Download Habitat Fragmentation

Habitat Fragmentation
Many times, natural
habitats show a
“patchy”
distribution.
This affects the
organisms that live
there.
However, in today’s 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.
Activities such as “clearcutting” have created a mosaic of forested and
unforested areas in many regions that were once completely covered with
forests.
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.
The most noteworthy effects of the
fragmentation of natural habitats are:
1. The formation of isolated patches of
habitat.
2. The increasing significance of edge
effects.
Since the remaining habitat begins to
resemble an island, the ideas of
island biogeography theory are
applied to them.
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.
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.
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”).
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.
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.
Typically, species-area curves for islands
have steeper slopes than those for nonisolated areas.
What does this mean?
It means that, when we reduce the
size of a habitat we can expect to
lose species.
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.
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.
Some species seem to be “areasensitive”, 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, whitebreasted nuthatches, and scarlet
tanagers.
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)
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.
The study of these dynamics has
introduced modeling into conservation
biology.
Spatially explicity models combine
population models with landscape
maps to make mathematical
predictions.
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.
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
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. 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.
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
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.
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?
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%.
Biological Dynamics of
Forest Fragments
Project
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.
So, what does it all mean?
Expect to have to address that, in
your own thoughts and words.