flows across the
landscape. They exist at opposite ends of a continuum
of flow regulation, differing in their effect on rates and
direction of flow (Fig. 1a; Allen & Hoekstra 1992); their
function is analogous to that of biological membranes as
they filter materials (Wiens et al. 1985; Allen & Hoekstra
1992; Forman & Moore 1992; Forman 1995). At one end
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Figure 1. Boundaries and corridors are at the opposite
ends of a permeability gradient. (a) Boundaries
stop and change the direction of flows, whereas corridors
channelize and change the speed of the flows. Permeability
affects not only the rate of flow between
patches but also the direction of the flow relative to the
patch boundary. (b) For aquatic movers, land between
two lakes may serve as an impermeable boundary,
an intermittent stream between the lakes provides
a semipermeable structure, and a permanent stream
acts as a corridor. (c) In the same system, terrestrial
movers experience the opposite effect: a permanent
stream is impermeable, an intermittent stream is
semipermeable, and a strip of land is permeable.
of the continuum, an impermeable boundary changes the
direction of flow by stopping, reflecting, or shuttling
flows along the boundary rather than allowing movement
through the boundary (this shuttling of flows transforms
the boundary into a corridor; Forman & Moore 1992;
Naiman and Décamps 1997; Haddad 1999). As with biological
membranes, boundaries can have varying degrees of
permeability to landscape fluxes (Forman 1995; Naiman
& Décamps 1997). An impermeable membrane contains
or redirects flows, whereas a permeable one changes
the rate of flow across the boundary (Forman 1995; Haddad
1999). With increasing permeability, the change in

Puth & Wilson Boundaries and Corridors
flow rate is greater than the change in flow direction. At
the other end of the continuum, corridors allow unimpeded
movement across boundaries (or even increase
the rate of flow; Haddad 1999); they are analogous to
pores in a membrane and function as connectors of
landscape patches (Allen & Hoekstra 1992). Like boundaries,
corridors once again direct flows, in this case by
funneling them across boundaries (Haddad 1999). Permeability
and directionality are mover-specific characteristics
of individual landscape structures (Fig. 1b & 1c). A
logging road, for example, might bar the movement of
interior forest birds, allow some sunlight to penetrate
the edge of the woods, and facilitate the flow of humans
across the forest. A cattail wetland at the land-water interface
may act as an impermeable barrier to terrestrial
animals and as a semipermeable filter for nutrients that
would otherwise flow directly into the water.
The position of a landscape structure along the boundary-corridor
continuum depends on the type of mover
and its spatial and temporal scales of movement. We explore
these ideas in the context of rivers and streams,
with occasional examples from terrestrial systems.
An Example: Streams and Rivers
Viewing boundaries and corridors as a continuum of
permeability and directionality of ecological flows gives
landscape features multiple roles in regulating ecological
flows. We examined the boundary-corridor continuum in
the context of rivers and streams. Theory, such as the
river-continuum concept (Vannote et al. 1980), considers
flow along streams as unidirectional and longitudinal.
Thus, streams operate as corridors for the passive
movement of materials and nutrients downstream (Forman
1995). But this view of streams and rivers functionally
isolates streams from the landscapes in which they
occur. A more holistic view sees the stream as a dynamic
landscape corridor and boundary that strongly affects
the landscape along the boundary-corridor continuum
(e.g., the flood-pulse concept: Junk et al. 1989; Naiman
et al. 1988; Ward 1989; Naiman & Décamps 1997).
Examining boundaries and corridors in the context of
streams and rivers offers several advantages to studying
terrestrial systems. Streams and rivers are easy to visualize
as traditional corridors because they exhibit obvious
directional flow and strong linearity (Forman 1995).
They also mirror traditional boundaries in their strong landwater
gradients and long edges (Forman 1995; Naiman &
Décamps 1997). Land-water and air-water interfaces tend
to bound aquatic systems more discretely than do many
land-land edges (Forman 1991), making aquatic systems
conceptually easier to delineate and often easier to study
(Carpenter et al. 1995). Flowing waters integrate entire
watersheds by combining both terrestrial and aquatic aspects
of the landscape. These characteristics make them
ideal natural laboratories in which to test ecosystem properties
(Allen & Hoekstra 1992; Carpenter et al. 1995).
Mover Specificity
The position of a landscape structure on the boundarycorridor
continuum—its function in the landscape—
depends on the movement characteristics of individual
movers (Noss 1991; Naiman & Décamps 1997), such that
a single landscape structure may function as a boundary,
corridor, or something intermediate (Allen & Hoekstra
1992; Naiman & Décamps 1997). Movers possess a wide
range of travel modes: they may walk, run, fly, swim,
crawl, or be carried by vectors. They may be biotic or
abiotic. They may move actively under their own power
or passively by vectors. These traits combine to produce
an enormous variety in movement characteristics, so
that locating a single stream along the boundary-corridor
continuum is impossible without first identifying the focus
mover (Fig. 1b & 1c).
Active movers, whether abiotic or biotic, may interact
differently with the same landscape structure. For biotic
movers, the effect of a boundary or corridor depends on
that organism’s dispersal rate and mode, home range, physical
size, and the presence or absence of predators and
competitors (Wiens et al. 1985). For abiotic movers, the
effect of a corridor or boundary varies with the structure’s
medium (e.g., land or air), the morphometry of
the boundary or corridor, and the rate at which the
mover is traveling. In a similar manner, the passive transport
of materials and energy across the landscape by
biotic and abiotic vectors depends on the interactions
between the vector and the boundary or corridor. For
example, riparian zones often reduce the water-borne flow
of nutrients from terrestrial systems to wetland areas,
but overwintering Snow Geese (Chen caerulescens)
and Sandhill Cranes (Grus canadensis) in New Mexico
move ecologically significant amounts of nitrogen and
phosphorus across riparian areas from agricultural fields
to wetland roosting areas (Post et al. 1998). As seasonally
important vectors for the nutrients, the birds function as
a corridor connecting the two areas. Mover specificity is
a function of the movement characteristics of whatever
actively travels across the landscape, whether this is a
mover or a vector carrying the mover (Fig. 1b & 1c).
Streams are well-defined corridors for the downstream
movement of nutrients, energy, and matter; but they
function in many other capacities on the landscape, as
defined by the mover(s) of interest. Streams funnel nutrients
and plant propagules ( Johansson et al. 1996;
Parendes & Jones 2000) but tend to redirect the movement
of terrestrial rodents, functioning as boundaries for
terrestrial organisms (Fig. 1b & 1c; Storm et al. 1976;
Knaapen et al. 1992) and flows, such as fire. In addition
to their role in transporting flows downstream, streams
and rivers also act as longitudinal corridors for a multi-
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Boundaries and Corridors Puth & Wilson
tide of upstream flows. Spawning salmon move nutrients
upstream from the oceans and enrich both the
headwaters and forest ecosystems (Donaldson 1967;
Mathisen et al. 1988; Cederholm et al. 1989; Kline et al.
1990; 1993; Schuldt 1995). Most adult airborne aquatic
insects fly in an upstream direction (Müller 1982), and
many aquatic organisms regularly travel upstream to
spawn or recolonize pools after floods or other local extinction
events (Chapman & Kramer 1991; Meadow &
Matthews 1992; Sheldon & Meffe 1995; Covich et al.
1996). As boundaries, streams separate some terrestrial
mammal populations but not others (Patton et al. 2000).
Scale
Movers and vectors interact with the landscape at significantly
different spatial and temporal grains and extents.
The scale at which movers or vectors encounter the land
defines the position of a landscape structure along the
boundary-corridor continuum (Wiens & Milne 1989; Noss
1991; Allen & Hoekstra 1992; Holling 1992). For example,
small mammals respond to river and canal boundaries
at smaller grains and extents than do large mammals
(Knaapen et al 1992). A stream that because of its
width represents an insurmountable barrier to a mouse
is mere steps to a deer. To the mouse, the stream is a
boundary limiting its movements; to the deer, the stream
is one of many water sources in the matrix of streams
and forest in which it forages. In a similar manner, a
stream corridor used by a hunting bat will represent a
habitat patch for a stonefly larva that perceives its environment
at the scale of meters (Wiens & Milne 1989).
Conversely, the position of a landscape structure on
the boundary-corridor continuum changes the scaling of
a landscape by linking some landscape patches and dividing
others. Processes occurring on a landscape composed
of patches connected by corridors operate at a
larger scale than on a landscape consisting of isolated
patches. These same corridors, may concurrently function
as boundaries, however, dividing other landscape
processes. For example, a canal dug between two otherwise
isolated lakes allows the interlake transfer of nutrients
and organisms; at the same time, it splits the local
terrestrial ecosystem and populations (Fig. 1b & 1c).
Through these effects on landscape connectivity, boundaries
and corridors delimit ecological systems.
Streams as Terrestrial Boundaries and Corridors
Streams and rivers act as boundaries and corridors for
terrestrial systems as well as for aquatic systems. Streams
and rivers can partition terrestrial landscapes by isolating
or reducing flows between landscape patches (Knaapen
et al. 1992; Lamborot & Eaton 1997; Patton et al. 2000).
Storm et al. (1976) found that midwestern rivers hindered
the movement of red foxes ( Vulpes fulva)
be-
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tween populations on opposite banks, with large rivers
providing lower permeability than small ones. Peres et al.
(1996) found that the function of rivers in the Amazon
basin as boundaries to gene flow for tamarins increased
with river size. Rivers may also stop terrestrial processes
such as fire (Forman 1995). For nonswimmers, rivers become
impermeable boundaries and direct movement
along the river bank, or back into the surrounding matrix.
Many terrestrial animals travel along riparian zones,
using them as movement corridors (Forman & Godron
1986), and flying organisms such as bats hunt above the
river channel (Forman 1995).
Leaky Stream Boundaries as Connections to
Terrestrial Systems
Streams and rivers do more than connect their sources
and endpoints; they exhibit “leaky” land-water boundaries,
acting as semipermeable filters to lateral flows (Naiman
et al. 1988; Junk et al. 1989; Forman 1995). Floods
transfer nutrients and materials from water to land ( Junk
et al. 1989; Power 1995), or from land to water ( Junk et
al. 1989). Biotic movers such as beavers (Naiman et al.
1994), emerging aquatic insects ( Jackson & Fisher 1986),
waterfowl (Post et al. 1998), and reproducing amphibians
(Seale 1980) connect terrestrial and aquatic ecosystems
and increase the permeability of the land-water interface
by moving laterally between the terrestrial and
aquatic systems.
Within-Stream Boundaries and Corridors
A continuum of boundaries and corridors exists within
streams and rivers, changing the direction and permeability
to flows within the stream or river channel itself
(Naiman et al. 1988; Forman 1995). Dams, culverts, and
waterfalls (Magnuson 1978; Holmquist et al. 1998; Jansson
et al. 2000), as well as riffles during periods of low water
(Power et al. 1985, Covich et al. 1996) physically
block within-stream movements. Biotic conditions of the
stream, such as the presence of predators or competitors,
limit movers (Bradford et al. 1993; Covich et al 1996;
García-Ramos et al. 2000), as do physical factors, including
water velocity, nutrient levels (Dent & Grimm
1999), temperature, dissolved oxygen, and water levels.
Reservoirs above dams, for example, tend to have
warmer, slower moving water than that below the dam,
and hence different community assemblages. In-stream
boundaries may be selectively permeable, as in waterfalls
that delimit adult goby populations but allow upstream
movement of larval fishes (Radtke & Kinzie 1996), and
low water conditions that block the movement of fishes
between deep pools. Interestingly, within-stream boundaries
such as shallow riffles or beaver dams may increase
the permeability of the stream boundary to terrestrial organisms
by increasing crossing points.

Puth & Wilson Boundaries and Corridors
Temporal Variability
Although the function of a stream or river along the
boundary-corridor continuum may be relatively permanent,
in other situations its role may be diurnal (tidal rivers),
seasonal (floods, droughts, and ice), stochastic (storm
events), ephemeral (temporary log jams), or related to
the life history of both movers and structures (size-refugia,
successional changes). Water levels in tidal rivers
may fluctuate several meters twice daily. In larger streams
and rivers, seasonal floods predictably deliver materials
and nutrients to lowland areas ( Junk et al. 1989). Sensitivity
to fluctuating lotic flow regimes can determine the
invasion success of exotic fish species (Brown & Moyle
1997). Ice allows terrestrial animals to cross large bodies
of water (Ferguson et al. 2000). Successional changes
along a corridor may dictate which movers can use it. A
stretch of seagrass that functions as a corridor allowing
the movement of crab predators (Micheli & Peterson
1999) is unlikely to have the same effect when the
shoots are young. A boundary defined by the presence
of predators in an adjacent patch will lose its effectiveness
if the mover obtains a size-refuge where it is too
large to be consumed (Werner & Hall 1988).
Interaction between Mover Specificity and Scale
Mover specificity and scale do not operate in isolation;
rather, they work together to determine the function of
a river or stream (Fig. 2). The scale of a river or stream is
related to its length and depth for flows parallel to it and
to its width and depth for flows perpendicular to it. The
scale of movers corresponds to their resolution of movement,
such as step length for terrestrial mammals. If the
width or length of a stream is smaller than that at which
a particular mover travels (e.g, moose [ Alces alces]
moving
tens of kilometers a day), the mover will not use the
stream as an independent landscape structure. If the
width of the stream matches the step or jump length of
the mover (e.g., bobcat [ Lynx rufus]
moving a few kilometers
a day) and the mover perceives the stream as inhospitable,
then the structure will act as a boundary. If
the scale of the stream matches that of the mover and
the mover (e.g., anadromous salmon moving a kilometer
a day) recognizes the stream as hospitable, the stream
will function as a corridor. When the width of the
stream is greater than the step length of the mover (e.g.,
mice moving tens of meters a day) and the mover perceives
the structure as inhospitable, the structure will
also operate as a boundary. If the width of the stream is
greater than that moved by the mover in a short period
of time and the mover recognizes the structure as hospitable,
the structure will act as habitat (caddisfly moving
tens of centimeters a day). This interaction reveals a fundamental
inequity between boundaries and corridors:
Figure 2. Scale and mover specificity interact to determine
the function of a structure. Mover specificity relies
on how well the mover can travel in a specific medium,
such as forest or water. This interaction reveals
a major inequity between the conditions that produce
corridors and boundaries, which suggests there will be
more boundaries than corridors on the landscape.
more landscape structures fall on the boundary side of
the boundary-corridor continuum, because more combinations
of scale and mover specificity lead to a structure
functioning as a boundary than as a corridor.
Levels of Ecological Organization
Because streams and rivers play numerous roles along
the boundary-corridor continuum, they have effects on
many types of ecological entities, including traditional
ecological levels of organization. For example, as a corridor,
a stream may allow the movement of breeding
aquatic organisms or the dispersal of juveniles, influencing
populations (Schlosser 1995). If the fish are predatory,
their movement along the stream corridor may
strongly affect stream communities (Covich et al. 1996).
The passage of sediments downstream and death of
anadromous fish upstream influence ecosystem processes
(Mathisen et al. 1988; Schuldt 1995). The pattern
of fish movement and energy flow up and down the
stream is a landscape consideration. Alternatively, the
same stream may separate terrestrial rodent populations
on either stream bank (populations), disconnect competitive
interactions between organisms (communities),
direct the flow of nutrients from eroded terrestrial sediments
(ecosystems), and prevent fires from crossing
from one bank to the other (landscapes). Changes in the
direction and permeability of streams and rivers will consequently
affect ecological flows important to all levels
of ecological organization.
27
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Volume 15, No. 1, February 2001

28
Boundaries and Corridors Puth & Wilson
Conclusions
Boundaries and corridors have traditionally been studied
as independent landscape structures. Based on the strong
similarities in landscape function, however, we argue
that boundaries and corridors are at polar ends of a continuum
based on permeability and directionality of ecological
flows. This viewpoint unites the often parallel
treatment of boundaries and corridors and emphasizes
the fact that a structure on the landscape may act as a
boundary or a corridor or may play an intermediate role,
depending on mover specificity and temporal and spatial
dynamics.
Researchers and managers should recognize that these
structures can be ephemeral features on the landscape,
and, conversely, that some landscape structures may
not always act as boundaries and corridors, although they
may look like boundaries and corridors. Moreover, the
movers that interact with boundaries and corridors may
also be ephemeral, depending on characteristics such as
life history and seasonality. The mover-specific qualities
of the boundary-corridor continuum underscore the dangers
of single-species management in which a structure’s
influence on one mover is emphasized over all others.
Understanding the similarities between corridors and
boundaries may help us interpret the movement of organisms
across the landscape and determine points of
vulnerability to the spread of exotics. A knowledge of
boundary and corridor function may also further the prediction
of likely consequences of natural disturbances,
anthropogenic perturbations, and management actions.
As areas of strong influence on ecological flows, boundaries
and corridors present a challenge to researchers, in
their detection and description and in their conservation.
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