Ecosystem
What is an Ecosystem?
Energy
transformations andbiogeochemical
cycling are the main processes that comprise the field
of ecosystem ecology. As we learned earlier, ecology generally is defined as
the interactions of organisms with one another and with the environment in
which they occur. We can study ecology at the level of the individual, the
population, the community, and the ecosystem.
Components of an Ecosystem
You are already familiar with the parts of an ecosystem. You have
learned about climate and soils from past lectures. From this course and from
general knowledge, you have a basic understanding of the diversity of plants
and animals, and how plants and animals and microbes obtain water, nutrients,
and food. We can clarify the parts of an ecosystem by listing them under the
headings "abiotic" and "biotic".
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ABIOTIC COMPONENTS
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BIOTIC COMPONENTS
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Sunlight
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Primary producers
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Temperature
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Herbivores
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Precipitation
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Carnivores
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Water or moisture
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Omnivores
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Soil or water chemistry (e.g., P, NH4+)
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Detritivores
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etc.
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etc.
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All of these vary over
space/time
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By and large, this set of environmental factors is important
almost everywhere, in all ecosystems.
Usually, biological communities include the "functional
groupings" shown above. A functional
group is a
biological category composed of organisms that perform mostly the same kind of
function in the system; for example, all the photosynthetic plants or primary
producers form a functional group. Membership in the functional group does not
depend very much on who the actual players (species) happen to be, only on what
function they perform in the ecosystem.
Processes of Ecosystems
This figure with the plants, zebra, lion, and so forth illustrates
the two main ideas about how ecosystems function: ecosystems
have energy flows and ecosystems cycle materials. These two processes are
linked, but they are not quite the same.
Energy enters the biological system as light energy, or photons,
is transformed into chemical energy in organic molecules by cellular processes
including photosynthesis and respiration, and ultimately is converted to heat
energy. This energy is dissipated, meaning it is lost to the system as heat;
once it is lost it cannot be recycled. Without the continued input of
solar energy, biological systems would quickly shut down. Thus the earth is an open system with
respect to energy.
Elements such as carbon, nitrogen, or phosphorus enter living
organisms in a variety of ways. Plants obtain elements from the surrounding
atmosphere, water, or soils. Animals may also obtain elements directly from the
physical environment, but usually they obtain these mainly as a consequence of
consuming other organisms. These materials are transformed biochemically within
the bodies of organisms, but sooner or later, due to excretion or
decomposition, they are returned to an inorganic state. Often bacteria complete
this process, through the process called decomposition or mineralization (see
previous lecture on microbes).
During decomposition these materials are not destroyed or lost, so
the earth is a closed
system with
respect to elements (with the exception of a meteorite entering the system now
and then). The elements are cycled endlessly between their biotic and abiotic
states within ecosystems. Those elements whose supply tends to limit biological
activity are callednutrients.
The Transformation of Energy
The transformations of energy
in an ecosystem begin first with the input of energy from the sun. Energy from
the sun is captured by the process of photosynthesis. Carbon dioxide is
combined with hydrogen (derived from the splitting of water molecules) to produce
carbohydrates (CHO). Energy is stored in the high energy bonds of adenosine
triphosphate, or ATP (see lecture on photosynthesis).
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