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How Forest Fragmentation Hurts Species
and What To Do About It
Daniel Simberloff1
HOW FRAGMENTATION HURTS
SPECIES
effect (Saunders et al. 1991). Of course light and moisture
regimes near an edge also differ from those of the interior. In
Wisconsin forests, increased light can pennit shade-intolemnt
vegetation to invade 30 m inwards (Ranney et al. 1981). In a
well-studied fragment of old-growth longleaf pine forest, every
introduced plant was within 2 m of a road or an artificially
maintained clearing (S. Hermann, pers. comm. 1991).
Animals can also penetrate far from an edge. For example,
tropical animals disperse seeds from secondary habitats into
pristine forest tree falls 5 km away (Janzen 1983). Similar effects
are known in other temperate and tropical forests (e.g., Janzen
1986; Wilcove et al. 1986). Most research on effects of animals
penetmting an edge is on how they eat forest interior organisms.
Fragmentation of habitat is the major global environmental
change occurring today and the one most likely to devastate
biodiversity and ecological pr<?cesses in the near future
(Simberloff 1993a). Fragmentatioq always accompanies habitat
destruction and the effects of fragmentation per se, as opposed
simply to the loss of area, have been intensively studied only
recently. An early suggestion that fragmentation could have
important consequences concerned forests in Wisconsin (Curtis
1956), and most of the maps of habitat fragmentation that pepper
conservation journals and texts are of forests. 1bere is no
satisfactOlY geneml theoretical framework for fragmentation
analogous to the species-area relationship for habitat destruction
(Simberloff 1993a). However, a number of intensive studies of
particular systems suggest certain potential effects should always
be considered. I will focus particularly on a system that
surrounds my home, the longleaf pine (Pinus pa/ustris)
ecosystem of the Southeast.
Increased Predation and Herbivory
Nest predation may increase greatly in a fragmented
landscape. Wilcove (1985, 1990) placed artificial nests with
quail eggs in eastern U.S. forests of different size, ranging from
small woodlots to the continuous forest of the Great Smoky
Mountains National Palk. In the latter, only 2% of nests were
preyed upon within a week, while nests in rural woodlots of
4-10 ha averaged 48% predation, and similar sized subutban
woodlots reached 70%. This study and similar results (e.g.,
Andren and Angelstam 1988, Small and Hunter 1988, Yahner
and Scott 1988) inspired the "intermediate predator hypothesis"
(cf. Terborgh 1988), which states that medium sized
predators-raccoons, squirrels, blue jays, crows, dogs, cats, etc.,
in eastern forest-are greatly increased in a patchwork quilt of
housing, farmland, second growth, and forest fragments. 1bese
predators, ~ tum, invade the forest fragments and prey on its
denizens. In the continuous forests of the past, according to this
hypothesis, large predators like wolves, mountain lions, and
raptors were much more numerous and greatly suppressed
populations of the intermediate predators.
In longleaf pine forests, nest predation of the state-listed
gopher tortoise (Gopherus polyphemus) is enhanced by habitat
destruction and fragmentation, as elevated populations of
skunks, raccoons, crows, and introduced fire ants (So/enopsis
invicta) thrive in the agricultural and second-growth matrix that
surrounds longleaf fragments but attack nests in the longleaf
(references in Simberloff 1993a). Nest predation of two common
Edge Effects
As fragments become smaller, they increasingly comprise
edge habitat. This is because areas within the fragment are
affected physically and biotically by the presence of the edge.
The "edge effect" (Moore 1962, Williamson 1975)-the
presence of species near an edge that chamcterize neither of the
adjacent habitats--<>ften results in increased diversity at an edge,
so wildlife biologists have traditionally viewed edges as
desirable (Harris 1988, Yahner 1988). However, species that
colonize edges are often common elsewhere, while forest interior
species that do not tolerate edges are often of special concem
Exactly how far inside a forest the existence of an edge is
manifested depends on the forest, but it can be surprisingly far.
Changes in wind currents, for example, can sometimes be
detected at a distance 100 times the height of the vegetation;
thus a forest with 20 m tall trees might need to be 2 km wide
before any part of it would not suffer a meteorological edge
1 Depattment of Biological Science, Florida State University,
Tallahassee, Florida 32306.
85
whether most species are maintained as metapopulations.
CursOly reviews (Harrison 1991, Simberloff 1993b) cast doubt
on the proposition Rather, it seems that many species are not
metapopulations at all and many others are metapopulations of
the sort envisioned by Boonnan and Levitt (1973) and Pulliam
(1988). In this model, large central populations are continuous
sources of colonists for smaller ephememl populations, which
are "sinks" in the sense that they are maintained only by this
recruitment and do not contribute to the persistence of the central
populations. Until many more data are available on movement,
any hypothesis of metapopulation-collapse induced by
fragmentation is just that: an hypothesis.
game species, bobwhite quail and tmkey, is apparently similarly
elevated, with many of the same culprits (references in
Simberloff 1993a). Not only nests are preyed upon Adult fox
squirrels (Sciurus niger), gopher tortoises, and tuIkeys are all
heavily preyed upon by species typical not of their favored old
growth pine habitats, but of the varied, distutbed landscape that
now prevails.
Herbivory of forest plants can be increased in a fragmented
landscape just as predation can, if numbers of herbivores and/or
their access to forest habitat are increased. The national forests
of northern Wisconsin were once ca. 80% old growth and
contained small fragments of earlier successional stages
genemted by fires and stonns. Nowadays, in the wake of
intensive logging, the landscape- is the reverse: a patchwOlK quilt
with 95% earlier successional stages dominated by aspen and
only 5% old growth fragments of 5 - 200 ha. Alverson et al.
(1988) have found that white-tailed deer populations have more
than doubled in this landscape of excellent browse, and their
browsing modifies even the Gld growth fragments. TIle deer
select many old growth ground cover plants as well as seedlings
of old growth trees like. eastern hemlock, white cedar, and
Canada yew. Alverson et aI. (1988) believe it will be impossible
to maintain old growth in small fragments unless deer
populations are controlled.
Seedlings of longleaf pine suffer a similar fate in some
regions (references in Simberloff 1993a). Pocket gophers
(Geomys pinetis) and especially wild hogs eat longleaf seedlings
and both animals thrive in the mixed agricultural/early
successional stage communities surrounding longleaf fmgments.
Other Effects
Other effects of fragmentation are not as geneml as edge
effects, increased predation and herbivory, and (potentially)
failure of metapopulation dynamics. Some, however, probably
occur in many systems.
Some species simply cannot maintain a population in a small
fragment and, if fragments are sufficiently isolated, cannot
maintain a population or metapopulation in the entire
constellation of fragments. For example, whatever forces
determine minimum viable population sizes (reviews by Shaffer
1981, Simberloff 1988), huge carnivores are likely to disappear
from small fragments for thermodynamic reasons alone unless
they are very good at getting from site to site. There simply is
insufficient food and space to support a population of bears or
bobcats in a 10 ha site.
Introduced species are likely to be a far greater problem
within forest fragments in a variegated landscape than they
would be in intact large expanses. Not only are some introduced
species highly adapted to the anthropogenous habitats that
surround forest fragments but these habitats provide access to
the forest proper (Simberloff 1994). Longleaf pine forests are
almost devoid of introduced fIre ants (Solenopsis invicta) except
along roads or edges (Tschinkel 1988). The same is true of
introduced plants.
Fmgmentation can disrupt a fIre regime and thereby change
an entire community. Longleaf pine forests are fIre disclimaxes
maintained by frequent fires. Previously, lightning-induced fIres
spread widely and every site was thus burned every few years
whether it was struck by lightning or not. Now the situation is
completely changed, because the forest fragments are widely
separated by farms, commercial plantations, roads, housing, etc.
(Simberloff 1993a). Managers must perform regular controlled
burns. Disruption of fue regimes can also be induced by
introduced plants, as has occurred with the introduction of
Melaleuca quinquenervia (Ewel 1986).
Failure of Metapopulation Dynamics
Metapopulation dynamics as a hedge against extinction are
all the rage nowadays. The fIrst model (Levins 1969) has been
supplemented by numerous others (Hanski and Gilpin 1991),
and the overall theoretical result is clear: populations that would
not persist in one large population might do so in a
metapopulation of populations, given sufficient mtes of intersite
movement. Metapopulation theory has superseded island
biogeogmphic theOIy as a way of thinking about nature among
conseIVation biologists (Merriam 1991). Many authors (e.g.,
Carter and Prince 1988, Wilson 1992, Noss 1993) contend that
most species are distributed as metapopulations, but there are
few data. If species are, in fact, maintained by continual
recolonization of tempomrily empty sites, it is easy to see how
fragmentation could cause a metapopulation to collapse. As
fragments get smaller and more isolated, the number of
individuals moving from site to site decreases and may surpass
a threshold below which the entire metapopulation collapses.
But this is the rub: actual rates of movement between sites is
mrely known, so it has proven almost impossible to assess
86
Uses" with countless signs. Thus there is no radical shift in
direction indicated in the letter, "Ecosystem Management of the
National Forests and Grasslands" sent by Chief ED. Robertson
on June 4, 1992. Rather, he says that now "an ecological
approach will be used to achieve the multiple-use management
of the National Forests and Grasslands." There seem to be two
main components to the new approach: more science and an
ecosystem focus. Will a more ecosystem-focussed management
and closer interaction with scientists lead to successful
maintenance of biodiversity while allowing continued other uses
of the habitat, such as recreation and harvest of wood, at levels
acceptable to all users? Only time will tell.
Just as with the rush to create corridors, however, there seems
to be an element of faith in the New Perspectives. That is, one
would expect a scientific approach to forest management to be
founded on a falsifiable hypothesis and a commitment to discard
the hypothesis if it is falsified. Neither the Chief's letter nor the
more formal statement of the New Perspectives (Kessler et aI.
1992) really presents the approach in this way. Neither considers
the possibility that adequate maintenance of biodiversity might
be incompatible with other uses at desired levels. Worse, the
terms of these manifestoes are sufficiently vague and general
that it is difficult to imagine a possible future result in some
specific ecosystem or landscape that would defInitively falsify
the hypothesis. That is, is there a particular set of observations
that could cause the Service, or its Chief, or its scientists, to
proclaim that the New Perspectives cannot achieve their desired
goal? The explicit method proposed by the scientists is adaptive
management, in which "information from monitoring is used to
continually evaluate and adjust management relative to predicted
responses, management objectives, and predetermined
thresholds of acceptable change" (Kessler et al. 1992, p. 225).
It is unclear in this approach exactly when the entire framewotk
for conceiving the problem might be rejected, if ever.
The Service itself clearly views the New Perspectives as
something vety different from what had gone before, and they
are "new" in the sense of" recent." So it is important to remain
optimistic and open-minded until some results are in However,
the histoty of conservation is littered with bright ideas of great
intuitive appeal that turned out not to solve many or any
conservation problems (Simberloff 1988), and one should take
a lesson from this fact: remain skeptical and conceive of evety
idea as an hypothesis. Partnerships play a key role in the New
Perspectives--R.obertson's letter speaks of "partnerships with
State and local governments, the private sector, conservation
organizations... " . For maintenance of biodiversity, it is clear in
some regions that partnerships are necessaty if only because the
Service (in fact the entire federal government) does not control
a large enough fraction of the land to ensure continued
persistence of all species. Half of all federally listed species and
subspecies are oot found on any federal lands; 64% of all
occurrence records for these taxa in Natural Heritage Data
Centers are oot from federal lands (Natural Heritage Data Center
Netwotk 1993). Consider the South: 90% of southern timberland
is privately owned (Norvell 1993). Corporate timber companies
HOW TO COUNTER THE EFFECTS OF
FRAGMENTATION
Corridors
The most highly publicized approach to mitigating problems
engendered by fragmentation is to connect the fragments by
corridors. 'These proposals range from rows of trees 10 m wide
(Hussey et al. 1990) through mega<orridor proposals such as
the Wildlands Project (Mann and Plummer 1993) or the
proposed corridors 300 meters wide 'and thousands of kilometers
long to provide for movement in the face of global warming
(Hunter et al. 1988). What these proposals almost all have in
common is a dearth of evidence that the target OIganiSms will
actually use the corridors and scant consideration of the cost of
the corridors relative to the cost of other possible conservation
measures (Simberloff and Cox 1987, Simberloff et al. 1992).
Because these problems have been tboroughly aired (e.g., Hobbs
1992), I will oot belabor them here.
Suffice it to say that even defenders of the proposition that
corridors will often be vety useful still qualify their defenses by
admitting there are few data showing this, and that corridor
proposals almost never include a discussion of possible
alternative uses of funds.
Landscape Management
Another way to mitigate fragmentation, or one that might
operate simultaneously with corridors, is to manage the entire
landscape so that, as a whole, it supports a large fraction of the
community. In other words, granted that small refuges are
important but insufficient and that large eoough refuges may oot
be attainable for ecooomic reasons, is there some way that the
land outside refuges can be managed so that the refuges do oot
appear, to the species of concern, as islands in an inhospitable
sea? This is the premise behind the "new forestry" (e.g.,
Franklin 1989, Swanson and Franklin 1992): can timber be
extracted from a major portion of the forest without major harm
to resident species? 'The idea of" habitat variegation" (McIntyre
and Barrett 1992) proposed for the oorthem tablelands of New
South Wales is very similar. In both instances, the goal is to
manage a landscape so that, even if it is far from pristine, and
even if many resources are extracted, the threat to all species is
vitiated. The Forest Service calls its version of this philosophy
"New Perspectives" (Kessler et al. 1992).
Is the new forestry truly new? It and related ideas seem to
be versions of a multiple-use strategy at the landscape level. The
Forest Service's planning regulations under the National Forest
Management Act of 1976 (36 C.ER. pt. 219) require that the
Service manage the land for multiple use (sec. 219.2(b)(l». Well
before then, the Forest Service applied a multiple-use philosophy
to forest management (Kessler et al. 1992), and the Service has
for years proclaimed the national forests the "Land of Many
87
management of private lands has generally been even more
inimical to the bird than management of the national forests.
Not all private lands have been poorly managed from the
standpoint of the woodpecker. The Red Hills Hunting
Plantations of southern Georgia contain the sixth largest
aggregation of birds (James 1994). This region is dominated by
uneven-age management and selective cutting (James 1994), the
antithesis of the methods primarily used on national forests and
large timber plantations (Jackson 1994).
The generally poor situation on private lands has led the U.S .
Fish and Wildlife Service to formulate a strategy for private
landholders that )they claim will aid the recovery effort (Costa
1993). They view small landowners as unlikely targets for this
effort because they feel the costs would be too high for them
to bear and the birds on their lands are doomed anyway, so they
focus on large landowners. The strategy has three parts: 1) a
procedures manual for private lands, 2) a rangewide habitat
conservation plan, and 3) individual habitat conservation plans
or memoranda of understanding. The last element, the
memoranda of understanding, is viewed as "probably the best
hope of maintaining the remaining, relatively large RCW
populations on private lands" (Costa 1993, p. 13).
Given the importance of these memoranda, it is not
surprising that the first one, with the Georgia Pacific
Corporation, by far the largest southern timberland holder
(Norvell 1993~ pulp and paper companies not included),
was front page news in the national press (e.g., Schneider
1993). Secretary of Interior Babbitt said that the most
important effect of this agreement "could well be the
precedent it sets in helping to establish a less politically
incendiary approach to safeguarding endangered wildlife
and their habitat" (Schneider 1993). It is thus crucial that
this agreement be a sound one.
The company agrees to restrict operations on some
20,000 ha in return for a government promise not to invoke
the Endangered Species Act to restrict logging on the
remaining 1.68 million ha of Georgia Pacific timberland in
the South. The restriction consists of not clearing at least
4 ha of land around each colony site on those 20,000 ha,
and reducing the stocking rate to 4.59 m2 basal area of
pine/ha over 61 ha. On the face of it, this agreement seems
quite remarkable, given that home ranges often exceed 80
ha and may exceed 400 ha in poor habitat (references in
Jackson 1994). In prime homogeneous habitat in Florida,
ca. 25 ha per social group apparently suffices for
population persistence (James, pers. comm. 1993). This
disparity is less mysterious, perhaps, in light of the heavily
criticized 1985 recovery plan (Lennartz and Henry 1985),
which calls for a 4 ha core area around each cavity tree.
The stocking rate is more impressive than the area, as it
is quite low and constitutes a 3-fold reduction of the
original plan of the company (Wood and K1einhofs 1992).
From the standpoint of the company, this agreement may
be acceptable in that the sacrifice of income does not
unduly affect profit margin (Wood and Kleinhofs 1992).
own 17% of forested areas (Doster 1993). Evidently the private
sector will have to be a partner if biodiversity is to be
maintained.
A Case-Study:
The Red-Cockaded Woodpecker
The red-cockaded woodpecker (Picoides borealis) is the most
publicized problematic species in southern forests. Probably it
would have achieved the global notoriety of the northern spotted
owl but for two facts: 1) The longleaf pine forests that are its
prime habitat, though beautiful, do not match in visual impact
the majestic rainforests of the· Northwest. 2) Almost all of the
southern old-growth was cut down long ago (Tebo 1985), before
conservation of biodiversity was even an issue and before heavy
logging operations moved to the Northwest. Of about 28 million
ha of original longleaf pine fQrest in the Southeast, fewer than
600 ha remain (references in Simberloff 1993a).
The woodpecker has been viewed as endangered since at least
1968 (U.S.D.!. 1968) and was listed as endangered in 1970
under the Endangered Species Conservation Act of 1969. It was
one of the flISt species listed under the Endangered Species Act
of 1973. Two recovery plans have been written; the flISt was
never implemented, and the second (Lennartz and Henry 1985)
was severely criticized by a committee appointed by the
American Ornithologists' Union but has not been revised
(Jackson 1994). The number of birds has declined more than
20% during the last decade, and much of the decline has been
in populations designated as recovery populations in the 1985
recovery plan, including populations in national forests (James
1994). The bird is important not only in its own right, but
because the cavities it laboriously constructs in large, diseased
trees are used by many other species (Engstrom 1993).
Human activity has affected the woodpecker primarily in two
ways. First, through loss of active and potential cavity trees, and
second, through fragmentation and loss of foraging habitat
through conversion of forest to other habitats or change in forest
type because of short-rotation, even-aged management or
limitation of fire (Jackson 1994). The bird does not regularly
disperse more than about 8 km (Walters et al. 1988), and small,
isolated sites that lose their woodpecker colonies yet appear to
constitute suitable habitat often remain without birds for a long
time. The decline of numbers is undoubtedly partly due to this
loss of isolated populations, but it is an open question whether
the failure of this aspect of metapopulation dynamics threatens
the larger aggregations~ over half of all sites are in six areas
(James 1994).
Given the large fraction of southern forests in private
ownership, it seems that recovery of this species could be greatly
aided by partnerships with private landholders. Of approximately
4,000 known active sites, half are on national forests, a fourth
on Department of Defense lands, and only an eighth on private
lands (Costa 1993). This disparity between fraction of privately
held lands and ftaction of woodpeckers reflects the fact that
88
Carter, RN., and S.D. Prince. 1988. Distribution limits from a
demographic viewpoint. pp. 165-184 in A.J. Davy, MJ.
Hutchings, and A.R. Watkinson (eds.), Plant Population
Ecology. Oxford: Blackwell.
Copeyon, CX. 1990. A teclmique for constructing cavities for tre
red-cockaded woodpecker. WildI. Soc. Bull. 18:303-311.
Costa, R. 1993. U.S. Fish and Wildlife SeIVice white paper on tre
issue of red-cockaded woodpeckers on private lands.
Unpublished.
Curtis, IT. 1956. 1k modification of mid-latitude grasslands and
forests by man. pp. 721-736 in W.L. 11x>mas (ed.), Man's Role
in Changing the Face of the Earth. Chicago: Univ. of Chicago
From the standpoint of the U.S.F.W.S., it is hard to imagine why
this agreement is acceptable; certainly it does not reflect an
abundance of scientific evidence.
The rangewide habitat conselVation plan of the U.S.F.W.S.
seems peculiar as well. It will apparently consist of memoranda
of understanding with large landholders, such as the one with
GeoiWa Pacific, plus a global agreement with smaII landholders
by which woodpeckers on their lands would be moved to federal
Iands or to larger private Iands (Costa 1993). It is again clear
that such cooperation benefits the landholders: it relieves them
of the onus of managing for an endangered species. It is again
not clear that the U.S.F.W.S. will benefit. The U.S.F.W.S. sees
these birds as a potential aid in designated, larger recovety
populations. 1\vo recent developments (reviewed by Jackson
1994) spur such reasoning: movement of young females from
natal sites to clans lacking a female (DeFazio et al. 1987), and
the construction of artificial cavities (Copeyon 1990, AIlen
1991). Although both techniques may be of great use in
recovety, they are sufficiently new~ that one cannot be certain
how well they will wotk in the long term, and they are
expensive.
Although the Endangered Species Act does not address the
fate of species that interact with a listed species, an ecosystem
approach to forest management surely would, and there has been
no substantial study of how artificial cavities are used by the
many species who depend on red-cockaded woodpecker holes.
In fact, if the new thrust of forest management is to be
ecosystemic, a goal proclaimed not only by the Forest SelVice
but by Secretaty Babbitt (U.S.D.I. 1993), species-specific
remedies such as movement of individuals and provision of
artificial habitat will likely play a reduced role. Rather, a real
ecosystem approach necessitates addressing entire native
ecosystems. The longleaf pine forests are ripe for such an
approach, with numerous species of special concern, some
understanding of the problems of many of them, and a good
scientific basis for management (Hermann 1993). One hears
much talk about "getting ahead of the extinction cUlVe"
nowadays; we will never get ahead of this CUlVe if we attempt
to save one species at a time.
Press.
DeFazio, IT., Jr., M.A. Hunnicutt, M.R. Lennartz, G.L. Chapman,
andIA. Jackson 1987. Red-cockaded woodpeckertrnnslocation
experiments in South Carolina. 1987 Proc. Annu. Conf.
Southeast Assoc. Fish and WildI. Agencies 41:311-317.
Doster, GL. 1993. Timber company biologists opposed to feral
swine. Southeastern Cooperative Wildlife Disease Study Briefs
9(1):3-4.
Engstrom, R.T. 1993. Characteristic birds and mammals of longleaf
pine forests. In press in S. Hermann (ed.), Proceedings of a
Conference on LongleafPine Forests. TaIIahassee, Florida: Tall
Timbers, Inc.
Ewe~ JJ. 1986. Invasibility: Lessons from south Florida pp.
214-230 in H.A Moorey and IA Drake (eds.), Ecology of
BiolOgical Invasions ofNorth America and Hawaii. New York:
Springer-Verlag.
Franklin, IF. 1989. Toward a rew forestty. Amer. Forests.
Nov.lDee. 1989: 1-8.
Hanski, 1., and M.E. Gilpin 1991. Metapopulation dynamics: brief
histoty and conceptual domain BioI. I Linn Soc. 42:3-16.
Harris, L.D. 1988. Edge effects and conselVation of biotic diversity.
ConselV. BioI. 2:330-332.
Harrison, S. 1991. LocaI extiIdion in a metapopulation context:
an empirical evaluation BioI. J. Linn Soc. '42:73-88.
Hennann, S. (ed.) 1993. Proceedings ofa Conforence on Longleaf
Pine Forests. In press. Tallahassee, Florida: Tall Timbers, Inc.
Hobbs, R.J. 1992. The role of corridors in conselVation: Solution
or bandwagon? Trends in Recent EcoI. EvoI. 7:389-392.
Hunter, M.L., G.L. Jacobson, Jr., and T. Webb m. 1988.
Paleoecology and the coarse-futer approach to maintaining
biological diversity. ConselV. BioI. 2:375-385.
Hussey, B.M.J., R.J. Hobbs, and D.A. Saunders. 1990.
Guidelines for Bush Corridors. Chipping Norton, N.S.W.:
Organizing Committee of conference "Nature ConselVation:
the Role of Corridors."
Jackson, J.A. 1994. Red-cockaded woodpecker. In press in A.
Poole, P. Stettenheim, and F. Gill (eds.), Birds of North
America. Philadelphia: A.O.U.
J~ F.e. 1994. The status oftre nrl-<:OCkackrl woodpecker in 19W
ani tre prospect for recovery. In p-ess in R Costa, D. Kullnvy, an:l
R H~ (aIs.), Proceedings of the Red-Codmded Woodpecker
Symposiwn m. Charleston, S.Car.: U.SDA FonS: Service.
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