Download There is a one-way flow of energy through the earth, but nutrients

3.7
FOOD WEBS
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There is a one-way flow of energy through the earth, but
nutrients are cycled through biota to sustain life.
The Sun is the ultimate
source of energy.
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In ecosystems, energy is cycled through these niches:
Autotrophs - produce
food by photosynthesis
Heterotrophs - consume
autotrophs and other
heterotrophs
Decomposers rearrange organic materials
into nutrients for autotrophs
The amount of energy available to support life is called the
carrying capacity.
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Food chains represent the steps in transferring energy through the
ecosystem.
Arrows indicate the
direction of flow of
energy from prey to
predator or plant to
animal.
Complex and integrated food
chains are called food webs.
The most common population in a community is called the
dominant. The food web is built around organisms called critical
biomass or keystone species.
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Energy transfer can be
representations by trophic
pyramids that compare the
number of organisms in a
population, or the weight
(biomass) of organisms, or the productivity (energy calories).
Biomass Pyramid
Trophic Pyramid
Numbers Pyramid
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The amount of energy changes even though energy is
never created or destroyed because:
1. Energy is converted
into unusable forms
such as heat.
Warm-blooded animals are “expensive” in an ecosystem
because they consume more calories to maintain body heat.
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2. Energy is converted for an organisms’ own respiration,
metabolism, digestion and predation.
USDA
3. Parts of an organism, such as bones and scales, do not provide
calories.
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As energy flows through each step of a food web, it becomes more
unusable.
The rate of conversion of solar energy into chemical energy is the
amount of energy available, calculated as productivity.
Gross productivity - respiration = net productivity
given in units of g/m2/yr.
G–R=N
Biomass is the dry weight of all
organic matter in living organisms it
is used to estimate gross
productivity.
Respiration is the rate of energy used at each level to maintain
life (estimated at 90% of the available calories).
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If one trophic level has 100,000 calories available, only 10,000 can
potentially be transferred to the next trophic level.
The more levels in the trophic pyramid the less energy is available
in a usable form at the next higher level.
Shorter food chains
are more efficient, but
they lack diversity.
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