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发布时间：2022-04-27 08:41
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时间：2023-10-07 00:30
托福阅读真题列举：
The Long-Term Stability of Ecosystems
Plant communities assemble themselves flexibly, and their
particular structure depends on the specific history of the area.
Ecologists use the term “succession” to refer to the changes that
happen in plant communities and ecosystems over time. The first
community in a succession is called a pioneer community, while the
long-lived community at the end of succession is called a climax
community. Pioneer and successional plant communities are said to change
over periods from 1 to 500 years. These changes—in plant numbers and
the mix of species—are cumulative. Climax communities themselves change
but over periods of time greater than about 500 years.
An ecologist who studies a pond today may well find it relatively
unchanged in a year’s time. Indivial fish may be replaced, but the
number of fish will tend to be the same from one year to the next. We
can say that the properties of an ecosystem are more stable than the
indivial organisms that compose the ecosystem.
At one time, ecologists believed that species diversity made
ecosystems stable. They believed that the greater the diversity the
more stable the ecosystem. Support for this idea came from the
observation that long-lasting climax communities usually have more
complex food webs and more species diversity than pioneer communities.
Ecologists concluded that the apparent stability of climax ecosystems
depended on their complexity. To take an extreme example, farmlands
dominated by a single crop are so unstable that one year of bad weather
or the invasion of a single pest can destroy the entire crop. In
contrast, a complex climax community, such as a temperate forest, will
tolerate considerable damage from weather to pests.
The question of ecosystem stability is complicated, however. The
first problem is that ecologists do not all agree what “stability”
means. Stability can be defined as simply lack of change. In that case,
the climax community would be considered the most stable, since, by
definition, it changes the least over time. Alternatively, stability
can be defined as the speed with which an ecosystem returns to a
particular form following a major disturbance, such as a fire. This
kind of stability is also called resilience. In that case, climax
communities would be the most fragile and the least stable, since they
can require hundreds of years to return to the climax state.
Even the kind of stability defined as simple lack of change is not
always associated with maximum diversity. At least in temperate zones,
maximum diversity is often found in mid-successional stages, not in the
climax community. Once a redwood forest matures, for example, the
kinds of species and the number of indivials growing on the forest
floor are reced. In general, diversity, by itself, does not ensure
stability. Mathematical models of ecosystems likewise suggest that
diversity does not guarantee ecosystem stability—just the opposite, in
fact. A more complicated system is, in general, more likely than a
simple system to break down. A fifteen-speed racing bicycle is more
likely to break down than a child’s tricycle.
Ecologists are especially interested to know what factors
contribute to the resilience of communities because climax communities
all over the world are being severely damaged or destroyed by human
activities. The destruction caused by the volcanic explosion of Mount
St. Helens, in the northwestern United States, for example, pales in
comparison to the destruction caused by humans. We need to know what
aspects of a community are most important to the community’s resistance
to destruction, as well as its recovery.
Many ecologists now think that the relative long-term stability of
climax communities comes not from diversity but from the “patchiness”
of the environment, an environment that varies from place to place
supports more kinds of organisms than an environment that is uniform. A
local population that goes extinct is quickly replaced by immigrants
from an adjacent community. Even if the new population is of a
different species, it can approximately fill the niche vacated by the
extinct population and keep the food web intact.
Paragraph 1: Plant communities assemble themselves flexibly, and
their particular structure depends on the specific history of the area.
Ecologists use the term “succession” to refer to the changes that
happen in plant communities and ecosystems over time. The first
community in a succession is called a pioneer community, while the
long-lived community at the end of succession is called a climax
community. Pioneer and successional plant communities are said to
change over periods from 1 to 500 years. These changes—in plant numbers
and the mix of species—are cumulative. Climax communities themselves
change but over periods of time greater than about 500 years.
