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Conservation Biology, Pages 1241–1252
Volume 12, No. 6, December 1998
Review

Do Habitat Corridors Provide Connectivity?

PAUL BEIER* AND REED F. NOSS†

*School of Forestry, Northern Arizona University, Flagstaff, AZ 86011–5018, U.S.A., email paul.beier@nau.edu
†Conservation Biology Institute, 800 NW Starker Avenue, Suite 31C, Corvallis, OR 97330, U.S.A.

Abstract:

Skeptics have questioned the empirical evidence that corridors provide landscape connectivity.
Some also have suggested dangers of corridors. We reviewed published studies that empirically addressed
whether corridors enhance or diminish the population viability of species in habitat patches connected by cor-
ridors. A randomized and replicated experimental design has not been used—and we argue is not required—
to make inferences about the conservation value of corridors. Rather, studies can use observational or exper-
imental analyses of parameters of target populations or movements of individual animals. Two of these ap-
proaches hold the greatest promise for progress, especially if the shortcomings of previous studies are reme-
died. First, experiments using demographic parameters as dependent variables—even if unreplicated—can
demonstrate the demographic effects of particular corridors in particular landscapes. Such studies should
measure demographic traits before and after treatment in both the treated area (corridor created or de-
stroyed) and an untreated area (habitat patches isolated from one another). This approach is superior to ob-
serving the demographic conditions in various landscapes because of the tendency for corridor presence to be
correlated with other variables, such as patch size, that can confound the analysis. Second, observations of
movements by naturally dispersing animals in fragmented landscapes can demonstrate the conservation
value of corridors more convincingly than can controlled experiments on animal movement. Such field obser-
vations relate directly to the type of animals (e.g., dispersing juveniles of target species) and the real land-
scapes that are the subject of decisions about corridor preservation. Future observational studies of animal
movements should attempt to detect extra-corridor movements and focus on fragmentation-sensitive species
for which corridors are likely to be proposed. Fewer than half of the 32 studies we reviewed provided persua-
sive data regarding the utility of corridors; other studies were inconclusive, largely due to design flaws. The
evidence from well-designed studies suggests that corridors are valuable conservation tools. Those who would
destroy the last remnants of natural connectivity should bear the burden of proving that corridor destruction
will not harm target populations.

Proveen Conectividad los Corredores de Hábitat?

Resumen:

Algunos escépticos han cuestionado la evidencia empírica de que los corredores proveen conectiv-
idad al paisaje. Otros han sugerido los peligros de los corredores. Revisamos estudios publicados que abord-
aron empíricamente si los corredores fomentan o disminuyen la viabilidad de poblaciones de especies en
parches de hábitat conectados por corredores. A la fecha no se ha llevado a cabo un diseño experimental ran-
domizado y con réplicas para realizar inferencias sobre el valor de los corresdores en la conservación—y nos-
otros argüímos que no es necesario. En cambio, los estudios pueden emplear análisis observacional o experi-
mental de parámetros de poblaciones de interés o movimientos individuales de animales. Dos de estas
aproximaciones son muy prometedoras y pueden progresar, especialmente si las limitantes de los estudios
previos son remediadas. Primero, los experimentos que usan parámetros demográficos como variables de-
pendientes—aún si no son replicados—pueden demostrar efectos demográficos de corredores en paisajes par-
ticulares. Estos estudios deberán medir características demográficas antes y después del tratamiento, tanto en
el área tratada (corredor creado o destruído) como en un área no tratada (parches de hábitat aislados unos
de otros). Esta aproximación es superior a observar las condiciones demográficas en varios paisajes puesto
que la presencia de un corredor tiende a estar correlacionada con otras variables, como lo es el tamaño del

Paper submitted January 28, 1998; revised manuscript accepted June 24, 1998.

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Corridors and Connectivity Beier & Noss

Conservation Biology
Volume 12, No. 6, December 1998

parche lo que puede confundir el análisis. Segundo, las observaciones de movimientos de animales que se
desplazan normalmente en paisajes fragmentados puede demostrar el valor de los corredores en la conser-
vación de manera mas convincente que los experimentos controlados sobre animales en movimiento. Este
tipo de observaciones de campo están directamente relacionades con el tipo de animal (e.g., juveniles de la es-
pecie de interés dispersándose) y con el tipo de paisajes que están sujetos a las decisiones de preservación de
corredores. Los estudios observacionales de movimientos de animales a futuro deberán tratar de detectar
movimientos extra-corredores y enfocarse a especies sensitivas a la fragmentación y para las cuales los corre-
dores son factibles a ser propuestos. Menos de la mitad de los 32 estudios revisados provee datos persuasivos
referentes a la utilidad de los corredores; otros estudios fueron inconclusos, mayormente debido a diseños de-
fectuosos. Las evidencias de estudios bien diseñados sugieren que los corredores son herramientas valiosas de
conservación. Aquellos que intentan destruir los últimos remanentes de conectividad natural deberían susten-

tarse demostrando que la destrucción de los corredores no afectará a poblaciones de interés.

Introduction

Conservation biologists generally agree that landscape
connectivity enhances population viability for many spe-
cies and that, until recently, most species lived in well-
connected landscapes (Gilpin & Soulé 1986; Noss 1987;
Primack 1993; Noss & Cooperrider 1994; Hunter 1996;
Meffe & Carroll 1997). Because urbanization and other
human activities often sever natural connections among
landscapes, many conservationists have advocated the
retention of habitat corridors. In part, this approach has
been justified by theoretical population models (e.g.,
metapopulation models, Gilpin & Hanski 1991). Such
models demonstrate the utility only of habitat connectiv-
ity, however, which benefits population viability via the
rescue effect (Brown & Kodric-Brown 1977) or other
mechanisms. Conservation value accrues to corridors
only if animals in real landscapes use corridors to bring
about connectivity. Simberloff et al. (1992) argued that
such evidence is lacking. Simberloff and Cox (1987), Sim-
berloff et al. (1992), and Hess (1994) also argued that
corridors might promote the spread of diseases, cata-
strophic disturbances (such as wildfires), or exotic spe-
cies into the areas connected by corridors or might
lure animals into areas—including the corridors them-
selves—where they experience high mortality (for a re-
view see Hobbs 1992). A central concern is that funds
spent acquiring corridors of questionable or unproven
value might be better spent acquiring habitat areas for
imperiled species, even if such areas are isolated (Sim-
berloff et al. 1992).
We reviewed empirical papers that appeared relevant
to the question, “Do corridors enhance or diminish the
population viability of target species in the habitat patches
connected by corridors?” Our goals were to make sug-
gestions for future research on these issues and to evalu-
ate scientific evidence that corridors serve as conduits
for movement in a way that justifies their use as a con-
servation tool or that corridors have negative effects on
target species.

Methods

We gathered papers on corridors (excluding modeling
exercises) by searching for the word

corridor

in titles,
abstracts, and keywords in all 1980–1997 volumes of

Auk, Biological Conservation, Condor, Conservation
Biology, Ecological Applications, Ecology, Journal of
Mammalogy, Journal of Wildlife Management, Wild-
life Society Bulletin, Wilson Bulletin

, and recent mono-
graphs (e.g., Saunders & Hobbs 1991). We gleaned addi-
tional citations from relevant papers.
We define

corridor

as a linear habitat, embedded in a
dissimilar matrix, that connects two or more larger
blocks of habitat and that is proposed for conservation
on the grounds that it will enhance or maintain the via-
bility of specific wildlife populations in the habitat blocks.
We define

passage

as travel via a corridor by individual
animals from one habitat patch to another. Our defini-
tion of

corridor

explicitly excludes those linear habi-
tats—such as riparian areas in agricultural landscapes—
that support breeding populations of many species but
do not connect larger habitat patches (e.g., Spackman &
Hughes 1995). There are important conservation issues
regarding nonconnective linear habitats, but we re-
stricted our attention to linear patches of land whose
conservation value is to allow passage between more
significant habitat patches.
Nicholls and Margules (1991) and Inglis and Under-
wood (1992) discussed the formidable difficulties in-
volved in designing a randomized and replicated experi-
ment to test whether corridors enhance recolonization
of habitat patches after local extinction. For such an ex-
periment to be realistic, each experimental unit is an en-
tire landscape, and there must be several replicate land-
scapes for each combination of treatments. Furthermore,
we suggest that the species studied must be those that
require connectivity on a landscape scale—fragmenta-
tion-sensitive species such as mammals with large home
ranges—and that each species be studied individually.
These requirements present staggering logistical and fi-