INFLUENCE OF SHORT-TERM VARIATION RAINFOREST DIVERSITY
and their diversity do not exist in a constant state, but are
the product of a series of impacts including fires, tree falls, small-scale
human clearing, and even lava flows. These events can increase forest
diversity by giving new species a chance to grow in the absence of the
towering canopy trees. The growth of new tree species spells new opportunities
for their symbiotic species (for example new pollinators or seed dispersers).
Forests that are regularly stressed, like those affected seasonally
by strong winds and storms, tend to be dwarfed with a less developed
canopy and reduced diversity. "Typical" tall rainforests
are typically found where
they are protected from strong winds, as
in valleys and certain geographical areas.
Within a relatively small area there can be great variations in forest
dynamics. For example, in the terra firme rainforests of the Central
Amazon—where average canopy tree age can exceed 300 years
and some trees can be more than one thousand years old
forest turnover rates can be extremely low. In contrast, nearby floodplain
forests may have turnover rates of less than 70 years due to migrating
river channels that periodically undercut river banks and trees.
Diversity is usually sharply reduced in forests degraded by activities
such as logging, burning, and agricultural development. Generally, when
forest is logged, the dense canopy structure is disturbed, allowing
more sunlight to penetrate to the forest floor. The forest is more likely
to dry out, and less water can be recycled through the system of evaporation
and transpiration. Many rainforest species are unable to cope with the
changes in the forest microclimate and either move on or gradually perish.
In addition, the loss of certain valuable hardwood trees to logging
has a major impact on species with which they have interdependent relationships.
Studies suggest that logging in any form reduces tropical forest diversity—studies around the world show declines of certain species, especially
primates, birds, and insects in degraded forests. While there may be
a local increase in the abundance and diversity of certain species,
there is an overall regional or global decline in biodiversity due to
the loss of species specially adapted to the conditions of undisturbed
forest. Degraded forest is also more prone to be developed or burned
by humans, severely reducing diversity. Heavy logging in the forests
of Indonesia and Brazil was partly responsible for creating
the dry forest conditions that drove the widespread forest fires of 1997-1998
Fallen tree in Borneo. (Photo by R. Butler)
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Continued / Next: Diversities of Image
Eldredge, N. and Gould S. ("Punctuated equilibrium: an alternative to phyletic gradualism." in T. Schopf, Models in Paleobiology, New York: WH Freeman 1972) introduce the idea of punctuated equilibrium as a new theory for evolution.
"Doomsday genes" which may enable species to undergo radical structural changes in mere generations in response to sudden environmental changes are discussed in Rutherford, S.L. and S. Lindquist, "HSP90 as a capacitor for morphological evolution," Nature 396: 336-342, 1998.