Chapter 3
Section 3Energy Flow
in Ecosystems
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THINK ABOUT IT
What happens to energy stored in
body tissues when one organism eats
another?
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Energy moves from the “eaten” to
the “eater.” Where it goes from
there depends on who eats whom!
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Feeding Relationships
Energy flows through an
ecosystem in one direction,
from the sun or inorganic
compounds to autotrophs
(producers) and then to various
heterotrophs (consumers).
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Feeding Relationships
Food Chains
A food chain is a series of steps in
which organisms transfer energy by
eating and being eaten.
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Feeding Relationships
Food chains can vary in length.
An example of a marine food chain is shown.
Phytoplankton
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In some aquatic food chains, such as the example
shown, primary producers are a mixture of floating
algae called phytoplankton and attached algae.
These producers are eaten by small, swimming
animals called zooplankton.
Note that there are four steps in the food chain
shown.
The top carnivore is four steps removed from the
primary producer.
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Generally, consumers are distinguished from one
another by their distance from the producer in the
flow of energy.
Consumers that eat producers are called first level
or primary consumers (i.e. the zooplankton).
Consumers that eat primary consumers are called
second level or secondary consumers (i.e. the
small fish)
What do 3rd level or tertiary consumers (i.e. the
squid) eat? What is the quaternary consumer?
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Feeding Relationships
Food Webs
Ecologists describe a feeding relationship in
an ecosystem that forms a network of complex
interactions as a food web.
Each path through a food web is a food
chain. A food web links all the food chains in an
ecosystem together.
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This food web shows some of the
feeding relationships in a salt-marsh community.
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Decomposers and Detritivores in Food Webs
Decomposers and detritivores are as important in food webs
as another other consumer is. Unfortunately they are often
left out of the “picture.”
Most producers die without being eaten. In the detritus
pathway, decomposers convert that dead material to detritus,
which is eaten by detritivores.
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At the same time, decomposition and the
action of detrivores releases nutrients that can
be used by primary producers. Dead and
decaying matter is broken down into forms that
can be reused by organisms, similar to the way
a recycling center works.
Without decomposers, nutrients would
remain locked in dead organisms.
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Food Webs and Disturbance
When disturbances to food webs happen, their effects can
be dramatic.
For example, all of the animals in this food web depend
directly or indirectly on shrimplike animals called krill. Krill
are an example of zooplankton.
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Food Webs and Disturbance
In recent years, krill populations have dropped
substantially. Given the structure of this food web,
a drop in the krill population can cause drops in
the populations of all other members of the food
web shown.
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Trophic Levels
Each step in a food chain or food web is
called a trophic level.
Producers make up the first trophic level.
Consumers make up the second, third, and
higher trophic levels.
Each consumer depends on the trophic level
below it for energy.
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Ecological Pyramids
Ecological Pyramids
The amount of energy or matter in an ecosystem
can be represented by an ecological pyramid.
An ecological pyramid is a diagram that shows
the relative amounts of energy, numbers of
organisms, or matter contained within each trophic
level in a food chain or food web.
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Ecological Pyramids
Ecologists recognize three
different types of ecological
pyramids:
•
energy pyramids
•
pyramids of numbers
•
biomass pyramids
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Ecological Pyramids
Energy Pyramid:
0.1% Thirdlevel
1% Secondconsumers
level
10% Firstconsumers
level
consumers
(pg. 77) level.
100% Produc
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Ecological Pyramids
There is theoretically no limit to the number of
trophic levels in a food web or the number of
organisms that live on each level.
However, as seen on the energy pyramid in the
previous slide, only a small portion of the energy
that passes through any given trophic level is
ultimately stored in the bodies of organisms at the
next level. Organisms expend much of the energy
they acquire on life processes, such as respiration,
movement, growth, and reproduction. Most of the
remaining energy is released into the environment
as heat—a byproduct of these activities.
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The more levels that exist between a
producer and a top-level consumer in an
ecosystem, the less energy that remains
from the original amount.
Only about 10 percent of the energy
available within one trophic level is
transferred to organisms at the next
trophic level.
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Ecological Pyramids
Biomass Pyramid:
(pg. 78)
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Ecological Pyramids
Biomass Pyramid
The total amount of living tissue within a given
trophic level is called biomass.
Biomass is usually expressed in terms of grams of
organic matter per unit area.
A biomass pyramid represents the relative amount
of potential food available for each trophic level in
an ecosystem. Typically, it is greatest at the base.
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Ecological Pyramids
Pyramid of
Numbers:
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Ecological Pyramids
Pyramid of Numbers
A pyramid of numbers shows the relative
number of individual organisms at each trophic
level.
Typically, the number of individuals at the
base is the greatest.
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In some ecosystems, however, there are
fewer producers than there are consumers.
Such an example is the forest ecosystem.
Here the pyramid of numbers would not
resemble a typical pyramid at all (though the
biomass pyramid probably would).
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