Environmental Engineering Reference
In-Depth Information
perimental research at landscape scales, identify the cause and magnitude of
risks to species diversity in ways that clearly illuminate the consequences
of different policy options for habitat fragment size.
The Ferraz et al. study implicitly assumes that loss of species from a habi-
tat fragment is a random event in the sense that each species of bird has an
equal likelihood of going extinct and that it is only the loss of absolute liv-
ing space that drives species in this system to local extinction. But, species
do not live in “splendid isolation” of other species (Lawton 1999).They are
embedded in food webs in which they are linked to competitor and pred-
ator species. In such cases, habitat fragmentation may disrupt important lines
of dependency in those food webs and precipitate nonrandom extinctions
due to altered competition or predation. A case example, which we now
consider, is the attendant consequences of top predator loss from another
ecosystem.
Fragmentation, Mesopredator Release,
and Loss of Bird Species Diversity
Southern California has been trans-
formed from a largely native sage-
scrub chaparral landscape to a highly
urbanized area.Vestiges of the native
habitat are relegated to steep-sided
canyons and small fragments within
the urban matrix.As expected from the species-area relationship, bird species
that normally depend on large, intact tracts of native sage-scrub habitat have
undergone significant declines in population size or have become extinct
(Crooks and Soulé 1999). Habitat fragmentation certainly is an important
factor in bird species decline. But, closer inspection of the ecological
processes underlying the dynamics of this system has revealed a more insid-
ious cause for bird species decline.
In the original sage-scrub habitat, birds species were part of a food web
(figure 6.2) in which the top predator, the coyote (
Canis latrans
), preyed
upon several species of natural middle-of-the-food-web predators (meso-
predators) including the striped skunk (
Mephitis mephitis
), the raccoon (
Pro-
cyon lotor
), and the gray fox (
Urocyon cinereoargenteus
), as well as more recent
exotic predators (domestic cat, Opossum [
Didelphis virginanus
]).These meso-
predator species in turn preyed upon birds' eggs, nestlings, and sometimes
juvenile and adult birds. In this system, the coyote had an
indirect
beneficial
Habitat fragmentation may disrupt
important lines of dependency in
those food webs and precipitate
nonrandom extinctions due to
altered competition or predation.
