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The living portion of an ecosystem is best described in terms of feeding levels known as trophic levels.
Green plants make up the first trophic level and are known as primary producers. Plants are able to convert en-
ergy from the sun into food in a process known as photosynthesis. In the second trophic level, the primary con-
sumers – known as herbivores – are animals and insects that obtain their energy solely by eating the green
plants. The third trophic level is composed of the secondary consumers, flesh-eating or carnivorous animals that
feed on herbivores. At the fourth level are the tertiary consumers, carnivores that feed on other carnivores. Fi-
nally, the fifth trophic level consists of the decomposers, organisms such as fungi and bacteria that break down
dead or dying matter into nutrients that can be used again.
Some or all of these trophic levels combine to form what is known as a food web, the ecosystem’s mecha-
nism for circulating and recycling energy and materials. For example, in an aquatic ecosystem algae and other
aquatic plants use sunlight to produce energy in the form of carbohydrates. Primary consumers such as insects
and small fish may feed on some of this plant matter, and are in turn eaten by secondary consumers, such as
salmon. A brown bear may play the role of the tertiary consumer by catching and eating salmon. Bacteria and
fungi may then feed upon and decompose the salmon carcass left behind by the bear, enabling the valuable
nonliving components of the ecosystem, such as chemical nutrients, to leach back into the soil and water, where
they can be absorbed by the roots of plants. In this way, nutrients and the energy that green plants derive from
sunlight are efficiently transferred and recycled throughout the ecosystem.
In addition to the exchange of energy, ecosystems are characterized by several other cycles. Elements such
as carbon and nitrogen travel throughout the biotic and abiotic components of an ecosystem in processes known
as nutrient cycles. For example, nitrogen traveling in the air may be snatched by tree-dwelling, or epiphytic,
lichen that converts it to a form useful to plants. When rain drips through the lichen and falls to the ground, or
the lichen itself falls to the forest floor, the nitrogen from the raindrops or the lichen is leached into the soil to
be used by plants and trees. Another process important to ecosystems is the water cycle, the movement of water
from ocean to atmosphere, to land and eventually back to the ocean. An ecosystem such as a forest or wetland
plays a significant role in this cycle by storing, releasing, or filtering the water as it passes through the system.
Every ecosystem is also characterized by a disturbance cycle, a regular cycle of events such as fires,
storms, floods, and landslides that keeps the ecosystem in a constant state of change and adaptation. Some spe-
cies even depend on the disturbance cycle for survival or reproduction. For example, longleaf pine forests de-
pend on frequent low-intensity fires for reproduction. The cones of the trees, which contain the reproductive
structures, are sealed shut with a resin that melts away to release the seeds only under high heat.
Humans benefit from these smooth-functioning ecosystems in many ways. Healthy forests, streams, and
wetlands contribute to clean air and clean water by trapping fast-moving air and water, enabling impurities to
settle out or be converted to harmless compounds by plants or soil. The diversity of organisms, or biodiversity,
in an ecosystem provides essential foods, medicines, and other materials. But as human populations increase
and their encroachment on natural habitats expand, humans are having detrimental effects on the very ecosys-
tems on which they depend. The survival of natural ecosystems around the world is threatened by many human
activities: bulldozing wetlands and clear-cutting forests – the systematic cutting of all trees in a specific area –
to make room for new housing and agricultural land; damming rivers to harness the energy for electricity and
water for irrigation; and polluting the air, soil, and water.
Many organizations and government agencies have adopted a new approach to managing natural resources
– naturally occurring materials that have economic or cultural value, such as commercial fisheries, timber, and
water, in order to prevent their catastrophic depletion. This strategy, known as ecosystem management, treats
resources as interdependent ecosystems rather than simply commodities to be extracted. Using advances in the
study of ecology to protect the biodiversity of an ecosystem, ecosystem management encourages practices that
enable humans to obtain necessary resources using methods that protect the whole ecosystem. Because regional
economic prosperity may be linked to ecosystem health, the needs of the human community are also consid-
ered.
Ecosystem management often requires special measures to protect threatened or endangered species that
play key roles in the ecosystem. In the commercial shrimp trawling industry, for example, ecosystem manage-
ment techniques protect loggerhead sea turtles. In the last thirty years, populations of loggerhead turtles on the
southeastern coasts of the United States have been declining at alarming rates due to beach development and
the ensuing erosion, bright lights, and traffic, which make it nearly impossible for female turtles to build nests
on beaches. At sea, loggerheads are threatened by oil spills and plastic debris, offshore dredging, injury from
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