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Some
ecologists assert that ecosystem stability is influenced by
species diversity. Roughly speaking, species diversity is a
measure of the number of species living in a community. The higher
the diversity, the greater the stability. Observations that
extremely complex ecosystems, such as tropical rain forests,
remain unchanged almost indefinitely if undisturbed support this
idea. Simpler ecosystems, such as tundra, are less stable, that
is, they experience sudden, drastic shifts in |
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population size. Other simplified ecosystems, such as fields of
wheat and corn are also extremely vulnerable to change, and they
deteriorate rapidly if biotic factors shift very much.
To understand why ecologists think there may be a connection between
stability and diversity, consider the food webs in simple and complex
ecosystems. The number of species in a food web in a mature
ecosystem is large. So is the number of interactions among these
organisms. In a complex ecosystem, the elimination of one species
would probably have little effect on the ecosystem. In sharp
contrast, the number of species in the food web of a simple ecosystem is
small. The elimination of one species could have repercussions on
all other species.
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In the frozen northern regions of Canada and Alaska, species
diversity is low. Heading south, diversity increases until
one reaches the tropics of Central America, where diversity is
highest. The relationship between latitude is also found in
plants and virtually all other kingdoms. Latitude,
therefore, is an important factor affecting species diversity. The
connection between latitude and species diversity is climate. |
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Quite clearly, the milder the climate, the more species live
there.
Simplifying ecosystems by reducing species diversity makes systems less
stable and more vulnerable to outside influences. Some researchers
maintain that populations and ecosystems rarely, if ever, return to
equilibrium once disturbed. Returning to equilibrium, if possible,
would depend in large part on the nature and severity of the
disturbance. Evidence suggests, that ecosystems can recover from
small perturbations, such as changes in rainfall or short-term drought.
More severe alterations, such as deforestation of the tropics, may
render a system unable to recover.
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