Environmental Engineering Reference
In-Depth Information
creasing their home range size. Ecological theory, which we consider next,
predicts that such alteration in a species' ecology can affect the fundamen-
tal structure and long-term dynamics of species populations and ecologi-
cal communities.
Habitat Fragmentation and Population and
Community Processes
Populations of species that live in large tracts of intact habitat are expected
qualitatively to undergo the kinds of population dynamics described in
chapter 4. Namely, individuals of a population vie for their share of resources
and the maximum limit on population size is determined by the capacity
of the intact habitat to supply resources. Populations of species that live in
patches of habitat that are separated on the landscape undergo different dy-
namics because they not only survive and reproduce within local habitat
patches but they migrate among habitat patches on the landscape.This col-
lection of local populations connected on the landscape by migration is
known as a metapopulation (Levins 1969).A powerful metaphor for describ-
ing metapopulation dynamics is to liken habitat patches to lights on a
Christmas tree in which patches occupied by the species are lit and patches
that are unoccupied are dim.The ensuing dynamics can be imagined as the
tree lights winking on and off.That is, some occupied patches become ex-
tinct due to random effects on small populations; however, some unoccu-
pied patches are colonized by migrating individuals. In theory, the perpetual
balance between patch extinction and recolonization allows the population
as a whole to persist on the landscape even though it may not be present
in any one patch at any one time (Kareivea and Wennergren 1995).
Fragmentation of large parcels of land into smaller, localized patches can
thus change the fundamental character of population process because it
forces individuals normally belonging to a large, contiguous population to
be relegated to small, local populations scattered among small habitat pock-
ets. An important feature of the population dynamics in fragmented habi-
tats is that landscape-scale persistence and maximum size of the newly
created metapopulation depends not only on the number and size of habi-
tat patches but importantly on the ability of individuals to migrate among
the local habitat patches (Kareivea and Wennergren 1995). Species will de-
scend to extinction once the number of available habitat patches falls below
a species-specific threshold. But, even if an ample number of patches is avail-
