BIODIVERSITY PROJECT SCIENCE BRIEFS
June 2000
PIECES OF NATURE: FRAGMENTATION AND WIDE-RANGING WILDLIFE
By Eric Kurtz
Human activities have cut up the landscape that once existed in
northeastern North America. Through agriculture, deforestation and urban
sprawl, we have whittled the once large tracts of forest down to isolated
fragments. The effects on individual species have been well-documented.
But Dr. Justina Ray says it's important to look at the whole picture, and
especially at wide-ranging species, to see the effects of fragmentation on
a range of different species and to be able to respond at the appropriate
scale.
Dr. Ray heads the Northeastern Mesocarnivore Initiative for the Wildlife
Conservation Society and is an adjunct professor at the University of
Toronto. In a talk given in April at the American Museum of Natural
History's conference "Nature in Fragments: The Legacy of Urban Sprawl,"
she outlined the effects of fragmentation on three groups of animals:
mammalian carnivores, raptors, and migratory songbirds.
The most obvious impact is outright habitat loss. Species like the puma,
marten, and red-shouldered hawk that require have wide home ranges are
finding fewer choices when looking for a place to live. But, Ray says a
more insidious effect might be the isolated nature of fragmented habitat,
due either to the large gaps between forest fragments or to barriers such
as roads or other development.
Ray says these barriers pose problems for individuals of wide-ranging
species during the course of their lifetimes, and over evolutionary time
for the entire species. For example, carnivores such as bobcats and lynx
leave their home ranges in search of new territory when they come of age.
They may need to range especially far in the face of shortages of suitable
habitat, which can result from obstacles imposed by humans. Barriers also
block migration patterns for some species. For instance, caribou in Alaska
have found the pipeline a barrier to their movement. And barriers can also
block genetic interchange between isolated sub-populations.
Besides reducing the amount of available habitat and making habitat patches
more isolated - significant effects by themselves - Ray says fragmentation
also erodes habitat quality. That mostly occurs due to edge effects. Edge
effects refer to changes in the micro-climate and structure of the forest
at its edges, which compromise the integrity of the interior forest conditions. According to Ray, generalist species - raccoons, skunks, and
coyotes, for instance - do well on edges, while specialists like the marten
often don't. As a result, edge effects often lead to increased
colonization by generalist species, which has implications for such
essential ecological processes as competition, predation, and parasitism.
For migratory songbirds, edges pose a special threat: nest parasitism by
Brown-headed Cowbirds and nest predation by raccoons, crows, and other
generalist predators. Ray says a recent study of wood thrushes showed that
in forests with less than 55 percent cover, nest parasitism took a heavy
toll, while in more forested landscapes, parasitism almost never occurred
(Robinson et al. 1995). "Brown-headed Cowbirds have parasitized over 220
host species," she says. "While not all hosts make good foster parents - a
number of species reject cowbirds - they've been successfully reared by
over 150 host species, with neotropical migrant songbirds comprising the
majority." Examples of neotropical migrant songbirds include: ovenbirds,
warblers, vireos, tanagers, flycatchers, thrushes and other species.
Ray says the point of studying wide-ranging species is "not just
documenting what's going on and the adverse impacts brought about by
fragmentation, but it's also being able to use this information." She says
the information should lead us to plan development on a broad scale to
address the adverse effects of fragmentation on carnivores, raptors, and
songbirds. "Even just the town level is not good enough. We really have
to look at the township level or the county level in terms of how we
redesign our landscape or how we conserve what is left."
Finally, Ray says looking through the eyes of wide-ranging species may also
bring benefits to species with a smaller range. "If we look at the animals
that view the landscape at a broad scale, then we may have hope of bringing
along other species along their coat tails, if we are able to conserve
wide-ranging keystone species."
For more information, contact:
Dr. Justina C. Ray
Faculty of Forestry
University of Toronto
33 Willcocks St.
Toronto, ON M5S 3B3
Canada
Or consult the following source (to be published shortly):
Ray, J.C. In press. "Mesocarnivores of the northeast: Status,
distribution, and conservation issues." New York: Wildlife Conservation
Society Issues Paper.
The paper will be available on-line at www.wcs.org