HONORS LIVING ENVIRONMENT
MS. ETRI
TOPIC 23: ECOLOGY
PART 2: FOOD CHAINS & FLOW OF MATERIALS THROUGH AN ECOSYSTEM
IMPORTANT TERMS (DEFINE THE FOLLOWING & PROVIDE AN EXAMPLE)
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AUTOTROPHS:
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HETEROTROPHS:
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HERBIVORES:
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CARNIVORES:
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OMNIVORES:
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PREDATOR/PREY:
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SCAVENGERS:
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DECOMPOSERS:
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FOOD CHAIN:
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FOOD WEB:
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ENERGY PYRAMID:
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HONORS LIVING ENVIRONMENT
FOOD CHAINS
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MS. ETRI
Every community has a TROPHIC STRUCTURE—a pattern of feeding relationships consisting of
several different levels.
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FOOD CHAIN:
o
The sequence of food transfer up the trophic levels.
o
Show the one-way transfer of energy from one organism to the next in an ecosystem.
o
This transfer of food moves chemical nutrients and energy from organism to organism up the
trophic levels in a community.
O
RDER IN WHICH ENERGY FLOWS THROUGH AN ECOSYSTEM
o
The arrows in the diagram below point from the food to the consumer (in the direction of
nutrient and energy transfer).
o
Energy is transferred from one organism to the
next when one organism eats another
organism.
A. The INITIAL SOURCE OF ENERGY for all
organisms in a food chain is solar energy which
comes from the sun!
B. FIRST TROPHIC LEVEL: (PRODUCERS)
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The trophic level that supports all others
consists of autotrophs “self-feeders,” which
ecologists call PRODUCERS.
o
Plants are the main producers on land.
o
Photosynthetic phytoplankton are the main
producers in water.
C. SECOND TROPHIC LEVEL: (PRIMARY
CONSUMERS)
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 All organisms in trophic levels above the producers are heterotrophs “other feeders), or
consumers.
o
All consumers are directly or indirectly dependent on the output of producers.

Primary consumers are herbivores, which eat plants, algae or phytoplankton.

EXAMPLES: grasshoppers, insects, snails, and grazing mammals and birds that eat seeds
and fruits.
D. THIRD TROPHIC LEVEL: (SECONDARY CONSUMERS)
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These organisms eat the consumers from the level below (primary consumers)
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Secondary consumers are carnivores.
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EXAMPLES: birds, frogs, spiders, lions, large carnivores, small fishes.
E. FOURTH TROPHIC LEVEL: (TERTIARY CONSUMERS)
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Organisms feed on secondary consumers.
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EXAMPLES: snakes that eat mice, hawks, wolves.
**NOTE: Depending on the food chain, these levels may vary. For example, an organism can be a secondary
consumer in one food chain, whereas it may be a tertiary consumer in another food chain.
FLOW OF MATERIALS THROUGH AN ECOSYSTEM
THE CARBON-OXYGEN CYCLE
o
Depends on photosynthesis and respiration.
o
Carbon also resides in fossil fuels and sedimentary rocks, and as dissolved carbon
compounds (carbonic acid) in the oceans.
1. The process of photosynthesis removes carbon dioxide from the atmosphere and
incorporates it into organic molecules.
2. The organic molecules are passed along the food chain by consumers.
3. The process of cellular respiration by both producers and consumers returns carbon
dioxide to the atmosphere.
4. Decomposers break down the carbon compounds in detritus (waste/debris); that
carbon, too, is eventually released as carbon dioxide.
What would happen to the carbon cycle if all the decomposers suddenly “went on strike” and stopped
working?
Carbon would accumulate in organic mass, the atmospheric reservoir of carbon would decline, and plants
would eventually be starved for carbon dioxide.
**HUMAN IMPACT: The increased burning of fossil fuels (coal and petroleum) is raising the level of carbon
dioxide in the atmosphere.
DRAW THE CARBON-OXYGEN CYCLE IN THE BOX BELOW (USE COLORED PENCILS & LABEL)
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THE WATER CYCLE
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 Water is continuously moved between the land, oceans, and atmosphere by precipitation, evaporation
and transpiration from plants.

Over the oceans, evaporation exceeds precipitation—the result?
o
A net movement of water vapor to clouds that are carried by winds from the oceans across the
land.

On land, precipitation exceeds evaporation and transpiration—the result?
o
Excess precipitation forms systems of surface water (such as lakes and rivers) and
groundwater, all of which flow back to the sea.
DRAW THE WATER CYCLE IN THE BOX BELOW (USE COLORED PENCILS AND LABEL
THE NITROGEN CYCLE
o
Depends on bacteria.
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o As an ingredient of proteins and nucleic acids, nitrogen is essential to the structure and
functioning of all organisms.
o
Nitrogen has two abiotic reservoirs, the atmosphere and the soil.
o
ATMOSPHERE—almost 80% of atmosphere is nitrogen gas (N2), however plants
cannot absorb nitrogen in this form.
o
NITROGEN FIXATION (performed by some bacteria), converts N2 to compounds of nitrogen
that can be used by plants.
TWO TYPES OF NITROGEN-FIXING BACTERIA
1. Some bacteria live symbiotically in the roots of certain species of plants, supplying
their hosts with a direct source of usable nitrogen. (Largest group of plants with this
mutualistic relationship is the legumes; a number of non-legume plants that live in
nitrogen-poor soils have a similar relationship with bacteria.
2. Free-living nitrogen-fixing bacteria in soil or water convert N2 to NH3 (ammonia),
which then picks up another H+ to become ammonium (NH4+).
a. After nitrogen is “fixed,” some of the NH4+ is taken up and used by plants.
b. Nitrifying bacteria in the soil also convert some of the NH4+ to nitrate (NO3). This form of nitrogen is more readily absorbed by plants.
What do plants use the nitrogen for? Plants use nitrogen to synthesize
molecules such as amino acids, which are then incorporated into
proteins.
3. When an herbivore eats a plant, it digests the proteins into amino acids, and then
uses the amino acids to built the proteins it needs.
a. NOTE: higher-order consumers gain nitrogen from their prey.
b. RECALL: nitrogen-containing waste products are formed during protein
metabolism, consumers excrete some nitrogen as well as incorporate
some into their body tissues. Mammals excrete nitrogen as urea.
(Industrially produced urea is used as an agricultural fertilizer).
4. Organisms that are not consumed eventually die and become detritus which is
decomposed by prokaryotes and fungi.
a. DECOMPOSITION releases NH4+ from organic compounds back into the
soil, and with the help of nitrifying bacteria, it also converts ammonium to
nitrate (NO3-).
NOTE: Under low-oxygen conditions, soil bacteria known as denitrifiers strip the oxygens from NO3releasing N2 back into the atmosphere and depleting the soil reservoir of usable nitrogen.
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HUMAN IMPACT: Modern agricultural practices are another major source of nitrogen. Animal wastes
from intensive livestock production release ammonia into the atmosphere. Farmers use enormous
amounts of nitrogen fertilizer to supplement natural nitrogen fixation by bacteria.
o
**Less than half the fertilizer is taken up by the crop plants. Some nitrogen escapes to the
atmosphere, where it forms nitrous oxide, an inert gas that lingers in the atmosphere and
contributes to global warming.
DRAW THE NITROGEN CYCLE IN THE BOX BELOW. (USE COLORED PENCILS AND LABEL)
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HONORS LIVING ENVIRONMENT
FOOD WEBS
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MS. ETRI
A graphical model depicting the many food chains linked together to show the feeding relationships of
organisms in a community.
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Is generally a more complex network of what-eats-what. In a particular ecosystem.
ENERGY PYRAMID
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