Note
Toward a Theory of Inter-Refuge Corridor Design
ROBERT L. HARRISON
Department of Biology
University of New Mexico
Albuquerque, N.M. 87131, U.S.A.
Introduction
The accelerating fragmentation and isolation of wildlife
populations as a consequence of habitat alteration has
resulted in increased interest in the preservation of
travel corridors between populations to minimize local
extinction and genetic isolation (Harris 1984; Noss &
Harris 1986). Landscape architects and other planners
now frequently include such wildlife corridors in development
designs. However, little work has been done on
the theory underlying the parameters of effective corridors,
and there are little data on the details of movements
of animals through landscapes that would be useful
to the development of corridor design. Except for
riparian strips, natural corridors have rarely been mentioned
in the literature (see Berger 1987). In this paper,
I review natal dispersal patterns to begin to build a general
theory of corridor design for mammals. I also make
recommendations for future research and suggest minimum
corridor widths for representative mammals.
Natal Dispersal Patterns of Mammals
It is very difficult to experimentally test any theory underlying
corridor design by providing corridors for freeranging
mammals. The number of dispersing animals
that find and use a specific corridor within a reasonable
time period is likely to be very small. Precise replication
is unlikely to be available. A more fruitful approach is to
locate and describe natural corridors and to search for
patterns in animal movement across unfamiliar landscapes.
Mammals will be most likely to utilize an interrefuge
corridor during natal dispersal, because once dispersal
is completed most adult mammals, with the
exception of migrators, remain in the same general area
Paper submitted October 10, 1990; revised manuscript accepted September
27, 1991.
for the duration of their lives (Gaines & McClenaghan
1980). Hence, examination of movement patterns during
natal dispersal can provide insights into the requirements
for corridors.
Mammals that are potentially subject to predation,
such as white-tailed deer (Odocoileus virginianus),
wild horses (Equus caballus), and dwarf mongoose
(Helogaleparvula), reduce their vulnerability by making
rapid transfers between groups and by joining neighboring
groups (Berger 1987; Nelson & Mech 1987;
Rood 1987). Mammals that are less subject to predation,
such as black bears (Ursus americanus), bobcats (Lynx
rufus), red foxes (Vulpes vulpes), and wolves (Canis
lupus), disperse until they locate unoccupied areas with
suitable habitats (Storm et al. 1976; Griffith & Fendley
1982; Mech 1987; Rogers 1987). (Suitable habitat is
defined here as habitat in which the species concerned
may maintain permanent populations). Thus, level of
predation risk strongly affects dispersal patterns and
must be considered in corridor design.
In some species, such as black bears, white-tailed
deer, dwarf mongoose, and tigers (Panthera tigris), the
majority of females settle in a portion of their mother's
home range, essentially not dispersing at all (Nelson &
Mech 1987; Rogers 1987; Rood 1987; Kitchener 1991).
Populations of such species expand into new areas
slowly, through a series of overlapping home ranges established
by the small number of females that do disperse
(Mathews 1991).
There is evidence that topographic features such as
lakes, mountain passes, and valleys may affect dispersal
paths locally (Shirer and Downhower 1968; Seidensticker
et al. 1973; Storm et al. 1976), but at present it
is not possible to predict precisely the direction or distance
of dispersal (see Robinette 1966). Few individuals
disperse farther than necessary to find an accepting
group or unoccupied habitat. Mammals typically disperse
less than five home-range diameters (Chepko-Sade
293
Conservation Biology
Volume 6, No. 2, June 1992
This content downloaded from 147.251.87.220 on Tue, 03 Dec 2019 11:59:50 UTC
All use subject to https://about.jstor.org/terms
294 Theory of Inter-refuge Corridor Design Harrison
and Halpin 1987), but while moving in search of these
situations they may enter unsuitable habitat where they
will experience high mortality. A strong tendency to
remain within suitable habitat while dispersing has been
observed in several species of rodents (Holekamp 1984;
Garrett & Franklin 1988; Wiggett & Boag 1989). Dispersers
will cross the territories of conspecifics, although
they may prefer to remain on territory edges and
thus in less suitable habitats (Fritts & Mech 1981).
Design Parameters of Corridors
Prey species that make rapid transfers between groups
and that do not live alone and species in which females
remain in or near their mother's home range require
contiguous areas of suitable habitat for normal dispersal.
Potential depredators, such as wolves, bears, and mountain
lions, will also require suitable habitat and prey in
order to remain within the bounds of the corridor and
prevent mortality from owners of domestic livestock in
areas adjacent to corridors. These dispersal patterns indicate
that effective corridors must contain suitable habitat
for the species of interest to reside permanently
within the corridor. The habitats within the corridor
will greatly influence the extent to which dispersers
utilize it. Gaps between suitable habitat should be small
relative to dispersal distances. Habitat thus emerges as a
critical design parameter of corridors.
If a corridor must contain enough suitable habitat for
a given species to permanently occupy the corridor,
then the width of the corridor may be estimated from
data on home-range sizes and shapes. Minimum corridor
widths for seven representative mammals are presented
in Table 1, assuming that corridors are one home range
wide and contain rectangular home ranges that are
twice as long as they are wide. In addition to this minimum
width, the corridor must be wide enough to maintain
the desired habitat against penetration of other vegetation
types from edges.
The width required for a corridor to be effective may
depend upon its length. Beier (1991) observed dispersing
mountain lions effectively using a corridor of suitable
habitat only .5 to 1.0 km wide, which is much less
than the width suggested in Table 1. However, this corridor
was only 6 km long, which is less than the length
of one average mountain lion home range (approximately
12 km, based on home range sizes in Nowak &
Paradiso 1983). Thus, effective corridors may be narrow
if they are short enough that dispersers may pass
through without foraging.
The effectiveness of corridors will be affected by the
type and extent of human activities and land use practices
both within and adjacent to the corridor. The impact
of hunting and trapping, both legal and illegal, intrusion
of domestic dogs, livestock grazing, and
Conservation Biology
Volume 6, No. 2, June 1992
Table 1. Examples of minimum corridor widths based on
average female or pack home-range sizes.
Minimum
corridor
width Home range size
Species Location (km) source
Wolves Minnesota 12 Nowak & Paradiso
(1983)
Wolves Alaska 22 Ballard & Spraker
(1979)
Black Bears Minnesota 2 Rogers (1987)
Mountain Lions California 5 Hopkins et al.
(1982)
Bobcats South Carolina 2.5 Griffith & Fendley
(1982)
White-Tailed Deer Minnesota 0.6 Nelson & Mech
(1987)
Dwarf Mongoose Tanzania 0.6 Rood (1987)
aHome ranges are designed to be rectangul
wide.
disturbance due simply to human presence must be
considered. The greatest human impact will occur near
towns and along roads and edges where access to the
corridor is easily available. Schoen et al. (1990) estimated
that habitats on Admiralty Island, Alaska, lose
one-half of their suitability for brown bears if they are
within 8 km of communities of more than 500 people,
within 1.6 km of any community of more than ten people,
or within 1.6 km of arterial and collector roads.
Also, the type of human development, such as agrarian
or industrial, in the vicinity of corridors will affect the
extent of illegal activities (Berger & Daneke 1988).
Thus, corridors design may have to include buffer zones
to reduce undesirable human impact.
Finally, the location of a corridor may be affected by
the relationship between seasonal movement patterns
and the specific purpose of the corridor. For example,
white-tailed deer migrate between winter and summer
ranges. Males disperse from June to November, but data
are lacking on the timing of female dispersal (Nelson &
Mech 1987). Corridors designed for gene flow via dispersing
males may be located by summer or fall ranges.
Corridors designed for reestablishment of populations
must accommodate both sexes and may have to be designed
for movement at other times.
Conclusion and Priorities for Future Research
Our knowledge of the basic principles determining the
effectiveness of corridors is extremely limited. I have
identified habitat, width, length, human activities, and
location as critical variables, but we need much more
data on dispersal patterns and the use of natural corridors
before we can realistically attempt to direct the
movement of free-ranging mammals across landscapes.
The following list of critical research needs was easy to
This content downloaded from 147.251.87.220 on Tue, 03 Dec 2019 11:59:50 UTC
All use subject to https://about.jstor.org/terms
Harrison Theoiy of Inter-refuge Corridor Design 295
generate because we have so little knowledge in any of
these areas:
1. Monitor the movements of dispersing mammals
continuously rather than intermittently in relation
to habitat type, topographic features, and territories
of conspecifics.
2. Investigate cues used to determine dispersal direc-
tion.
3. Investigate mortality and movement patterns in
habitats that are not suitable to determine the effect
of gaps between refuges.
4. Identify and monitor potential natural corridors for
dispersal movements. It should not be assumed
without observation that riparian zones, for example,
are used as corridors.
5. Determine the minimum width of effective corridors,
possibly by measuring the minimum widths of
home ranges.
6. Quantify the impact of human activities on wildlife
populations as a function of activity and distance
from roads and other developed sites, in the manner
of Schoen et al. (1990).
Literature Cited
Ballard, W. B., and T. Spraker. 1979. Unit 13 Wolf Studies,
Alaska Department of Fish and Game Projects W-17-9 and
W-17-10 progress report.
Berger, J. 1987. Reproductive fates of dispersers in a haremdwelling
ungulate: the wild horse. Pages in B. D. Chepko-Sade
and Z. T. Halpin, editors. Mammalian dispersal patterns. University
of Chicago Press, Chicago, Illinois.
Berger, J., and D. Daneke. 1988. Effects of agricultural, industrial,
and recreational expansion on frequency of wildlife law
violations in the central Rocky Mountains, U.S.A. Conservation
Biology 2:283-289.
Chepko-Sade, B. D., and Z. T. Halpin. 1987. Mammalian dispersal
patterns. University of Chicago Press, Chicago, Illinois.
Fritts, S. H., and L. D. Mech. 1981. Dynamics, movements, and
feeding ecology of a newly protected wolf population in
northwest Minnesota, Wildlife Monographs 80:1-79.
Gaines, M. S., and L. R. McClenaghan. Dispersal in small mammals.
Annual Review of Ecological Systems 11:163-196.
Garrett, M. G., and W. L. Franklin. 1988. Behavioral ecology of
dispersal in the black-tailed prairie dog. Journal of Mammals
69:236-250.
Griffith, M. A., and T. T. Fendley. 1982. Pre- and post-dispersal
movement behavior of subadult bobcats on the Savannah
River Plant. Pages in S. D. Miller and D. D. Everett, editors. Cats
of the World. National Wildlife Federation, Washington, D.C.
Harris, L. 1984. The fragmented forest, University of Chicago
Press, Chicago, Illinois.
Holekamp, K E. 1984. Natal dispersal in Belding's ground
squirrel (Spermopbilus beldingi). Behav. Ecol. Sociobiology
16:21-30.
Hopkins, R. A., M. J. Kutilek, and G. L. Shreve. 1982. Density
and home range characteristics of mountain lions in the Diablo
Range of California. Pages in S. D. Miller and D. D. Everett,
editors. Cats of the World. National Wildlife Federation, Washington,
D.C.
Kitchener, A. 1991. The natural history of wild cats. Comstock
Publishing Association, Ithaca, New York.
Mathews, N. E. 1991. The rose-petal metaphor of deer population
expansion. 76th Meeting of the Ecological Society of
America, San Antonio, Texas.
Mech, L. D. 1987. Age, season, distance, direction, and social
aspects of wolf dispersal from a Minnesota pack. Pages in B. D.
Chepko-Sade and Z. T. Halpin, editors. Mammalian dispersal
patterns. University of Chicago Press, Chicago, Illinois.
Nelson, M. E., and L. D. Mech. 1987. Demes within a northeastern
Minnesota deer population. Pages in B. D. ChepkoSade
and Z. T. Halpin, editors. Mammalian Dispersal Patterns
University of Chicago Press, Chicago, Illinois.
Noss, R. F., and L. D. Harris. 1986. Nodes, networks, and MUMs:
preserving diversity at all scales. Environmental Management
10:299-309.
Nowak, R. M., and J. L. Paradiso. 1983. Walker's mammals of
the world. 4th edition. Johns Hopkins University Press, Baltimore,
Maryland.
Robinette, W. L. 1966. Mule deer home range and dispersal in
Utah. Journal of Wildlife Management 30:335-349.
Rogers, L. L. 1987. Factors influencing dispersal in the black
bear. Pages in B. D. Chepko-Sade and Z. T. Halpin, editors.
Mammalian dispersal patterns. University of Chicago Press,
Chicago, Illinois.
Rood,J. P. 1987. Dispersal and intergroup transfer in the dwarf
mongoose. Pages in B. D. Chepko-Sade and Z. T. Halpin, editors.
Mammalian dispersal patterns. University of Chicago
Press, Chicago, Illinois.
Schoen, J. W., R. W. Flynn, L. H. Suring, and L. R. Beier. 1990.
Brown bear habitat preferences and brown bear logging and
mining relationships in southeast Alaska. Alaska Department of
Fish and Game Federal Aid in Wildlife Restoration Final Report,
Project W-23-2.
Seidensticker, J. C., M. G. Hornocker, W. V. Wiles, and J. P. Messick.
1973. Mountain lion social organization in the Idaho
Primitive area. Wildlife Monographs 35:1-60.
Shirer, H. W., and J. F. Downhower. 1968. Radio tracking of
dispersing yellow-bellied marmots. Transactions of the Kansas
Academy of Science 71:463-479.
Storm, G. L., R. D. Andrews, R. L. Phillips, R. A. Bishop, D. B.
Siniff, and J. R. Tester. 1976. Morphology, reproduction, dispersal,
and mortality of Midwestern red fox.populations. Wildlife
Monographs 49:5-82.
Wiggett, D. R., and D. A. Boag. 1989. Intercolony natal dispersal
in the Columbian ground squirrel. Canadian Journal of
Zoology 67:42-50.
Conservation Biology
Volume 6, No. 2, June 1992
This content downloaded from 147.251.87.220 on Tue, 03 Dec 2019 11:59:50 UTC
All use subject to https://about.jstor.org/terms