Habitat Fragmentation

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Habitat Fragmentation
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Many times, natural habitats show a patchy
distribution. This affects the organisms that
live there.
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However, in todays world the effect of
anthropogenic habitat fragmentation is probably
much more significant. Equally significant is the
fact that many of the organisms in these habitats
are not adapted for such fragmentation.
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Activities such as clearcutting have created a
mosaic of forested and unforested areas in many
regions that were once completely covered with
forests.
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With the growth of human population and the
increasing removal of natural habitat, the
remaining wild areas begin to take the form of
habitat islands surrounded by relatively
uninhabitable areas. This habitat fragmentation
is an increasingly serious problem in biological
conservation.
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• The most noteworthy effects of the fragmentation
  of natural habitats are
• The formation of isolated patches of habitat.
• The increasing significance of edge effects.

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Since the remaining habitat begins to resemble an
island, the ideas of island biogeography theory
are applied to them.
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On small islands, the number of species results
primarily from the interaction of two processes
Colonization Extinction The point at which these
two rates are in equilibrium will determine the
number of species found on the island.
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In theory, the colonization rate will start out
high and decline, since there the initial number
of species is low (or none). The extinction rate
will start out low (for the same reason) and
rise. Eventually, the two will reach an
equilibrium.
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Colonization and extinction rates vary with a
number of factors. Among the most important are
the size of the island and its distance from the
species pool (the mainland).
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So, what happens if we form an island from an
area that was once part of a larger habitat. It
will, initially, probably contain more species
than the equilibrium of colonization and
extinction can support. This, in theory, would
lead to biotic relaxation. Biotic relaxation is
simply a decline in the number of species when a
formerly connected region becomes isolated as
it approaches a new equilibrium.
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The effect of area and isolation on the number of
species can be examined using species-area
curves. These curves plot the number of species
versus the size of the area examined. Typically,
both are plotted on a log scale. The result is
usually a straight line, the slope of which is
an indication of how fast the number of species
changes with increasing area.
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Typically, species-area curves for islands have
steeper slopes than those for non-isolated areas.
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What does this mean? It means that, when we
reduce the size of a habitat we can expect to
lose species.
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In habitat islands, the relatively high rate of
colonization and extinction result in a rapid
turnover of species. In other words, although
the number of species may remain relatively
constant, the actual species that are present may
change.
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National Parks are habitat islands, and often
show biotic relaxation. This is often most
pronounced in the smaller parks. Mount Rainier
National Park in Washington has seen a reduction
in the number of mammals found there from 68 to
37 species.
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Some species seem to be area-sensitive, meaning
that they are at a high risk of extinction if the
habitat reaches some critical minimum level. A
study by Robbins et. Al (1989) identified a
number of such species of birds, including
pileated woodpeckers, white-breasted nuthatches,
and scarlet tanagers.
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Habitat fragmentation may create a
metapopulation, a group of subpopulations varying
in size. Some of the smaller of these will tend
to go extinct often (sink populations), and are
only repopulated by individuals dispersing from
larger subpopulations (source populations)
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In such a situation, the survival of the species
in a region depends on the dynamics of the
subpopulations. If too many populations are
small, and become sinks, then the whole
population will move toward extinction.
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The study of these dynamics has introduced
modeling into conservation biology. Spatially
explicity models combine population models with
landscape maps to make mathematical predictions.
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The creation of habitat patches from formerly
continuous woodlands modifies the microclimate of
the forest, resulting in increased edge effects.
Environmental conditions around edges are
different from those in the interior. This
results in differences in the types of trees, the
understory, and even the animals.
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One effect is the ability of predators from open
habitats to penetrate forest edges and prey on
the animals there.
Striped skunk
Blue jay
Common crow
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The brown-headed cowbird is a nest parasite that
frequents edge habitats.
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A study in California looked at the number of
chaparral bird species in isolated canyons in an
urban setting. The number of bird species
declined as the size of the canyon decreased.
Interesting, canyons visited by coyotes had more
bird species than those that were not. The
coyotes apparently helped control the abundance
of bird predators, like skunks and domestic cats.
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How strong is the island effect for different
types of organisms? Study in Western Australia
Slope for birds 0.18
Slope for lizards 0.25
Slope for non-flying mammals 0.39 Many mammals
were extinct on the smaller preserves.
Bandicoot
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Estimates are that the Mkomazi Game Reserve in
Tanzania would lose 17 of its 39 species of large
mammals in the next 300 years if it is separated
from surrounding reserves.
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Species-area curves can be used to predict how
large a reserve must be to preserve its
biological diversity. For the Australian
wheatbelt region, estimates are that a reserve of
43,000 hectares would be necessary to preserve
all 25 species, and a reserve ¾ that large to
preserve 90 of them. The largest current
preserve is 5119 hectares. What are the problems
with estimates like these?
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Based on work initiated by Terborgh, 5000 square
kilometers has been adopted as a rough minimum
size for major tropical forest preserve in the
Amazon Basin. Based on very speculative
reasoning, it is thought that this might reduce
extinction rates to less than 1.
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Biological Dynamics of Forest Fragments Project
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Near Manaus, Brazil, forest patches have been
established 1, 10, 100, 1000, and 10 000
hectares. Censused prior to isolation, and will
be studied for at least 20 years.
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So, what does it all mean? Expect to have to
address that, in your own thoughts and words.