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Biology
Ecosystems and Biomes
The Flow of Energy
Unlike matter that is recycled, energy enters the system, is used by
organisms, and is lost as heat. All of the Earth’s energy is initially provided in
the form of solar energy. The amount of solar energy that reaches each individual
ecosystem varies, and ecosystems are limited by the amount of energy they receive.
Energy first enters an ecosystem when photosynthetic organisms use sunlight to
produce food. The flow of energy continues through the ecosystem as photosynthetic
organisms are consumed by other organisms. As these organisms break down the food
molecules to obtain nutrients, the majority of the energy contained in them is
converted to heat and lost to the atmosphere. However, the organisms use
approximately 10 percent of the energy to produce new molecules. As organisms are
consumed by other organisms, the breakdown and loss of energy continues.
Trophic Levels
Organisms in an ecosystem are divided into two major trophic levels based
on their source of energy:
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Autotrophs manufacture their own food. Autotrophs, which
include plants, algae, and some bacteria, generally use light energy to
manufacture food via photosynthesis. Autotrophs are also known as
primary producers. Chemoautotrophs can manufacture their
food without the use of solar energy. They live in extremely hostile
deep-sea hydrothermal vents and use carbon dioxide to oxidize sulfur and
other inorganic compounds for the production of food, in a process known as
chemosynthesis.
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Heterotrophs are unable to produce their own food. They
gain their energy by consuming autotrophs or other heterotrophs that feed on
autotrophs. All animals and fungi, as well as most protists and prokaryotes,
are heterotrophs. Heterotrophs are also known as consumers.
Heterotrophs
Heterotrophs are divided into several trophic levels depending on
their source of energy.
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Primary consumers, or herbivores, feed on plants.
Examples include rabbits and caterpillars.
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Secondary consumers, or carnivores, feed on
herbivores. Examples include tigers, foxes, and sharks.
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Tertiary consumers feed on secondary consumers.
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Quaternary consumers feed on tertiary consumers.
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Detritivores feed on the waste of an ecosystem.
Detritivores include scavengers, which feed on the remains
of dead animals. Vultures and jackals are scavengers. Detritivores also
include decomposers, which break down dead plant and animal
remains. Decomposers include bacteria and fungi.
The Food Chain
A food chain is a diagram showing how organisms from each
trophic level feed on each other. A food web is a more
complicated diagram that recognizes that one organism rarely feeds only on
one type of organism. A food web shows all the relationships between
organisms in the ecosystem.
Food Chain
Food Web
Productivity in Ecosystems
The flow of energy through an ecosystem is not an efficient process. Only
around 1 percent of the solar energy that reaches a plant is used to produce
food. From there, at each level that organisms consume each other for food,
energy is lost as heat. Scientists use three calculations for measuring plant
productivity in an ecosystem:
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Primary productivity measures how much energy is produced
by the photosynthetic organisms in an ecosystem.
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Gross primary productivity measures the total amount of
organic matter produced by the photosynthetic organisms in the ecosystem.
Plants also use some of the matter they produce for respiration. The
remainder is available for heterotrophs.
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Net primary productivity (NPP) measures the amount of
matter produced that is available for heterotrophs. NPP for an ecosystem is
generally measured in grams per square meter
(g/m2).
EXAMPLE:
In an environment such as a rainforest or wetland, NPP can be as high
as 3,000 g/m2. In an environment such as a
desert, it may be less than 100 g/m2.
Biomass is the total weight of all an ecosystem’s organisms.
Biomass increases as net primary productivity increases. Secondary
productivity is a measure of the rate of biomass production by an
ecosystem’s heterotrophs. Only around 10 percent of the energy available at one
trophic level is available at the next. Energy may be lost as fecal matter or
converted to heat. As energy passes from consumer to consumer down the food
chain, the amount of available energy is continually diminished, limiting the
number of steps a given ecosystem’s food chain can sustain.
Ecological Pyramids
Ecological pyramids are diagrams that show the relationships
between the trophic levels of an ecosystem. The pyramid shape reflects the loss
of energy that occurs from one trophic level to the next. There will generally
be many individuals at the lowest trophic level, where energy levels are more
abundant. Since only 10 percent of the energy is passed on to the next level,
there will be fewer individuals at the second level, and even fewer at the
third.
Ecological pyramids can be represented in three ways:
- A pyramid of numbers shows the number of individuals at
each level.
- A pyramid of biomass shows the amount of biomass produced
at each level.
- A pyramid of energy shows the amount of energy available
at each level.
Ecological Pyramids
EXAMPLE:
Photosynthetic algae fix 1,000 calories of potential energy. Of
these, 850 calories are used by the algae, and 150 calories are transferred
to a heterotroph that eats algae. Of the 150 calories, around 120 calories
are used by the heterotroph, and 30 calories are transferred to a consumer
of the heterotroph.
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