JUNE1987
Endangered Species
School of Natural Resources
The University of Michigan
MAINTAINING BIOTIC DIVERSITY IN NATIONAL FORESTS:
The Necessity for Large Blocks of Mature Forest
S.L.
Solheim, W.S. Alverson, and D.M. Waller
Ever since forest managers began planning
for wildlife, their strategy has been to create openings and edge conditions
in forests to favor a few selected species that thrive in those conditions and
are particularly desired by hunters. Indeed, the term "wildlife" has become practically
synonymous with "game animals."
Fortunately for foresters, the requirements of many game animals conform
well with timber harvest activities. However,
non-game wildlife is often affected as well and sometimes adversely so.
A new model for wildlife conservation is now emerging due to increased
understanding of the needs of a wider array of plants and animals.
Central
to the problem of conserving biotic diversity is an understanding of the role
of scale. Conditions which lead
to maximizing local diversity within a habitat (alpha diversity) may lead to a
decrease in diversity if applied throughout a large area. The
diversity of a large area is the total (or gamma) diversity resulting from
diversity both within and between habitats. Thus
for large land management areas like a national forest, it would be a mistake
to sacrifice gamma diversity by over-replicating one habitat type because it
is particularly diverse. Yet this
has been the centerpiece of "game management" strategies for many
years: the intentional creation of "edge" in order to locally
increase the number of favored species. Although
old growth forest is not necessarily more diverse than second growth as is
sometimes mistakenly presumed (Middleton and Merriam 1985), if there are
plants and animals that depend on mature forests for long term survival, then
old growth forests are an important element in maintaining the overall
diversity of the National Forests.
Ecologists and foresters are becoming increasingly concerned with the
degradation of habitat for species that require less disturbed conditions.
The cumulative effects of road building, timber harvests, and
intentional creation of openings for game have fragmented most forest lands
into relatively small areas of mature vegetation embedded in a matrix of
younger forest.
Biological communities are dynamic and permeable and respond to events at and
outside their boundaries. Whenever
one type of vegetation (in this case mature forest) is isolated as a
relatively small patch, both small size per se (area effects), and an
increased edge/area ratio (edge effects), can lead to deterioration of the
biotic community. The area of a
habitat clearly influences the species that can be maintained within it.
Animals with large "home ranges and/or relatively narrow habitat
requirements are especially sensitive to area and become scarce or disappear
from isolated patches of forest.
Perhaps it is less obvious that other animals with smaller home ranges and
even sedentary plants also require minimum areas of suitable habitat. This is
partly because numbers of organisms are not static over time. Natural
fluctuations due to environmental variance and random changes in plant and
animal survival and reproduction occur constantly and imply that a large
enough population of the organisms (usually called its "minimum viable
population" or "MVP") must be present in an area to ensure
continued presence despite these factors.
One alternative means of preventing local extinctions is to maximize the
likelihood of recolonization from similar, nearby areas.
Biologists find that this immigration is becoming less likely for
many organisms in our fragmented landscapes and that biological preserves need
to be large enough to be self-sustaining.
The exact effects of area and isolation on survival are currently the object
of much study under the rubric of "Island Biogeography," a set of
ecological principles that relates size and isolation of areas (including
habitat "islands" such as mature forest) to the number and kinds of
species that can be supported indefinitely in those areas (MacArthur and
Wilson 1967; Harris 1984; Newmark 1987).
The analogy of nature preserves to islands, however, has been
criticized. Nature preserves may
not be strictly analogous to islands because of impacts from surrounding land.
This may point to the need for
even larger areas for effective preservation. (Eternal External Threats,
Janzen. 1983,1986).
Even
where the analogy holds, research on fragmentation of natural areas is a
difficult problem because organisms persist in an area for varying periods of
time after habitat conditions required for long-term maintenance are gone and
the population is doomed. For
example, the survival of adult individuals in a population does not
necessarily mean that these individuals can successfully reproduce under
present conditions. The lag
between habitat degradation and complete disappearance of the organisms
(called the "relaxation time") may be quite long and lead to
unwarranted optimism about the ability of small areas to maintain viable
populations.
Edge
effects at small scales are well known: increased light levels and decreased
humidity are common problems of small woodlots surrounded by agricultural
land. It is now becoming evident
that edge effects significant to many species occur at previously
unanticipated scales. Such edge
effects have been demonstrated to occur over tens of thousands of acres, even
when older forest is embedded in younger growth. The
result often involves animals from the disturbed matrix adversely affecting
plants and animals adapted to undisturbed habitat.
In
the Great Lakes region, for example, increased herbivory by deer has
contributed to a serious decline in regeneration of once important forest
community members such as Eastern Hemlock and Canada Yew (Anderson &
Loucks 1979; Frelich & Lorimer 1985).
Similarly, the increase in nest predation and parasitism by birds and
mammals favored in the disturbed forest matrix has caused a decline in
breeding populations of migratory songbirds.
Studies show that the rate of predation declines significantly with
increased forest area (Brittingham and Temple 1983; Wilcove 1985).
Adding to the problems encountered when too much reliance is placed on small
areas for preservation is the possibility of catastrophic disturbance.
Small-scale disturbances such as tree falls are part of the dynamics of
any natural community. Larger-scale disturbances like fires or blowdowns are
common over a longer time span and must also be accounted for in natural area
management. Studies of
pre-settlement forests in Wisconsin suggest that at any one time 17 to 25 of
Wisconsin's northern woods were recovering from windthrows (Canham &
Loucks 1984). In 1977, Wisconsin
lost its largest remaining stand of virgin forest to windthrow.
Natural areas must be large enough to retain internal sources for
recolonization following disturbance. This
"minimum dynamic area" (Pickett and Thompson 1978) is determined
both by the potential size of disturbances and by the biological
characteristics of the organisms affected.
Clearly, small patches of pristine vegetation such as Research Natural Areas (the Forest Service's land classification category that includes small (50-100 ha.) tracts of exemplary natural communities) will not suffice to maintain many sensitive species for the long term. Although no one knows with any precision how large protected blocks must be, in the Great Lakes region, areas of at least 20,000 ha. (approximately 50,000 acres) in a single large block may be necessary to provide sufficient habitat and to minimize edge effects. Areas of 40,000 ha. may even prove to be more appropriate when more data on MVP's, extinction, colonization rates, and edge effects become available.
This, however, need not imply great losses to forest economic productivity. There is no shortage of disturbed forest in most of North America. Hence, an intensification of timber harvest on non-protected areas (i.e. the majority of the remaining timber base) may be acceptable. This is exactly what we have proposed for the Chequamegon and Nicolet National Forests of Wisconsin: not a decrease in harvest levels, but a "zoning" of the forests so that 100% of the logging activities and traditional wildlife management occur on 80% of the land. This is fully compatible with the multiple use mandate of the Forest Service, which specifically states that not all forest lands should be used for all purposes. We have proposed specific "Diversity Maintenance Areas" using guidelines derived from general ecological principles (e.g. minimizing the edge/area ratio) and site-specific concerns such as "embedding" currently protected but small reserves and rare plant sites. In this way, rather than having a homogeneous patchwork of disturbed lands and ineffective small old-growth stands, true biological diversity can be maintained by providing sufficient habitat for all kinds of plants and animals.
The common but specious "ecology vs. economics" argument can be sidestepped by wisely planning the spatial distribution of forest activities for the benefit of all.
This article is extracted from the "Statements of Reason", documents
written in response to the Chequamegon and Nicolet Forest Plans.
These Statements (800 pgs.) provide a much fuller exposition of the
above concerns, and may be obtained at cost from the authors:
Department
of Botany
University
of Wisconsin
430
Lincoln Drive
Madison,
Wl 53706
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