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Landscape Characterization / Forest Fragmentation / What is Habitat Fragmentation? / The Process of Habitat Fragmentation
What is Habitat Fragmentation?
The Process of Habitat Fragmentation
Fragmentation occurs when a large region of habitat has
been broken down, or fragmented, into a collection of smaller patches
of habitat. Fragmentation typically occurs when land is converted from
one type of habitat to another. For example, a forest habitat may become
fragmented when a highway is built across the forest. The highway would
split a single, large, continuous patch of forest into two smaller patches.
History diagram
Stage 1: the
clearing of forest to make fields. Patches of cleared land appear within
the matrix of forest.
Stage 2: as
farms expand and a region develops, more land is cleared of forest. The
patches expand until there is as much forest as there is open agricultural
fields.
Stage 3: Development
continues; the farm fields coalesce into large open areas of land. Patches
of forest now exist within a matrix of agricultural fields.
Figure 2. Series of aerial photographs showing the stages of the fragmentation
process. The process proceeds from development of small patches within
a large area of continuous forest (a) to expansion of the developed patches
(b), and ultimately to conversion of the dominant land cover type from
forest to human land use (c).
Effects on wildlife population
Forest fragmentation affects plant and animal populations at several scales.
Over the larger landscape of the Chesapeake Bay watershed, once continuous
populations of forest dwelling species were broken up into smaller subpopulation
occupying the remaining forest fragments. Scientists believe these subpopulations
may act as metapopulations. A metapopulation is a collection of small populations
occupying a number of habitat patches (Figure 3).
Figure 3. A collection of forest patches occupied by populations of a
forest dwelling species. The weight of arrows indicates the rate of occasional
exchange of individuals among populations (heavier arrows indicate more
exchange). Note that the rate of exchange is correlated with how close
patches are in space.
 Individuals
occasionally move between patches, and populations can go extinct in individual
patches as a result of chance events. For example, the two males in a
population of six females may be eaten by a predator. If no other males
immigrate into the patch from other patches, the population in that patch
will go extinct. Small populations are particularly susceptible to this
type of chance extinction. However, because individuals occasional move
between patches, the vacant patch will eventually be colonized and occupied
again in the future (Figure 4). If colonization rates of vacant patches
are higher than extinction rates, the metapopulation will persist. This
is because while some patches are experiencing extinction others are being
colonized. In effect, the movement of individuals among populations ties
all of the populations into a metapopulation capable of persisting in
the altered landscape.
Figure 4. Occupancy of collection of forest patches supporting a metapopulation
of a forest dwelling species at two different time. Solid patches are
occupied and open patches are not. Note that the occupancy of individuals
patches changes through time (i.e. patches are colonized and populations
in patches go extinct), but the number of occupied patches remains the
same through time.
 The
problem for land planners is that the rates of movement of individuals
among patches is determined by how close or far apart they are. Patches
that are far from other patches will not exchange individuals with other
patches, and the small population remaining in the patch will eventually
go extinct. What this tells us is that as patches become more and more
isolated from each other the colonization rates go down to the point that
extinction rates are higher than colonization rates, and the whole metapopulation
will go extinct. This will occur before all of the patches in a landscape
are gone. The bottom line is that patches that are close to each other
provide better habitat than isolated patches of the same size.
At the scale of the individual forest patch, several factors affect its
value as plant and wildlife habitat. In general, larger patches support
more species. This is because larger forest patches have more different
kinds of habitats and support larger populations that are less vulnerable
to chance extinction. Additionally, only larger patches are likely to
contain enough habitat to support species like larger mammals that requiring
larger areas.
Individual forest patches are also affected by their surrounding. At
the forest edge, wind and sun light result in dryer conditions than are
found in the interior of the forest patch. Forest edges are also more
accessible to predators and parasites that may occur in adjacent fields
or developed areas. For example, house cats that kill small birds are
often more common in forest edges adjacent to residential developments.
Cowbirds, which are nest parasites, are also more common in forests adjacent
to the open fields where they feed. Cowbirds lay their eggs in the nests
of other birds. The host birds will care for the cow bird eggs. When the
eggs hatch, the larger cow bird nestlings will out compete the host nestlings
for food, and may even push the host nestlings out of the nest.
Some species of birds, known as forest interior species, are not tolerant
of the dryer conditions or the predators and parasites that occur at the
forest edge. These species only occur in the core habitat of forest patches
(Figure 5).
Figure 5. Graph showing the relationship between soil moisture and edge
and core habitat in a forest patch. Many species of birds cannot tolerate
conditions in the edge habitat and are only found in core habitat.
Summary-Why is fragmentation a problem?
The fragmentation of a species' habitat can threaten that
species survival for a variety of reasons. These include:
Reduction of total habitat area. As a patch of habitat is cut into smaller
and smaller pieces, there is less habitat available for a species. This
reduced amount of habitat will support a correspondingly smaller population.
Vulnerability during dispersal to other patches. As a habitat becomes
fragmented into many smaller patches, these patches become separated from
one another by relatively inhospitable terrain. Any individual that attempts
to cross between patches of habitat becomes temporarily vulnerable to
predators, harsh environmental conditions, or simply to starvation. For
example, salamanders may respond to a drying pond or to overpopulated
environment by moving on to a new pond. As the amphibian moves across
an open field, pavement, or a forest floor in search of a new patch of
habitat, it runs the risk of being eaten, starvation, or dying in an unsuitable
environment. These migrations become more difficult as fragmentation progresses.
Isolation of a population. Populations can become isolated within their
patch when all of their surrounding patches of habitat are destroyed.
This makes migration into different patches difficult and hazardous. Isolated
populations are prone to decline due to inbreeding, chaotic swings in
numbers due to overexploitation of their habitat, and sudden removal from
the patch due to small disturbances.
Edge effects: As habitat becomes fragmented into smaller patches, more
of the habitat will end up adjacent to a different type of habitat. Land
that is deep in the middle of a forest is different than land that is
at the edge of a forest- more light penetrates, and there is a different
collection of species that prefer the edge of a forest to the core of
a forest.
Edge effects- changes in microclimate. Many habitats have such an impact
on the physical environment that they can create their own microclimate.
For example, dense forests tend to be shadier, more humid, and less windy
than adjacent unforested land. This pattern becomes more pronounced the
farther you go away from the edge of the forest. If a patch of forest
becomes small enough, then you will always be near the forest's edge.
Some species require the solitude, deep shade and protection from wind
that you can only find in the middle of a dense old-growth forest.
Edge effects-vulnerability to external competition and predation. If a
prey species wanders too close to the edge of their protective habitat,
they can be quickly captured and eaten. Other species may be excellent
competitors deep within their own specialized habitat, but are less successful
against species found at the edge of their habitat that take advantage
of the wide variety of conditions found between two dissimilar habitats.
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