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The first thing to realize about fragmentation is that there are some
important differences between distributions that are naturally patchy
and those that become patchy because of habitat fragmentation. Groom
et al. [3, p. 219] mention four in particular:
- Fragmentation reduces the extent and connectivity of remaining
habitats, and species may or may not be able to persist as a result
of those changes.
- Natural patches have a rich internal patch structure versus relatively a simple, uniform patch structure in
patches associated with human alteration of the environment.
- Natural patches typically have a smaller contrast between
interior and exterior of patch in natural patches (largely because
of 2).
- The matrix in which anthropogenically produced fragments are
embedded may harbor activities and produce byproducts that are a
direct threat to the patches that remain, while the threats from
patch exteriors to natural fragments is less severe.
Another, less obvious, consequence of fragmentation is that typically
the size of fragments becomes increasingly skewed as habitat
destruction proceeds. Even if the total area of relatively undisturbed
habitat remains fairly large, the size of remaining fragments may be
quite small.
And remember what we said in one of the earliest lectures in this
course. The number of species found in any place is related to its
geographical area. Smaller fragments will contain a smaller number of
species. Moreover, there are some ``area-sensitive'' species that
require large areas to persist. They will be lost from all
fragments. Consider the red-eyed vireo or the scarlet tanager, for
example (Figure 2.) For either species fragments
must be on the order of 10ha for a breeding pair to be found, and
fragments must be on the order of 30-100ha to have a 90% chance of
being occupied.
Figure 2:
Relationship between size of forest fragment and probability
of occupancy in red-eyed vireo and scarlet tanager (Figure 7.6
in [3]). Notice that the x-axis is on a
logarithmic scale.
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Wilcove et al. [10] use observations like these to
predict how species diversity changes as a function of habitat loss
and fragmentation(Figure 3). There are two very
important facts to note in this figure:
Figure 3:
Loss of species diversity for species with large area
requirements and low vagility (closed circles) and for species with
less stringent area requirements and greater vagility (open circles).
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- Loss of species doesn't occur until a significant fraction of
the habitat has been lost.
- Once species begin to be lost from the system, they are lost
very rapidly (see 3).
Halting fragmentation is always worthwhile if it is
feasible. Each additional increment of habitat fragmentation leads to
loss of more species than did the last. Moreover, Seabloom et
al. [8] illustrate that realistic scenarios of
habitat loss, i.e., those in which habitat is aggregated into certain
regions rather than spread randomly across a landscape, lead to even
faster rates of species loss.
Subsections
Next: Edge effects
Up: Habitat fragmentation
Previous: What will be affected
Kent Holsinger
2009-11-03