Corridors in Real Landscapes:
A Reply to Simberloff and Cox
REED F. NOSS
Department of Wildlife and Range Sciences
School of Forest Resources and Conservation
University of Florida
Gainesville, Florida 32611
Abstract: Habitat corridors have become popular in landuse
plans and conservation strategies, yetfew data are available
to either support or refute their value. Simberloff and
Cox (1987) have criticized what they consider an uncritical
acceptance of corridors in conservation planning
Any reasonable conservation strategy must address the
overwhelming problem of habitat fragmentation. Although
Simberloff and Cox use island analogies to illustrate advantages
of isolation, these analogies do not apply directly
to problems in landscape planning. Genetics also does not
offer unequivocal advice, but the life histories of wide-ranging
animals (eg, theFlorida panther) suggest that the maintenance
or restoration of connectivity in the landscape is a
prudent strategy. Translocation of individuals among reserves-considered
by Simberloff and Cox a viable alternative
to natural dispersal-is impractical for whole
communities of species that are likely to suffer from probletns
related to fragmentation.
Many of the potential disadvantages of corridors could be
avoided or mitigated by enlarging corridor width or by applying
ecologically sound zoning regulations. Corridors are
not the solution to all of our conservation problems, nor
should they be used as a justification for small reserves. But
corridors can be a cost-effective complement to the strategy
of large and multiple reserves in real-life landscapes.
Corridors are a hot topic, perhaps even a fad, in conservation
planning these days. Planners and environPaper
submitted 12/9/86; revised manuscript accepted 3/31/87.
Resumen: Los corredores naturales se ban becho muy comunes
en proyectos de uso de terreno y estrategias de conservacion,
pero hay muypocos datos disponibles quepuedan
apoyar o rechazar su valor. Simberloff and Cox (1987) critican
lo que ellos consideran es una aceptaci6n poco critica
de los corredores naturales en la planificaci6n de estrategias
de conservaci6n.
Cualquier estrategia razonable tiene que dirigirse al problema
preponderante de fragmentaci6n de habitat. Aunque
Simberloffy Cox usan el ejemplo de las islas como analogia
para ilustrar las ventajas del aislamiento, estas analogias
no aplican directamente aproblemas en laplanificacion del
uso de la tierra La genetica tampoco ofrece consejo inequivoco,
pero la historia natural de animales con ambitos extensos
(e]em. Florida panther) sugiere que el mantenimiento
o restauraci6n de una conexi6n entre acreas naturales es una
estrategia prudente. El movimiento de individuos de una u
otra especie entre las reservas, considerado por Simberloff y
Cox como una alternativa viable a la dispersion natural, es
impractico para comunidades enteras de especies que son
vulnerables a la fragmentaci6n.
Muchas de las desventajas potenciales de corredores naturales
se pueden evitar o mitigar ensanchando el corredor,
o estableciendo reglamentos de zonificacion que sean congruentes
con principios ecol6gicos establecidos. Los corredores
naturales no son la soluci6n a todos los problemas de
la conservacion natural, ni tampoco deberian ser usados
como una justificacion para reservas pequenias. Pero los
corredores naturales pueden ser un complemento de bajo
costo y rendimiento efectivo a la estrategia de establecer
muchas reservas de gran extensi6n.
mentalists from county to federal levels are busy drawing
"greenbelts" and other habitat corridors into their designs,
sometimes with only a vague awareness of the
biological issues underlying the corridor strategy. Re-
159
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160 Corridors in Real Landscapes Noss
cently, some biologists have expressed concern that the
corridor idea has been thrown into the political arena
prematurely, without adequate field research or discussion
among conservation biologists.
Soule and Simberloff (1986), briefly, and Simberloff
and Cox (1987), in more depth, have discussed some
possible advantages and disadvantages of the corridor
strategy. Simberloff and Cox (1987) place particular emphasis
on disadvantages, because they believe that "much
of the current literature concerning corridors fails to
consider potential disadvantages and often assumes potential
benefits without the support of sufficient biological
data, or even explicit recognition that such data are
needed." No doubt more research is needed to develop
optimal connectivity strategies, but the continuing severance
of natural linkages in many landscapes suggests
that active strategies to combat the process and the
consequences of fragmentation must proceed quickly,
with or without "sufficient" data.
Many conservation biologists agree with Wilcox and
Murphy (1985) that "habitat fragmentation is the most
serious threat to biological diversity and is the primary
cause of the present extinction crisis." Conservation
strategies, therefore, might be evaluated on the basis of
how well they counter the effects of fragmentation in
real landscapes. The fragmentation problem has essentially
two components: 1) a decrease in total habitat
area, and 2) an apportionment of the remaining area into
ever more isolated pieces (Wilcove et al. 1986). The
two ways to counter fragmentation, then, are 1) increase
effective habitat area, and 2) increase connectivity. My
purpose in this note is to offer an alternative viewpoint
to Simberloff and Cox (1987) and to evaluate potential
advantages and disadvantages of corridors in the context
of an integrated landscape conservation strategy.
Potential Advantages and Disadvantages of
Corridors in Human-Dominated Landscapes
Some potential advantages and disadvantages of corridors
in human-dominated landscapes, with particular
reference to conservation of terrestrial species and habitats,
are listed in Figure 1. These lists are not comprehensive,
as many important functional attributes of
corridors will not be discussed here. For example, vegetated
riparian corridors are important in maintaining
water quality in streams (Karr & Schlosser 1978, Schlosser
& Karr 1981), and hedgerows and shelterbelts have
well-known advantages in inhibiting soil erosion (Forman
& Baudry 1984).
Simberloff and Cox (1987) propose that corridors be
evaluated individually on their own merits, and that theoretical
considerations cannot be applied universally.
Few ecologists would quarrel with that statement. But
although Simberloff and Cox (1987) criticize the use of
biogeographic analogies in pro-corridor arguments, much
of their argument against corridors (or in favor of habitat
subdivision) is based on island analogies. The common
goal in this debate-conservation of biodiversity-might
be served best if all parties, whenever possible, refrain
from arguments based on theory and analogy, and devote
their efforts to solving concrete problems in real-world
landscapes.
The extent to which a habitat corridor might facilitate
dispersal and thus increase immigration rates to reserves
is strictly an empirical matter, and would depend upon
habitat structure within the corridor, corridor width and
length, and the autecologies of the particular organisms
in question (Forman 1983, Harris 1984, Forman & Godron
1986, Noss & Harris 1986). If we determine that
immigration rate is enhanced, we still do not know
Potential Advantages of Corridors Potential Disadvantages of Corridors
1. Increase immigration rate to a reserve, which could 1. Increase immigration rate to a reserve, which could
A increase or maintain species richness and diversity (as predicted by A facilitate the spread of epidemic diseases, insect pests, exotic spe
island biogeography theory); weeds, and other undesirable species into reserves and across the landB
increase population sizes of particular species and decrease probability scape;
of extinction (provide a "rescue effect") or permit re-establishment of B decrease the level of genetic variation among population or
extinct local populations; lations, or disrupt local adaptations and coadapted gene complexes
C prevent inbreeding depression and maintain genetic variation within ("outbreeding depression").
populations. 2. Facilitate spread of fire and other abiotic disturbances ("contagious catas-
2. Provide increased foraging area for wide-ranging species. trophes").
3. Provide predator-escape cover for movements between patches. 3. Increase exposure of wildlife to hunters, poachers, and
4. Provide a mix of habitats and successional stages accessible to species that 4. Riparian strips, often recommended as corrido
require a variety of habitats for different activities or stages of their life- dispersal or survival of upland species.
cycles. 5. Cost, and conflictswith conventional land preservation strategy to preserve
5. Provide alternative refugia from large disturbances (a "fire escape"). endangered species habitat
6. Provide "greenbelts" to limit urban sprawl, abate pollution, provide rec- low).
reational opportunities, and enhance scenery and land values.
Figure 1. Potential advantages and disadvantages of conservation corridors.
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Noss Corridors in Real Landscapes 161
whether the net effect of this increased immigration is
good or bad for conservation. (Fig. 1; item number 1).
According to island biogeography theory (MacArthur &
Wilson 1967), increased immigration should result in a
higher equilibrium species number. But higher species
richness at the local scale may not be a goal in conservation,
particularly if the species that invade are alien
to the landscape or not in need of reserves for survival
(Diamond 1976, Noss 1983). Augmentation of local population
size with immigrants from the same species
("rescue effect"; Brown & Kodric-Brown 1977) and reestablishment
of extinct local populations (Fahrig &
Merriam 1985) might be considered an advantage of
increased connectivity, but not if the organism so benefitted
is a competitor, predator, parasite, or pathogen
of a species of greater conservation concern.
The genetic consequences of increased immigration
rate are also controversial, as discussed by Simberloff
and Cox (1987). On the one hand, inbreeding depression
and genetic drift might be minimized with the influx
of genetically different individuals, resulting in increased
fitness for the average individual and increased genetic
variation in the population. Evidence for immigrants
contributing much to heterozygosity or fitness in a small
population is meager, however (Frankel & Soule 1981).
Furthermore, increased gene flow between demes might
lead to genetic swamping and homogenization of the
gene pool. Corridors have never been implicated in this
problem, although the possibility worries Simberloff and
Cox (1987).
Gene flow might also disrupt local adaptation and
coadapted gene complexes, resulting in outbreeding
depression. Although outbreeding depression is likely
to be a temporary phenomenon, rapidly eliminated by
selection and sometimes replaced by a superior coadapted
gene complex, it could greatly increase the chances
of extinction in small populations (Templeton 1986).
Outbreeding depression is a potential problem in captive
breeding programs, when animals from distinct
populations are mated (e.g., de Boer 1983, Templeton
et al. 1986). On the other hand, maintaining or restoring
natural landscape connectivity never has been shown to
cause outbreeding depression in populations.
On the genetics issue, Simberloff and Cox (1987) suggest
that "the subdivision of a larger population allows
the effects of genetic drift to assist in the maintenance
of genetic variability in the species" (see Chesser 1983).
When inbreeding becomes a problem, manual translocation
of individuals between isolated populations is
considered preferable ("genetically advantageous and
much less expensive"; Simberloff & Cox 1987) to protecting
corridors for natural dispersal. The esthetic and
perhaps ethical question of whether shipping animals
around in crates is a satisfactory substitute for natural
movements warrants philosophical discussion.
Although translocation might fulfill genetic management
objectives for a few focal species, moving all the
species of a community among reserves is clearly impractical.
Fragmentation thus far has been documented
to have deleterious effects in a relatively few sensitive
species, but many secondary extinctions may be forthcoming
(Wilcove et al. 1986). Small populations of organisms
that today seem relatively secure, such as longlived
forest herbs with limited dispersal capabilities and
slow responses to potentially significant forest changes,
may prove vulnerable in the long term (Middleton &
Merriam 1983). The same may be true for many forest
songbirds (D. Wilcove, personal communication). Furthermore,
because of mutualistic dependencies among
certain species and the natural shifting of habitat patches
over time and space, levels of biological organization
above the population may depend on habitat connectivity
for long-term persistence (Noss & Harris 1986).
In the face of scientific ignorance about these phenomena,
maintenance of habitat connectivity would seem to
be the prudent course.
Simberloff and Cox (1987) choose the Florida panther
(Felis concolor coryi) as an example of a species where
proposed corridor strategies (Cristoffer & Eisenberg
1985, Noss 1985a, Noss & Harris 1986) are inappropriate.
Unfortunately, they fail to consider the most critical
component of that strategy: the re-creation of
wilderness landscapes by closing roads and limiting access
in large core areas of public lands in the present
range of the panther in south Florida, and in proposed
reintroduction sites in north Florida, as detailed in Noss
(1985a, 1985b) and summarized in Noss and Harris
(1986). In this admittedly idealistic strategy, large roadless
areas would be surrounded by multiple-use buffer
zones and connected by broad habitat corridors (revised
map in Noss 1987a). Thus, corridors are envisioned as
one critical component of an integrated landscape conservation
strategy, not as panaceas. Wilderness restoration
may be the only hope for the Florida panther, an
animal demonstrably intolerant of habitat fragmentation.
Like other pumas, it does not prosper in human-dominated
landscapes (Noss 1985a, usFws 1986, Van Dyke
et al. 1986).
Simberloff and Cox (1987) propose that "a better
method of preserving extant levels of genetic variability
might be to isolate new translocated populations" of
panthers, but their reasoning is unconvincing in light of
Felis concolor's natural history. F c. coryi was originally
distributed continuously from Florida northeast to South
Carolina, and west to Arkansas and east Texas or west
Louisiana (Goldman 1946, Hall 1981). Habitat fragmentation
and direct persecution subsequently eliminated
this subspecies from all of its range except south Florida,
which until recently was remote wilderness (Belden, in
press; usFws 1986). Although the current level of genetic
variation in the 20 to 50 remaining panthers and how
this relates to fitness is unknown, this may be a classic
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162 Corridors in Real Landscapes Noss
case of inbreeding depression. All five males examined
to date have had greater than 93 percent abnormal sperm
(Roelke 1986, usFws 1986). The Florida panther as we
know it today is an "inbred subset" of the F. c. coryi
recognized by Goldman (1946), and strategies to enhance
genetic variation are being discussed (Eisenberg
1986).
Given what is known about the former continuity of
panther distribution and the long-distance movements
of individual Felis concolor (often over 100 km; Young
1946, Hornocker 1970, Dewar 1976, Hemker et al. 1984,
Logan et al. 1986), it is possible that a single deme may
have occupied all of peninsular Florida. Documented
dispersal of juveniles and immigration of transients suggest
that F. concolor cannot adequately be managed siteby-site,
but instead requires management on a regional
basis (Logan et al. 1986). Because several individuals
often travel common corridors that are influenced by
topography (e.g., Young 1946), the safest strategy may
be to provide numerous, carefully selected, and wellprotected
swaths of habitat (including highway underpasses)
for this movement. The alternative of isolated
reserves would virtually assure that individuals will continue
to be shot and run over in the mortality sink of
the developed landscape.
Evidence that corridors on a finer scale can help animals
avoid predation is accumulating, contrary to the
suggestion of Simberloff and Cox (1987) that thin corridors
may increase the exposure of animals to predators.
Studies of fall movements of blue jays in Wisconsin
have demonstrated that jays usually follow wooded fencerows
in crossing open farmland. Apparently this is a response
to predation from numerous migrating hawks, for
jays frequently dive into fencerow cover when hawks
approach (Johnson & Adkisson 1985). Experimental and
radio-telemetry studies of Peromyscus movements along
fencerows in Ontario farmland are providing data on
what type of cover is preferred by these mice (Merriam
1986). Simberloff and Cox (1987) correctly note that
the question of whether a corridor represents safety or
a threat to an animal can be answered only by considering
ecological factors specific to the organism and the
site.
Most of the other potential advantages and disadvantages
of corridors listed in Figure 1 are self-explanatory.
Two, however, warrant further comment. Although biologists
seldom consider the anthropocentric functions
of "greenbelts" or "open space" in developed landscapes,
these quality-of-life factors are of utmost importance
to landscape architects and planners. Scenery,
recreation, pollution abatement, and land value enhancement
are what usually motivate planners to draw
corridors into their designs (various human uses of corridors
are mentioned in Forman & Godron 1986). And
many of these corridors are being drawn. It would be
auspicious for biologists and planners to work together
to develop corridor designs that can optimize the quality
of both the human and the nonhuman environment.
Finally, a major concern of Simberloff and Cox (1987)
is that the cost of corridors will conflict with conventional
conservation objectives to preserve endangered
species habitat. I prefer to think of the corridor strategy
as a complement to efforts to save "the last of the least
and the best of the rest," as in The Nature Conservancy's
heritage approach (Jenkins 1985). Whereas the heritage
approach generally focuses on relatively small, discrete
sites chosen for their occurrences of endangered elements
(often plant species), corridors are an element of
a landscape-level approach designed to restore and protect
intact ecosystems and wide-ranging animals, many
of which are critically endangered (Noss 1983, 1987a,
b). I share Soule's concern (personal communication)
that corridors might be prescribed as an answer to every
problem, or as a justification for preserves that are too
small. But the fact remains that almost all existing reserves
are far too small to maintain natural ecological
processes and viable populations of the species with the
largest home ranges (Pickett & Thompson 1978, White
& Bratton 1980, Lovejoy & Oren 1981, Schonewald-Cox
et al. 1983, Harris 1984, Noss & Harris 1986).
The corridor strategy can be an important complement
to the strategy of large and multiple reserves (cf. Soule
& Simberloff 1986). Although money for conservation
is never easy to come by, conservationists probably have
not made strong enough demands for funds. For example,
assuming that 26,000 ha of Florida wildland can
be purchased for $20 million (i.e., a recent state acquisition
of a critical coastal corridor), then over 4.5 million
ha could be purchased for the cost of one $3.5 billion
space shuttle. Furthermore, many corridors can be protected
by conservation easements, tax incentives, management
agreements, registry programs, and other lessthan-fee
negotiations (e.g., Noss & Harris 1986), although
these options all involve their own complications
(J. Cox, personal communication).
Conclusions
Perhaps the best argument for corridors is that the original
landscape was interconnected. This is not to deny
that dispersal barriers such as rivers and mountain ranges
have been important in biogeography and evolution, or
that naturally isolated habitats such as lakes, caves,
mountain tops, and edaphic patches are important features
of natural landscapes. But as can be observed
readily in aerial photographs of undeveloped land,
presettlement landscapes in general are interdigitating
mosaics with high connectivity of similar habitats.
Connectivity declines with human modification of the
landscape (Godron & Forman 1983). Hence, wide-ranging
animals such as large predators that once were disConservation
Biology
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Noss Corridors in Real Landscapes 163
tributed almost continuously over entire continents are
now confined to the few remaining pockets of unfragmented
land. Corridors are simply an attempt to maintain
or restore some of the natural landscape connectivity.
No one, to my knowledge, is suggesting that we build
corridors or other connections between naturally isolated
habitats.
Certainly, humans and associated disturbances will
impinge on corridors, just as they impinge on small nature
reserves. For this reason, corridors should generally
be as wide as possible. Planners and conservationists
often ask how wide corridors need to be, and corridor
widths (especially for riparian corridors) are often specified
in land-use plans. In reality, the necessary width
will vary depending on habitat structure and quality
within the corridor, the nature of the surrounding habitat,
human use patterns, and the particular species that
we expect to use the corridor. Narrow fencerows might
suffice for many farmland species, but wilderness species
such as large carnivores may require corridors many
miles wide to travel safely among reserves. A wide enough
swath, of course, effectively creates one large reserve
out of two or more smaller reserves.
Corridors are not tbe answer to our conservation
problems. Undoubtedly, in many situations acquiring
marginal habitat for corridors should be of lower priority
than preserving isolated sites for endemic or endangered
species. The major area of common ground among the
various conservation biologists involved in this debate
is that we are all interested in maintaining biodiversity,
and most of us are wary of facile generalizations and
analogies as guides to conservation. We furthermore
agree that conservation actions must be based on the
autecologies of the species concerned, and on other sitespecific
attributes. But a holistic, "top-down" framework
can be useful in providing context to autecology. When
money is limited, as it always will be, alternative actions
should be weighed carefully. Weighing alternatives is
made easier by evaluating their potential contributions
to a landscape conservation strategy that addresses the
overwhelming problem of habitat fragmentation.
Acknowledgments
My thinking on corridors has benefitted from discussions
and correspondence with Jim Cox, Richard Forman, Larry
Harris, Michael Soule, Jack Stout, David Wilcove, and
many others. I particularly thank Jim Cox, David Ehrenfeld,
Robert May, and an anonymous referee for their
helpful comments on an earlier draft of this paper.
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