Water Pollution 55Three categories of organisms make up an ecosystem. The producers use energy
from the sun and nutrients like nitrogen and phosphorus from the soil to produce high-
energy chemical compounds by the process of photosynthesis. The energy from the sun
is stored in the molecular structure of these compounds. Producers are often referred
to as being in the first trophic (growth) level and are called autotrophs. The second
category of organisms in an ecosystem includes the consumers, who use the energy
stored during photosynthesis by ingesting the high-energy compounds. Consumers
in the second trophic level use the energy of the producers directly. There may be
several more trophic levels of consumers, each using the level below it as an energy
source. A simplified ecosystem showing various trophic levels is illustrated in Fig. 4- 1,
which also shows the progressive use of energy through the trophic levels. The third
category of organisms, the decomposers or decay organisms, use the energy in animal
wastes, along with dead animals and plants, converting the organic compounds to stable
inorganic compounds (e.g., nitrate) that can be used as nutrients by the producers.
Ecosystems exhibit a flow of both energy and nutrients. The original energy source
for nearly all ecosystems is the sun (the only notable exception is oceanic hydrothermal
vent communities, which derive energy from geothermal activity). Energy flows in only
one direction: from the sun and through each trophic level. Nutrient flow, on the other
hand, is cyclic: nutrients are used by plants to make high-energy molecules that are
eventually decomposed to the original inorganic nutrients, ready to be used again.
Most ecosystems are sufficiently complex that small changes in plant or animal
populations will not result in long-term damage to the ecosystem. Ecosystems are con-
stantly changing, even without human intervention, so ecosystem stability is best
defined by its ability to return to its original rate of change following a disturbance.
For example, it is unrealistic to expect to find the exact same numbers and species
of aquatic invertebrates in a “restored” stream ecosystem as were present before any
disturbance. Stream invertebrate populations vary markedly from year to year, even
in undisturbed streams. Instead, we should look for the return of similar types of
invertebrates, in about the same relative proportion as would be found in undisturbed
streams.
The amount of perturbation that an ecosystem can absorb is called resistance.
Communities dominated by large, long-lived plants (e.g., old growth forests) tend to
be fairly resistant to perturbation (unless the perturbation is a chain saw!). Ecosystem
resistance is partially based on which species are most sensitive to the particular dis-
turbance. Even relatively small changes in the populations of “top of the food chain”
predators (including humans) or critical plant types (e.g., plants that provide irreplace-
able habitat) can have a substantial impact on the structure of the ecosystem. The
ongoing attempt to limit the logging of old-growth forests in the Pacific Northwest is
an attempt to preserve critical habitat for species that depend on old growth, such as
the spotted owl and the marbled murmlet.
The rate at which the ecosystem recovers from perturbation is called resilience.
Resilient ecosystems are usually populated with species that have rapid colonization
and growth rates. Most aquatic ecosystems are very resilient (but not particularly
resistant). For example, during storm events, the stream bottom is scoured, removing
most of the attached algae that serve as food for small invertebrates. The algae grow