Environmental Engineering Reference
In-Depth Information
Loss of Tropical Bird Diversity
To decrease this uncertainty, a team of
ecologists (Ferraz et al. 2003) initiated
in 1980 a large-scale fragmentation
experiment in the heart of the Ama-
zon basin near Manaus, Brazil, to
evaluate the consequences of forest
loss on biodiversity.The project cre-
ated eleven isolated fragments that differed in size by orders of magnitude
(two roughly one hundred hectare areas, four roughly ten hectare areas, and
five roughly one hectare areas) with the matrix between the experimental
patches comprised of cattle pasture (Ferraz et al. 2003).The study evalu-
ated how quickly understory bird diversity, measured as species richness,
disappeared from the fragments.The study employed standard bird mist net
sampling for the ten-year duration of the experiment.The study discovered
that smaller fragments tended initially to harbor fewer bird species than
larger fragments, consistent with the species-area relationship (figure 6.1a).
During the ten-year period, some bird species appeared to go extinct, some
declined in abundance, and others remained stable.The rate of loss of a given
proportion of species was higher in small fragments than in larger ones,
again consistent with predictions of a species-area relationship (figure 6.1b).
Ferraz et al.'s analysis of data also indicates that in order to increase by a fac-
tor of ten the time it takes for a fragment to lose 50 percent of its species,
one must correspondingly increase the size of the fragment by a factor of
one thousand. Even the hundred hectare (one square kilometer) fragments
stand to lose half of their species within a decade or so.To guarantee long-
term persistence of understory bird diversity in this tropical forest system
for one hundred years, conservation efforts need to ensure that fragments
do not become smaller than one to ten thousand hectares (ten to one hun-
dred square kilometers).
Whether or not the insights from this research will be put into practice
is beyond the scope of the science.The decision to require forest fragments
in this system to remain larger than one thousand hectares to ensure long-
term persistence of bird species diversity rests within the domain of gov-
ernment policy making. Government must also reconcile the interests of
agriculture and logging with conservation. Nevertheless, the scientific study
serves as an excellent example of how ecologists can, through careful ex-
The species-area relationship pre-
dicts that habitat fragmentation
qualitatively should lead to loss of
valued tropical biodiversity, once
fragments become small.
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