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Landscape
Permeability
(Adapted from Rewilding North
America)
Conservation biologists Bruce Wilcox and Dennis Murphy warned in
1985 that “habitat fragmentation is the most serious threat to
biological diversity and is the primary cause of the present
extinction crisis.” Reed Noss and Larry Harris at the University of
Florida acted on their warning by designing a conceptual nature
reserve system for Florida consisting of core reserves surrounded by
buffer zones and linked by habitat corridors. Most of the core
reserves were existing big chunks of federal land.
In a paper presented to the 1986 Natural Areas Conference, Noss
said, “The problems of habitat isolation that arise from
fragmentation can be mitigated by connecting natural areas by
corridors or zones of suitable habitat.” In other words,
protecting and restoring connective habitat in a fragmented
landscape can mitigate the problem of island-like nature reserves.
From the Noss proposal, we can trace a new approach to protected
area design that now dominates the field of nature reserve planning.
[For general information on fragmentation, see the Wounds page, and
for information on connectivity and linkages, see the Wildlands
Network Design page.]
Continental planning for rewilding tweaks the way we see
connectivity and fragmentation on the land. For continental
rewilding, the classic model of core-corridor-core becomes general
landscape permeability in which large core habitats are
embedded. Peter Singleton and his Forest Service colleagues
considered landscape permeability for wolf, lynx, grizzly, and
wolverine in the state of Washington and extreme southern British
Columbia. They explain the shift as follows:
The early theoretical work in [the field of habitat fragmentation]
was largely based on island biogeography theory, which emphasized
perceptions of “islands” of suitable habitat in a “hostile sea” of
nonhabitat. Concepts of habitat corridors providing linear
connections through this “hostile sea” developed from the
application of island biogeography theory to conservation problems.
Several more recent discussions of this issue have pointed out that
these approaches focusing on “suitable” corridors through “hostile”
landscapes may be overly simplistic, and have proposed that
different conditions on the landscape create different levels of
resistance to movement for different species. Landscapes between
patches may encompass either habitats through which an animal can
move easily or barriers that prevent or redirect movement. It is the
composition and configuration of these characteristics that define
the permeability of a landscape.
Singleton and company define landscape permeability as “the quality
of a heterogeneous land area [a landscape] to provide for passage of
animals.” They explain the shift in how they view the landscape as,
“In contrast to focusing on the identification of corridors or
connected habitat patches, the evaluation of landscape permeability
should provide a broader measure of resistance to animal movement
and give a consistent estimate of the relative potential for animal
passage across entire landscapes.” They explain that “the broader
evaluation of landscape permeability provides an estimation of
relative potential for animal passage across the entire landscape,
including the identification of potential barriers to animal
movement.”
Nonetheless, the core-corridor (linkage) model applies fully in more
fragmented regions and through barriers in a generally permeable
landscape. Landscape permeability is not a replacement for cores and
linkages; but a complementary approach. Nor is it something entirely
new. In 1992, for example, Reed Noss wrote that “multiple-use zones”
could often serve a corridor function.
Animal movements can be either intraterritorial, within a
home range for daily or seasonal travel, or interterritorial,
“long-distance dispersal or exploratory movements outside of an
established home range.” Intraterritorial movement affects
“individual survival and reproduction.” Interterritorial travel
“influences the level of gene flow between groups (subpopulations)
of animals, the ability of animal populations to become established
in unoccupied suitable habitat, and other metapopulation functions.”
Certain species are more particular about permeability than are
others. Singleton and company explain:
Some species, e.g., wolves and lynx, are able to move long distances
through diverse habitats. For these species, maintaining landscape
linkages that have relatively few landscape barriers but do not
support breeding individuals may be adequate to provide for movement
between areas where populations of those species persist. However,
other species (e.g., grizzly bears) have not been documented to make
long distance movements through marginal habitat areas. For those
species that are not inclined to make long-distance interterritorial
movements, maintaining breeding habitat for at least a few
individuals in the linkage area may be necessary to achieve a
functional linkage between blocks of habitat supporting larger
groups of animals.
For these reasons and because different species have different
habitat needs for travel, evaluating landscape permeability needs to
be on a species by species basis—which is what Singleton and his
fellow researchers do in their study.
Areas “of reduced landscape permeability between habitat
concentrations” are called “fracture zones.” Fracture zones can be
relatively small such as an interstate highway and associated
development between wild mountain ranges, or moderately sized such
as the agribusiness-dominated Central Valley of California.
Singleton and company write of Washington, “fracture zones
between…blocks of habitat generally correspond to developed valley
bottoms where forest cover is often discontinuous, where human
population centers are usually located, and where road densities are
often high.”
Of course, fracture zones can also be huge, such as the U.S. Midwest
(excepting the Upper Great Lakes region). One does not need to be a
professional geographer or carnivore biologist to know that large
areas of North America are simply not now suitable for rewilding, at
least not in the foreseeable future. Dense human populations,
intensive agriculture, and a domesticated landscape make rewilding
highly unlikely for much of the U.S. East, Midwest, and South. Large
areas in the West are also unsuitable. Other highly settled and
domesticated areas unsuitable for rewilding are in southern Canada
and in much of Mexico and Central America.
(Adapted from
Rewilding North America by Dave Foreman [chapters 6 and 8].
Copyright © 2004 by the author. Reproduced by permission of Island
Press, Washington, D.C. Quotes are from the papers below.)
Books
Rewilding North
America: A Vision for Conservation in the 21st Century
by Dave Foreman (Island Press 2004).
Order from The Rewilding Institute.
Continental
Conservation: Scientific Foundations of Regional Reserve Networks
edited by Michael E. Soulè and John Terborgh (Island Press
1999). Particularly Chapter 6 “Connectivity: Maintaining Flows in
Fragmented Landscapes.”
Order from The Rewilding Institute.
The Fragmented
Forest: Island Biogeography Theory and the Preservation of Biotic
Diversity by Larry Harris (University of Chicago Press, 1984).
This is a conservation biology classic by a great pioneer in
conservation biology. Harris applies island biogeography to forest
management in the Pacific Northwest. Illustrated. Order from Amazon.
Landscape Linkages
and Biodiversity edited by Wendy E. Hudson, Defenders of
Wildlife (Island Press, Washington, DC, 1991). An early and credible
look at wildlife movement linkages with contributions from Michael
Soulè, Reed Noss, Larry Harris, Don Waller, and other experts. Order
from Amazon.
The Role of
Corridors: Nature Conservation 2 edited by Denis A. Saunders and
Richard J. Hobbs (Surrey Beatty & Sons, Chipping Norton, Australia).
A heavily illustrated anthology from a 1985 Western Australian
conference on corridors. Order from Amazon.
Articles
Available as a PDF:
Peter H. Singleton,
William L. Gaines, and John F. Lehmkuhl, Landscape Permeability
for Large Carnivores in Washington: A Geographic Information System
Weighted-Distance and Least-Cost Corridor Assessment (U.S.
Department of Agriculture, Forest Service, Pacific Northwest
Research Station, Research Paper PNW-RP-549, December 2002).
CLICK HERE FOR PDF
Available in a book
listed above:
Andy Dobson et al.
“Connectivity: Maintaining Flows in Fragmented Landscapes,” Chapter
6 in Continental Conservation.
PDFs not yet
available:
Bruce A. Wilcox and
Dennis D. Murphy, “Conservation Strategy: The Effects of
Fragmentation on Extinction,” American Naturalist 125 (1985):
879-887.
Reed F. Noss,
“Protecting Natural Areas in Fragmented Landscapes,” Natural
Areas Journal 7 no. 1 (1987): 2-13.
Reed Noss, “A Recipe
for Reserve System Design and Management,” special issue, Wild
Earth, 1992, 24.
Peter H. Singleton,
John F. Lehmkuhl, and William Gaines, “Using Weighted Distance and
Least-Cost Corridor Analysis to Evaluate Regional-Scale Large
Carnivore Habitat Connectivity in Washington,” A Time For Action:
2001 Proceedings ICOET (International Conference on Ecology and
Transportation, September 24-28, 2002, Keystone, Colo.), 583-594.
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