Essay
Movement Corridors: Conservation Bargains or
Poor Investments?
DANIEL SIMBERLOFF
JAMES A. FARR
Department of Biological Science
Florida State University
Talla~e, FL 32306, U.S.A.
JAMES COX
Florida Game and Fresh Water Fish Commission
620 S. Meridian Street
Tallahassee, FL 32399, U.S.A.
DAVID W. MEHLMAN
Department of Biological Science
Florida State University
Tallaha&~e, FL 32306, U.S.A.
and
Department of Biology
University of New Mexico
Albuquerque, NM 87131
Abstract: Corridorsfor movement of organisms between refuges
are confounded with corridors designed for otherfunction~
obscuring an assessment of cost.effectivenesx The rationales
for movement corridors are (1) to lower extinction
rate in the sense of the equilibrium theory, (2) to lessen
demographic stochastictty, (3) to stem inbreeding depression,
and (4) tofulfill an inherent needfor movement There
is a paucity of data showing how corridors are used and
whether this use lessens extinction by solving theseproblem~
Smal~ isolated populations need not be doomed to quick
extinction from endogenous forces such us inbreeding depression
or demographic stochasttctty, if their habitats are
protectedfrom human.~ In specific instance~ corridors could
have biological disadvantage~ Corridorproposals cannot be
adequately judged generically. In spite of weak theoretical
and empirical base~ numerous movement corridor projects
are planned In the State of Florid~ multi-million-dollar
corridorproposals are unsupported by data on which species
might use the corridors and to what effect Similarly, plans
for massive corridor netumrks to counter extinction caused
Paper subraitted August 13, 1991; revised manuscript acceptedFebruary
27, 1992.
Corredores para el movimiento: ~Gangas de la conservaci6n
o malas inversiones?
Resumen: Los corredores para el movimiento de organismos
entre refugios son confundidos con corredores designados
para otras funciones obscureciendo una evaluaci6n so.
bre costo-efectividad. Las funciones atribuidas a los
corredores para movimiento son (1) disminuir la tasa de
exttnci6n deflnida en td~minos de la teoria de equilibrio, (2)
disminuir la estocasticidad demogrdfica~ (3) contrarrestar
la deprest6n endogdmica y (4) satisfacer una necesidad innata
de movimiento. Existe una carencla de datos que demuestren
como son usados estos corredores y si este uso
minimtza las extinciones al resolver estosproblema~ Poblaclones
pequetias y aisladas no estdn necesarlamente condenadas
a una rdpida extinci6n causada por fuerzas en.
d6genas, como depresi6n endogdmica o estocasticidad
demogrdfic~ si sus dbitats estdn protejidos de los humano~
En instanclas particulare~ los corredores pueden tener desventaJas
biol6gica£ Las propuestas sobre corredores no
pueden serju~adas apropiadamente enforma gend~caL Numerosos
comedot~ para movtmiento estdn pmyectados a
pesar de fundamentos te6rtcos y emptricos ddbile~ En el
493
ConservationBiology
Volume6,No. 4, December1992
494 MovementCorridors Siraberloffet al.
by global warming are weakly supported Alternative approaches
not mutually exclusive of corridors might be more
effective, but such a judgment cannot be made without a
cost-benefit analysis.
estado de Florid~ propuestas multtmtllionarias para corre.
dores no estdn fundamentadas por datos que tndiquen que
especies usarian los corredores y con que prop6sito 1o hariar~
En forma semejant¢ los proyectos de redes masivas de
corredores para contrarrestar la extinci6n causada por el
calentamiento global estdn pobremente fundamentado~ Estrategias
alternativa~ que no son mutuamente excluyentes
con los corredore~ podrian ser mas efectiva~ pero tales
juicios no pueden ser hechos sin un andlisis de costo-
beneficio.
Introduction
A remarkable publicity campaign, much of it outside the
bounds of mainstream science, has promoted corridors
for conservation. Wilson and Willis (1975) originally
proposed corridors based on the equilibrium theory of
island biogeography (MacArthur & Wilson 1967); the
suggestion was reprinted in World Conservation StrateKF
(International Union for the Conservation of Nature
and Natural Resources [IUCN] 1980). With the imprimatur
of the IUCN, the United Nations Environmental
Program, and the World Wildlife Fund, the idea was
widely accepted. The popular promotion of corridors
entails lead articles in lay magazines (such as Arnold
1990), a videocasette (Suchy & Harris 1988), a pamphlet
(Anonymous 1990a), the entire December 1986
issue of ENFO, and a special publication by Defenders of
Wildlife (Mackintosh 1989). In Montana, a court has
ruled that corridors are scientifically established as important
(Breen 1991; Pace 1991 ). Keith Hay of the Conservation
Fund argued that corridors "hold more promise
for the conservation of the diversity of life than any
other management factor except stabilization of the human
population" (Chadwick 1990).
This hype is occurring in spite of a dearth of evidence
of whether corridors will be useful in specific situations.
"The workshops that discussed the values of corridors
were certain that they were positive features despite the
lack of supporting research" (Dendy 1987). That they
facilitate movement "is now almost an article of faith"
(Hobbs & Hopkins 1991). Saunders & Hobbs (1991a)
believe that we do not have time to test the efficacy of
corridors, while Noss (1992) argues that, in the face of
uncertainty, it is prudent to maintain or restore "natural"
kinds of corridors.
Discussion of corridors is confused (Anonymous
1986). We are concerned in this article with corridors
for movement, but at least six senses of "corridor" appear
in the conservation literature.
First, some habitats constitute corridors. A corridor
may deserve protection as a distinct habitat, whether or
not it aids movement (Simberloff & Cox 1987). For
example, riparian communities are very threatened in
some regions (Johnson 1989), but their value as habitats
is independent of whether they allow movement between
other habitats. To embed a discussion of riparian
habitats in an argument about movement corridors (see
Johnson 1989; Ames 1990) confounds assessment of
strategies for land acquisition. Some linear artificial habitats,
such as rights-of-way for highways (Adams & Dove
1989; Wilcox 1989), railroads (Noss 1992), and transmission
lines (Anderson et al. 1977; Kroodsma 1982;
Forman 1983), are also called "corridors," and can bolster
animal populations and enhance urban and suburban
green space (Adams & Dove 1989). Again, their
utility as habitats is separate from their importance for
movement.
Second, greenbelts and buffers are occasionally called
"corridors," and their function of ameliorating the human
environment is cited in discussions of movement
corridors (see Gilbrook 1986; Budd et al. 1987; Adams
& Dove 1989). Although such areas might aid dispersal,
some envisioned uses (such as hiking and horse trails,
boating, outfaU for storm sewers) could impede their
utility as either dispersal routes or habitats. In any event,
the value of such constructs as aesthetic amenities is
independent of their value for movement.
Third, Harris (1985) and Suchy and Harris (1988) call
biogeographic landbridges such as the Isthmus of Panama
"corridors." Such large regions have aided intercontinental
movement of entire communities (Brown &
Gibson 1983), but the relevance of this fact to maintaining
viable populations in refuges is obscure (Simberloff
& Cox 1987).
Fourth, a series of discrete refuges for migratory waterfowl
is occasionally called a corridor (see Harris
1985; Anonymous 1991). Below we will suggest that
such a system can be construed as an alternative to
corridors or complementary to a corridor networlc
Fifth, underpasses and tunnels are often now called
"corridors." They are commonly used to allow animals
to cross highways (reviewed by Bennett 1990b; Noss
1992), primarily to keep individuals from being killed
on the road rather than to decrease demographic stochasticity,
prevent inbreeding depression, or serve
other population-level functions. Costs of such strucConservation
Biology
Volume6,No,4,December1992
Stmberloff et al. Movement Corridors 495
tures should be construed as part of road construction
rather than as land acquisition.
Sixth, and our concern here, are strips of land intended
to facilitate movement between larger habitats.
Wilson and Willis (1975) and Harris and Scheck ( 1991 ),
in the context of equilibrium theory, envision such
movement as increasing immigration rates, thus raising
the equilibrium number of species at each site. Harris
(1984, 1985) suggested two other rationales for corridors.
First, individuals of some species typically range
widely; second, inbreeding depression will lead to extinction
in Small refuges. Recently, alleviation of demographic
stochasticity has been a suggested benefit of
corridors (see Merriam 1991; Thomas 1991; Noss
1992).
We are particularly concerned with the economic
cost of corridors. Hobbs and Hopkins (1991) talk of
corridors as adding an option to a sparse conservation
repertoire, but options may be foreclosed if a particular
corridor is reserved. Noss (1992 and personal communication)
believes it is prudent to retain existing corridors,
conceding that establishing new ones may be costineffective.
But even an existing corridor is not
necessarily free, and prudence when all options cannot
be pursued requires some sort of cost-benefit analysis of
each.
Rationales for Movemem Corridors
The EquilibriumTheory of Island Blogeography
The equilibrium theory of island biogeography states
that species number is constant, but that local turnover
changes composition. Thus, corridors are useful by virtue
of maintaining more species. An odd aspect of the
rush to found a technology of refuge design based on
island biogeographic theory is that, exactly when the
IUCN and others were popularizing refuge design based
on equilibrium theory, the theory was increasingly
heavily criticized (Gilbert 1980; Williamson 1981,
1989; Williams 1984, 1986) as inapplicable to most of
nature, largely because local population extinction was
not demonstrated.
DemographicStochasticityand the Metapopulation
Although not cited in the original recommendations for
corridors, demographic stochasticity has been widely
recognized as a potential threat to small populations
(references in Simberloff 1988), and its diminution is
now viewed as a rationale for corridors (see Merriam
1991; Thomas 1991; Noss 1992). No unified theory
combines genetic, demographic, and other forces
threatening small populations, nor is there accord on
the relative importance of these threats. Lande (1988)
believes that demography will usually be more important
than genetics to very small populations, while
Goodman (1987a) sees demographic stochasticity as
important only when just a "handful" of individuals re-
main.
The metapopulation paradigm (Levins 1970) has replaced
equilibrium theory for habitat islands (Merriam
1991 ), partly because of absence of evidence for turnover.
The metapopulation is seen as lessening or redressing
extinction by demographic stochasticity of the
component populations. As with the equilibrium theory,
however, there is now a tendency to take the metapopulation
paradigm as broadly representative of nature,
rather than as a new and untested hypothesis. Such
statements as, "Many or most species are distributed as
'metapopulations'" (Noss 1992) are simply unwarranted.
There are few empirical data, no specification of
the range of dispersal rates that qualifies a group of
populations as a metapopulation, and a variety of untested
metapopulation models (references in Hanski &
Gilpin 1991), none of which has been demonstrated to
represent many situations in nature. Even if the metapopulation
model should be shown to apply to a particular
situation, the model requires movement, not corridors
(Merriam 1991).
InbreedingDepression
A genetic argument for corridors extends the concept of
movement to genes as well as individuals. Harris (1984,
1985) argues that gene flow is required to prevent inbreeding
depression from causing extinction and that
corridors are needed for this gene flow.
Although a degree of inbreeding depression is usually
found in captive animal populations, its threat must be
established empirically. Some species with little genetic
variation suffer no inbreeding depression (such as Pere
David's deer and the European bison [Frankel & Soul6
1981] ). It is often said that such species were luckym
their populations probably shrank gradually, and natural
selection removed deleterious alleles that cause inbreeding
depression. This line of reasoning implies that
inbreeding depression is likely if normally outhred species
suddenly inbreed (Frankel & Soul6 1981 ), as might
be induced by rapid habitat fragmentation. However, in
these cases most of the population decline occurred in
a few generations. Therefore it remains mysterious why
some species have little or no inbreeding depression. In
nature, evidence from animal populations is extremely
scarce (Charlesworth &'Charlesworth 1987). The oftcited
evolution of behavior that reduces inbreeding
need not have been selected by inbreeding depression
(Charlesworth & Charlesworth 1987). In higher plants
in nature, a measure of inbreeding depression has almost
always been found when sought (Charlesworth &
Charlesworth 1987). However, adequate data have been
collected from far too few species.
Conservation Biology
Volume 6, No. 4, Decembex 1992
496 MowmentCorrMors Simberloffet al.
Finally, finding that small populations are threatened
by inbreeding depression is different from demonstrating
its existence (Lande 1988). To evaluate the threat
from inbreeding depression, it is important to bear in
mind that a loss in genetic fitness need not endanger a
population. "Inbreeding depression" means that more
inbred individuals are less fit than less inbred ones. Most
populations of most species, however, generation after
generation produce some individuals less fit than others,
yet are not endangered. So it is not axiomatic that inbreeding
even if it should lead to inbreeding depression,
is a major threat to small populations, relative to
other threats (Lande 1988). In any event, even if inbreeding
depression is found to threaten a population,
the key question is whether corridors are the best way
to stem it; we return to this point below.
Need of Individual Animals for Movement
The Northern Spotted Owl (Strix occidentalgs caurina )
has a median pair home range in most areas of 1200 to
2000 ha (Thomas et al. 1990). Home ranges are larger
in areas with less old-growth forest habitat (references
in Thomas et al. [1990] ), suggesting that the crucial
requirement is enough small mammal prey inhabiting
this habitat. There is an indication that owls that do not
get enough favored prey experience lowered reproduction
rates, but there is no suggestion that failure of space
for movement per se is limiting. Mortality of dispersing
juveniles is severe, but Thomas et al. (1990) argue
against even wide corridors specifically for owl movement
on the grounds that predators may thrive in them.
Spotted Owls appear to disperse in random directions,
which also argues against a corridor strategy (Noss
1992). Rather, Thomas et al. (1990) suggest management
of the entire matrix surrounding owl habitat conservation
areas to make it suitable for owl dispersal.
Harris (1985) and Noss and Harris (1986) have adduced
the large home ranges of Florida panthers (Felis
concolor coryi) and black bears (Ursus americanus ) as
a rationale for corridors. Most conservation areas in
Florida are too small to provide the approximately 80
km 2 covered by a male black bear (Lindzey & Meslow
1977; Wooding & Hardisky 1990) or the approximately
400 km2 covered by a male panther (Belden 1989) each
year, and Maehr and Harris (1986) believe that areas as
large as 800 km 2 may not support viable populations of
either species. However, black bear populations have
persisted despite confinement to small areas (Lindzey &
Meslow 1977). Corridors might allow some species to
avoid potentially fatal intraspecific encounters. Territorial
battles between male panthers and between male
black bears are sometimes lethal (Kemp 1976; Belden
1989). There are instances of black bear cannibalism
(Tietje et al. 1986), and young black bears establish
territories more readily at low densities (Kemp 1976;
Rogers 1987). Therefore if corridors actually aided dispersal,
they might be beneficial, but there is little evidence
that dispersal is mediated by such social interactions
(Rogers 1987) or that bears use corridors to
disperse.
No Florida panther restricts its movements to one
protected area, and panthers use a "hardwood strand"
corridor linking the Big Cypress National Preserve to
unprotected habitat (Maehr 1990). Hardwood strands
follow depressions and slow-moving watercourses in
Florida (Maehr & Cox, unpublished data). This particular
strand is less than 5 km long--much less than the
daily ambit of a panther (Belden 1989)---and bounded
by agricultural areas, which panthers avoid. How well
panthers would use a much longer corridor is unknown.
Long-distance dispersal by bears and panthers is not
nearly as well documented. Maehr et al. (1988) describe
one black bear dispersal of 120 km, but they also
contend that well-defined corridors are not necessary
for bears so long as habitat does not fully impede move-
ment.
In sum, the need for movement per se is doubtful,
though animals may be driven to move for specific purposes,
such as finding food or avoiding conspecifics. The
key questions are whether such movement is necessary
in a specific refuge system and, if it is, whether corridors
of specified characteristics are the best strategy to facilitate
such movement.
How Inevitable is Quick Extinction of Small Populations?
Given the attention paid in recent conservation literature
to threats to small populations, it might seem that
any small, isolated population is doomed in the short
term. Many go extinct (see Petterson 1985). But many
small populations persist and cannot easily be dismissed
as recently-reduced populations en route to extinction.
Numerous endemic species of small islands are endangered
primarily by either habitat destruction or introduced
species (references in Simberloff 1986; see Craig
1991) but apparently thrived for millennia in the absence
of humans, though many probably numbered at
most in the low hundreds. Species in the highest trophic
levels are particularly striking. For example, the RedTailed
Hawk (Buteo jamaicensis socorroensis) of Socorro
Island (southwest of Baja California) has had a
stable population of about 20 pairs for at least the last
few decades, and there is no reason to think the population
was larger in antiquity (Walter 1990). Of course,
we do not even know the fates of many isolated populations
that disappeared in the absence of human interference,
but rapid extinction is not automatic.
Evidence for Corridor Use
Simberloff & Cox (1987) found few empirical data on
corridor use and discovered that most reported obserConservation
Biology
Volume 6, No. 4, December 1992
Simberloffet ~1. MovementCorridors 497
vations were ambiguous. These problems persist
(Hobbs & Hopkins 1991; Nicholls & Margules 1991;
Saunders & Hobbs 1991a). There are still few data, and
many widely cited reports are unconvincing.
Probably the first advocacy of corridors based on specific.
data was for birds of forest patches in the northeastern
U.S. (MacClintock et al. 1977). This study is
often cited (see Greenberg 1990) as showing that corridors
increase diversity. The study was uncontrolled;
there were no isolated sites not connected by a corridor
to a larger forest, and no data on corridor use for sites
that were connected to a larger forest. A frequently
cited (see Noss 1992) recent paper (Saunders & Ingram
1987) contends that "Comparison of breeding results
from all five populations studied and the amount of native
vegetation remaining in each area showed that
Carnaby's Cockatoo [Calyptorhynchus funereus latirostr/s]
can breed successfully in areas which have been
extensively cleared provided there are corridors of native
vegetation connecting patches of remnant vegetation."
In fact, data from this paper do not show that
corridors are more important than amount of vegetation
in the remnant (Saunders, personal communication
1987). However, additional uncited data (Saunders
1980) showed that, at the one site where the cockatoo
disappeared, feeding areas near railway and road verges
were used but did not connect to a nearby reserve.
Bennett (1987a), studying the long-nosed potoroo
(Potorous tridactylus) in Australian forest patches, concluded
that forest strips along roads and creeks aid dispersal
and allow persistence in a fragmented landscape.
No data in this paper addressed this question, but the
paper cited a dissertation (Bennett 1987b). There the
conclusion rested solely on two animals trapped in forested
corridors, while there were no traps outside of
corridors. Bennett (1990a) similarly felt that narrow
forested corridors "facilitate continuity between populations"
of eight mammals in Australia but did not study
movement outside of corridors. Suckling (1984) argued
that forested roadside strips connecting woodland
patches prevent local extinction of the sugar glider (Petaurus
breviceps) in Australia. However, he did not trap
outside corridors and forest patches, and the only unconnected
patch of the three studied was less than half
the size of the others. Thus there are no data on increased
dispersal with corridors or increased extinction
without them. Bennett (1990b) argued that "all known
dispersal movements" of the sugar glider involved corridors;
in fact, none were sought otherwise.
Assessing these claims was difficult, entailing correspondence
and examination of an unpublished document
(Bennett 1987b) in Australia that could not be
procured by interlibrary loan in Florida. One cannot
expect management personnel to expend such effort,
yet without this examination one would be left with a
misleading impression. For example, Ogle (1989) noted
that a 1986 corridor proposal in New Zealand was questioned
on the grounds of lack of evidence on corridor
use, "notwithstanding the plausibility of the concept
and a growing body of supportive data from overseas."
There were, at that time, virtually no supportive empirical
data, only a plethora of statements advocating the
concept (Simberloff & Cox 1987). Ogle (1989) went on
to cite new supporting data:
Biological corridors have been the subject of considerable
field research in Australia, and their importance for
movements of fauna between otherwise discrete
patches of habitat has been established on, for example,
small marsupials (Bennett 1987) and Cockatoos (Saunders
& Ingrain 1987). Bennett showed that narrow
strips of forest along road verges and water courses provide
routes for the dispersal of adult long-nosed potoroo
(Potorous) between remnants of native vegetation in
agricultural lands of south-west Victoria. Saunders & Ingram
have shown that Carnaby's Cockatoos (Calyptorhynchusfunereus
latirostris) in Western Australia nest
in isolated forest remnants in agricultural land, and feed
in remnants of native heathland. Native vegetation of
road verges provides some feeding habitat and the
routes (corridors) between nesting and feeding areas.
Ogle was at pains to show that corridors are critical for
these species, but neither study cited demonstrated
their importance.
Nicholls and Margules ( 1991 ) detail statistical difficulties
of experiments to show that corridors enhance
movement. Largely because of problems achieving sufficient
experimental sample sizes, they acknowledge
that observational studies could be useful. As the examples
above show, however, many such studies are inconclusive,
particularly because of failure to examine
movement without corridors. That an animal uses corridors
when these are present need not mean movement
without them is impossible, or even less frequent.
Of 36 papers in "The Role of Corridors" (Saunders &
Hobbs 1991b), five present new data on animal movement
(Arnold et al. 1991; Catterall et al. 1991; Date et al.
1991; Prevett 1991; Saunders & de Rebeira 1991). Of
these, only Arnold et al. (1991) gathered data on movement
between habitat patches without corridors. Only
three (Arnold et al. 1991; Date et al. 1991; Prevett
1991 ) concluded that corridors have a very small role in
conserving a particular taxon.
Regional Corridors to Alleviate the Effects of
Global Warming
The interaction of fragmentation with global warming
has led to a different sort of corridor proposal. Global
warming can cause a species' present sites to change
more quickly than the species can evolve (Peters &
Darling 1985). During earlier climatic changes many
species could shift their geographic ranges to remain in
their favored habitat. As habitats have become fragmented,
routes are increasingly blocked (Peters 1988).
Conservation Biology
Volume 6, No. 4, December 1992
498 MoveraentCorridors Simberloffet al.
Graham (1988), Hunter et al. (1988), and Harris and
Gallagher (1989) suggest a vast network of corridors
over North America. Hunter et al. (1988) feel that even
a 300-meter-wide corridor stretching thousands of kilometers
could be very useful. Few data are available to
assess this plan. Whether a strip 300 meters wide would
allow the range shifts that typified the Pleistocene has
barely been discussed. Noss (1992) worries that the
anticipated speed of temperature increase may render
such corridors of little use for many species. Hobbs and
Hopkins (1991) admit that the utility of corridors in
counteracting effects of global warming is very uncertain,
but they argue for going ahead anyway.
If a corridor is intended to preserve an entire community,
as in the global warming recommendation, it is
particularly important that it be wide enough to permit
breeding as well as movement. One need only consider
the limited mobility of many soil invertebrates and
plants to realize that a range shift would be painfully
slow and require many generations.
A corridor to permit survival and breeding is serving
as more than a corridor for movement; it is a habitat in
its own right. Such a requirement raises the stakes considerably.
One must know if the interior of a corridor
functions as the intended habitat rather than as edge.
Both micrometeorological effects and biotic intrusions
can propagate an edge effect far into the forest (see
Levenson 1981;Janzen 1983, 1986; Wilcove et al. 1986;
Kapos 1989). Nest predation studies (such as Wilcove
1985; Andren & Angelstam 1988) also point to a substantial
edge effect. The modified habitat in thin corridors
is the motivation for the Klamath Corridor Proposal
(Pace 1991), which envisions a corridor 5.5 km
wide, connecting two wilderness areas 26 km apart in
the Klamath National Forest. This proposal specifically
aims at a corridor that is a habitat, not just a travel route.
ment would occur with and without corridors. Depending
on metapopulation structure, as little as one
breeding migrant per generation can render a population
effectively panmictic (Lande & Barrowclough
1987), which raises the possibility that even if a corridor
increased movement it might be unnecessary genet-
ically.
Spread of Catastrophes
Phenomena such as fires, diseases, or introduced species
can spread through a corridor (Simberloff & Cox 1987).
For example, in New Zealand, introduced feral pigs may
eliminate Paryphanta snails from large forest fragments
but do not invade small, isolated ones (Ogle 1987). Similarly,
introduced vertebrate browsers in New Zealand
have almost eliminated mistletoe from large forest fragments,
but not from isolated small groups of trees (Ogle
& Wilson 1985). In most instances, however, the added
risk of spreading catastrophes via corridors may be low
because introduced predators or diseases can reach
fragments without corridors.
Corridors as Reservoirs of Edge and Introduced Species
Several authors have suggested that corridors might be
inimical as a habitat in their own right. For example,
forest corridors have a high fraction of edge habitat and
might attract edge-inhabiting predators (Ambuel &
Temple 1983). The same concern is one reason for the
disenchantment with corridors on the part of Thomas et
al. (1990), and numerous other authors have pointed to
this potential liability in particular systems (see Catterall
et al. 1991). Others have warned that some corridors
may favor movement by introduced species (see Forman
1991; Hobbs & Hopkins 1991; Panetta & Hopkins
1991).
Potential Biological Disadvantages of Corridors
Genetic and Demographic Costs and Benefits
Although the possibility of inbreeding depression would
likely be lowered if corridors were used, the possibility
of loss of alleles to drift in an ensemble of refuges would
increase (references in Simberloff [1988] ). This tradeoff
is inevitable; which problem should be of more concern
depends on the severity of inbreeding depression
(an empirical matter) and how pronounced and dangerous
the slowing of future evolution by drift will be.
There is no unanimity on this matter (Simberloff 1988).
Goodman (1987b) has argued that effects of demographic
stochasticity can be greatly lessened by a modest
amount of migration among refuges, though he gives
no figure. Whether a corridor would be the only or even
the best way to provide whatever movement is necessary
would depend on information on how much moveCorridors
as Traps or Sinks
Henein and Merriam (1990) and Soul~ and Gilpin
( 1991 ) propose on the basis of simulation modeling that
low-quality corridors could act as sinks, decreasing the
size of a metapopulation. Field data on this proposition
are nonexistent.
Discussion
The notion that corridors can't hurt, even if the possible
biological costs could be discounted, is not necessarily
always true. Much would depend on the relative costs
and benefits of a proposed corridor and alternative uses
of the funds (Simberloff& Cox 1987). Because there are
so few data on the importance of movement through
corridors, such an analysis will be very dit~cult. We
believe no thorough analysis of this sort has ever been
conducted. Possibly the most that can be done today is
Conservation Biology
Volume 6, No. 4, December 1992
Simberloffet al. MovementCorridors 499
to say that some options are much less likely to be important
than others, but even this attempt is rarely
made. The enormous price of some corridor proposals
surely implies that other options would not bepursued.
For example, a 300-meter-wide corridor from south
Florida to Canada, proposed by Hunter et al. (1988),
would be approximately 720 km 2, enough for a very
large refuge. If it were possible to buy conservation
lands of this magnitude, would this corridor be the best
possible purchase?
Florida provides examples of the uncritical advocacy
of extremely expensive corridors. Florida's Conservation
and Recreation Lands (CARL) acquisition program
has an annual budget of approximately $50 million from
the sale of real estate stamps and an additional $135
million from the Landmark Preservation 2000 bond program.
Preservation 2000 bonds have been authorized
for the first two years only, although the program is
envisioned to extend ten years. Proposed acquisitions
still far exceed available funds. For example, of 93 CARL
proposals passing a stringent double review by 1991, a
priority list of 60 had a tax-assessed value of
$408,000,000 (fair market values are often twice taxassessed
ones). Thus, every acquisition means other
land will not be acquired. Citizens of several counties
have voted to tax themselves to acquire land. Many such
counties apply for joint acquisition projects to the CARL
program. County governments also face lists of projects
far exceeding available funds.
The Florida Natural Areas Inventory submitted the
Blackwater-Eglin Connector project to the CARL program
in 1988 (Anonymous 1988). This 2400-ha corridor
was intended to connect Eglin Air Force Base
(187,500 ha)with the Blackwater River State Forest and
the adjacent Conecuh National Forest (143,250 ha combined).
Anticipated fair market value exceeded $5 million.
The corridor was "to join three large publicly
owned areas ... into a single, uninterrupted ecological
unit of nearly one million acres; to assure perpetual opportunities
for gene flow among all populations in these
areas; to allow for direct movements of individuals of
more vagile species..." (Anonymous 1988). The application
spoke of the "urgency and desirability of developing
systems of interconnected reserves for maintaining
the full scope of biotic diversity" and stated that
"connections between reserves create buffers against
stochastic changes in populations, including local extinctions,
and provide suitable pathways for 'natural' inoculations
of new genetic material.... "
No data showed that any species needed to move
from one site to the other. Black bears are mentioned as
potential users of the corridor, but they do not occur
regularly in the Blackwater River State Forest or north of
it (Brady & Maehr 1985). The corridor is complicated
by Interstate 10, which separates Eglin from Blackwater.
While small culverts run under the highway, there is no
evidence that bears or other species use them or that
they would substantially decrease highway mortality.
Black bears are loath to cross interstate highways (Brady
& Pelton 1989).
The Blackwater-Eglin Connector was also proposed
to aid the movement of Red-cockaded Woodpeckers
(Picoides borealis) (Anonymous 1988). The woodpeckers
move up to 90 km (Waiters et al. 1988a), but
we doubt that this corridor would be effective. Of
breeding females, 19% move per year, fewer than 5% of
these move more than 5 km, and about 70% survive the
year (Waiters et al. 1988b). Therefore, the chance that
a female will move that far and survive is about 0.7% per
year. For all life-history categories together, the annual
chance that a bird will disperse and survive is 4--8%. For
breeding, this percentage is at least halved, because over
60% of dispersal is by first-year birds that produce only
20-67% as many fledglings as older ones.
Probabilities must be further reduced depending on
the chance that a bird finds and uses the corridor. The
distance traversed by the corridor is about 6 km, but
midpoints of the two refuges are 45 kin apart. Much less
than 1% of the population monitored by Waiters et al.
(1988b) moved this far, so the great majority of dispersers
will probably stay within their original refuge. The
corridor itself is unsuitable for the woodpecker. Greatly
altered by logging, it consists of streamside hardwoods
and commercial pinelands. Several streams through the
site have been impounded to create amenity lakes. It
would surprise us if even 1% of dispersing individuals
used this corridor, but if that many did, 0.04% of the
population might reach the adjacent refuge and contribute
to future generations. About 300 colonies inhabit
both refuges (Wood & Wenner 1983), and each colony
produces 1.2-1.7 fledglings per year (Waiters et al.
1988b). So one successful dispersal might occur every
5-7 years. The corridor would thus be a very expensive
way to move very few woodpeckers. It is unfair to ascribe
the entire cost of a corridor to one or two target
species when others would surely use it, but an expenditure
of this size merits discussion of the full roster of
species that would benefit.
This proposal was not approved by the CARL program.
This fate contrasts with that of the Pal-Mar project,
submitted to the CARL program in 1990 as a 8860
ha site in Martin and Palm Beach counties (Anonymous
1990b). Its fair market value was estimated at $22 million.
The site is a large, relatively intact system of mesic/
wet flatwoods and depression marsh/wet prairies containing
many listed plant and animal species--perhaps
the biggest relatively undisturbed functional wetland in
south Florida east of the Everglades.
After the original application had been submitted,
staff of the Florida Department of Natural Resources suggested
expanding the project (Timmerman et al. 1991 ).
They stated that the main thrust of a plan they had been
Conservation Biology
Volume 6, No. 4, December 1992
500 MorementCorndors Simberloffet al.
developing to protect biological diversity in southeast
Florida was establishment of two large corridors, and
they suggested adding a corridor connecting Pal-Mar to
Jonathan Dickinson State Park, which contains approximately
4850 ha. This corridor, subsequently added to
the proposed Pal-Mar project, is 1.6 km wide and 10 km
long, with one other section of land. The corridor is thus
18 km 2. It is bordered to the north by orange groves and
to the south by residences. No estimate of cost was
provided, but the proposed corridor contains substantial
developable uplands, and its tax-assessed value is
$21,000,000. Not only did the proposal request addition
of a corridor (without biological data on which species
would use it or why it is necessary), but it added that
the corridor was key to the whole project and should be
purchased before the original proposed acquisitions.
The letter failed to mention that this corridor is traversed
by two major highways. The final project design
with recommended acquisition phasing has not yet
been completed.
The detailed assessment of the Pal-Mar project, prepared
by staff of the CARL land acquisition selection
body (Anonymous 1990b), stated: "Acquisition of the
Pal-Mar proposal will secure a corridor connectingJ, w.
Corbett Wildlife Management Area to Jonathan Dickinson
State Park, and prevent otherwise inevitable development
that would isolate the state park and lead to
faunal extirpations" (our italics). Indeed, despite the
fact that acquisition of the site proposed initially would
protect a vast wetland system containing many stateand
federally-listed species, the project assessment said,
"Acquisition of the Pal-Mar tract would serve several
purposes of regional significance, theprimary one being
the protection of a large portion of a proposed wildlife
corridor that would link Jonathan Dickinson State Park
to J. w. Corbett..." (our italics). The assessment even
acknowledged that "Perhaps the most significant hurdle
that this plan must overcome is the fact that wildlife
routes between the state park and Pal-Mar are impeded
by 1-95 (six lanes) and the Florida Turnpike (four lanes),
which run side by side less than a mile west of the park."
Thus, even though large mammals have never been documented
on the site and would need to cross ten heavily
traveled lanes, the State of Florida is considering buying
an 18 km 2 corridor on very expensive developable
south Florida land.
We detail these examples because we feel that no
abstract discussion of corridors can adequately guide
acquisition and management decisions. There may well
be specific cases in which a proposed corridor would be
more effective than an alternative. For example, in parts
of the American West, where many potential highquality
corridors are already in public ownership, one
might expect acquisition costs to be low (Noss, personal
communication) and use high. Noss (personal
communication) suggests that elimination of below-cost
timber sales in such sites might even save public money.
However, the same benefits might derive from other
land that might be set aside in the same region; are these
the acres we would most wish a public agency to sequester?
Noss (1987) believes that even major corridor
costs can be met by the right sort of publicity and/or by
peace dividends. We hope he is correct, but if he is,
could not other conservation expenditures similarly increase?
Surely costs and benefits of alternative strategies
should be considered.
It is important to realize that there are general alternative
strategies to facilitate survival where refuges are
insufficient. Franklin (1989) has recently propounded
the "new forestry," a collection of methods by which
even logged forests in the American Northwest might be
more useful for conservation than they are now. The key
is that the entire landscape be managedas a matrix supporting
the entire biotic community. The specific costs
entailed in such management have not been tallied. In
the longleaf pine forests of the American Southeast, topography
and aspects of the biology of key species suggest
that such an approach would not be prohibitively
expensive (Simberloff 1992). The tablelands of northern
New South Wales can be managed so that the entire
landscape, not just reserves, contributes heavily to the
conservation of the community (McIntyre 1991).
Numerous authors have pointed to the importance of
small unconnected patches of forest or even single trees
for the persistence of populations (see Date et al. 1991;
Prevett 1991). McDowell et al. (1991) call for a network
of such "stepping stones" connected by corridors
to minimize extinctions in the South African fynbos
community. If resources are insufficient for both, which
would be more useful? How dense and how large must
a network of stepping stones be to be useful in this
system?
Of course these strategies are not incompatible; corridors
and stepping stones could be part of an entire
landscape managed for both extraction and conservation.
However, limited funds will almost certainly prevent
the simultaneous adoption of all possible approaches,
and it is simply not useful in practice to
advocate "strategically placed larger reserves complemented
by local or regional networks of smaller reserves
and short connecting links between them in as
many directions as possible" (Blyth 1991 ). One must be
willing to set priorities, and these should be based on
relative costs and benefits. We do not agree with Saunders
and Hobbs (1991a) that we should preserve all
corridors now because we do not have the time to find
out if they are useful. If, by preserving them, we fail to
pursue other options, such an approach is not prudent.
Finally, to the extent that this trend is encouraged by
the fact that the concept of corridors is easily understood
by the public and legislators (Harris & Gallagher
1989) or that "people feel they are doing something for
ConservationBiology
Volume6,No.4, December1992
Simberloff a al.
conservation" (Bennett, in Stolzenburg 1991), it is not a
scientific phenomenon at all; but it still has costs and
benefits, and these are not being addressed. Is it good
conservation biology to sell legislators and the public on
the easiest program for them to understand, in the absence
of evidence that it is the most effective one? Is it
beneficial for people to feel they are doing something
important for conservation by preserving narrow roadside
strips (Bennett's corridors) in the absence of evidence
that they really are doing something? Even if they
are, is preserving such corridors suf~cient? Does it foster
the belief that one has done enough and need not
preserve larger tracts of valuable habitat? We cannot
answer such questions, but we believe that it is important
to raise them.
Acknowledgments
Bob Jenkins, Reed Noss, Jack Ward Thomas, and three
anonymousreferees provided numerous suggestions for
improving this manuscript.
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CotmervationBiology
Volume6, No. 4, December1992