Habitat Fragmentation
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Habitat fragmentation is the process by which habitat loss results in the division of large,
continuous habitats into smaller, more isolated remnants.
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Many times, natural habitats show a “patchy” distribution.
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This affects the organisms that live there.
The effects of man-made habitat fragmentation are much more significant
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Organisms in these habitats are not “adapted” for fragmentation
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Activities such as “clearcutting” have created a mosaic of forested and deforested 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.
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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:
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The formation of isolated patches of habitat.
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The increasing significance of edge effects.
What does this mean?
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National Parks are habitat islands, and often show damage to animal populations. This is
often most pronounced in the smaller parks.
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It means that, when we reduce the size of a habitat we can expect to lose species.
Mount Rainier National Park in Washington has seen a reduction in the number of
mammals found there from 68 to 37 species.
The study of these dynamics has introduced modeling into conservation biology.
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Spatially explicit models combine population models with landscape maps to
make mathematical predictions.
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“Species-area curves” (special computer models) can be used to predict how large
a reserve must be to preserve its biological diversity.
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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.
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The largest current preserve is 5,119 hectares.
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What are the problems with estimates like these?
Edge Effects
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The creation of habitat patches from formerly continuous woodlands modifies the
microclimate of the forest, resulting in increased edge effects.
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Environmental conditions around edges are different from those in the interior.
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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.
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The brown-headed cowbird is a nest parasite that frequents edge habitats.
A study in California looked at the number of chaparral bird species in isolated canyons
in an urban setting.
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The number of bird species declined as the size of the canyon decreased.
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Interesting, canyons visited by coyotes had more bird species than those that were
not.
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The coyotes apparently helped control the abundance of bird predators, like
skunks and domestic cats.
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Estimates are that the Mkomazi Game Reserve in Tanzania will lose 17 of its 39 species
of large mammals if it is separated from surrounding reserves.
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5000 square kilometers has been adopted as a rough minimum size for major tropical
forest preserve in the Amazon Basin.
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Based on very speculative reasoning, it is thought that this might reduce
extinction rates to less than 1%.
Biological Dynamics of Forest Fragments Project
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Near Manaus, Brazil, forest patches have been created. They are made of
individual sections of:
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1, 10, 100, 1000, and 10 000 hectares.
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Observed prior to isolation, and will be studied for at least 20 years.
What are the key issues habitat fragmentation creates?
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Limits access to food, water, mates
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Creates “edge effects”
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Limits escape for prey and hunting range for predators
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Isolates organisms genetically
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Reduces overall habitat size
What causes habitat fragmentation in the following biomes?
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Rainforest
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Tundra
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Grasslands
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Desert
What can we do to help stop it?
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