Threats to Tropical Forests: Overview
Ecologists have identified many potential threats to the
integrity of tropical forests. These include continued
deforestation, forest degradation, forest fragmentation,
and increasing loss of biodiversity. Forest conservation
is a complex topic, because it is not just a matter of
forest clearance. Forests may not be fully cleared
but nonetheless be significantly degraded. Forest
degradation results from such activities as logging,
agriculture, cattle ranching, prospecting for minerals,
and building of hydroelectric dams (plate 18- 1). Forests
may be highly fragmented, which isolates small forest
tracts from one another, causing severe ecological
effects. Local hunting pressure may leave a forest
essentially intact but lacking in animal species essential
for seed dispersal. Another more subtle and complex
threat is the effect of climate change, a reality that will
likely alter forest dynamics, biotic interactions, trophic
dynamics, carbon sequestration, recycling of nutrients,
and land- use patterns.
Deforestation: How Much, How Bad?
It is well known that there has been extensive
deforestation throughout the global tropics, including
the Neotropics. Rates of deforestation throughout
the tropics are difficult to obtain and are constantly
changing. Anyone who travels throughout a tropical
region will bear witness to deforestation, but such
observations are anecdotal and difficult to quantify. The
method of choice for estimating large- scale regional
deforestation rates has been satellite imagery, because
it provides a large- scale look at forest changes over
time. One study, which used 30 m resolution satellite
imagery, concluded that from 1990 to 1997, a total of
5.8 million ha (14.3 million ac) of humid tropical forest
had been lost annually (an area about the size of the
state of Maryland) and that an additional 2.3 million
ha (5.7 million ac) were visibly degraded.
The once extensive Atlantic Forest of southeastern
Brazil has been reduced and fragmented to a mere 10%
of its original area. Approximately 70% of the animal
species considered endangered in Brazil are confined to
the Atlantic Forest. There are about 55 threatened bird
species (each of which is endemic), and 21 threatened
endemic mammal species. European colonists began
cutting the forest immediately upon their arrival,
because they believed that the forests had to be cleared
to allow for social progress (as did the European settlers
who colonized North America).
Deforestation in the global tropics is closely linked
with growth of human population (plate 18- 2). As
rural populations have burgeoned, forest loss has
increased. The United Nations Food and Agriculture
Organization (FAO), using data generated from
satellite observation, has documented that tropical
deforestation usually occurs first in the more dry and
open forests (which are easier for humans to access),
rather than humid rain forests, and that about 2.2% of
the potential closed- canopy tropical forest (all forest
types combined) is being removed each decade.
Forests are dynamic. If land that has been cleared
is abandoned, secondary forest will normally grow
on the site. So in some areas, such as areas cleared
for logging and subsequently abandoned, forests are
returning through the process of succession (chapter
7). But secondary forests are different from old- growth
forests. The dynamic between forest loss and forest
regrowth is complex, and it has major implications that
are essential to understand in considering the future
of tropical forests. It has become a topic of significant
debate and will be discussed further below.Fire, Degradation, and Deforestation
Human- caused fire has been shown to have a strong
impact on forest degradation, even to the point of
gradually changing forest into savanna in some areas.
Accidental fires are linked to road building, agriculture,
mining, and pretty much any human intrusion into
forest. Initially fires are of low intensity and travel along
the forest floor, not reaching the canopy. But though
the fires appear only to destroy leaf litter, they also kill
tree stems, because many tropical trees have thin bark.
A cascade of ecological effects ensues. The fire thins
the forest, and subsequent windthrow adds debris to
the litter layer. The increasingly open canopy permits
greater solar heating, and the litter dries, making it
more prone to fire. The greater openness results in
growth of various vines and herbaceous species that add
combustible fuel. Forests that have burned are therefore
likely to burn again and with greater intensity. A positive
feedback is established in which each subsequent fire is
more severe than the previous one, and recurrent firesChapter 18. The Future of the Neotropics
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