Environmental Science
Chapter 3, section 3.1
CH 3: How Ecosystems Work
NOTES
Name: ______________________
Energy Flow in Ecosystems
Life depends on the sun
 Organisms cannot survive without a supply of energy.
 Where does an organisms energy come from? Ultimately from the SUN
 Plants, algae and some kinds of bacteria capture solar energy and store it as food through a process
called PHOTOSYNTHESIS.
 The result is the production of sugar, an energy-rich food.
 Ex: When a rabbit eats a clover plant, the rabbit gets its energy from the food the clover made during
photosynthesis. A coyote eats the rabbit, some of the energy transfers from the rabbit to the coyote.
 The clover is a producer (can make its own food;) the rabbit and coyote are consumers (also called
heterotrophs or other feeders.) that get their energy by eating producers and other organisms.
An Exception to the Rule: Deep-Ocean Ecosystems
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In 1977, scientists discovered areas on the bottom of the ocean off the coast of Ecuador that had life
without sunlight.
Found were: communities of fish, worms, clams, mussels and barnacles living around cracks in the
ocean floor.
These communities exist in total darkness, where photosynthesis cannot occur.
They found bacteria that made food from hydrogen sulfide, which is present in the hot water that
escapes from the ocean floor.
The bacteria are considered producers, and are making food without sunlight.
Animals eat these bacteria, thus supporting this ecosystem.
What Eats What
 Herbivores (plant-eaters) are consumers that eat only producers:
o ex: rabbits, cows, sheep, deer, grasshoppers, etc.
 Carnivores (flesh-eaters) are consumers that eat only other consumers:
o ex: lions, hawks
 Omnivores (eaters of all) are consumers that eat both producers and other consumers:
o ex: humans, pigs, bears
 Decomposers are consumers that get their food by breaking down dead organisms, causing them to rot.
Decomposers make it possible to return nutrients to the soil or water:
o ex: bacteria, fungi
Respiration: Burning the Fuel
 The food you ate contains a lot of energy.
 Your body gets energy out of the food by using oxygen you breathe to break down the food molecules.
 The process of breaking down food to yield energy is called cellular respiration.
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 During cellular respiration, sugar and oxygen combine to yield carbon dioxide, water, and most
important, energy.
 A portion of the energy obtained through cellular respiration is used to carry out daily activities
(walking, breathing, reading, thinking) and to make more body tissue, so you can grow (some stored as
fat or sugar.)
 All living things use cellular respiration to get energy from food molecules.
Energy Transfer: Food Chains, Food Webs, and Trophic levels
 Each time one organism eats another organism, a transfer of energy occurs.
 We trace the paths that energy follows by studying food chains, food webs, and trophic levels.
 A food chain is a sequence in which energy is transferred from one organism to the next as each
organism eats another.
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 Ecosystems are more complicated than a simple food chain. There are many species that feed off of
more than one kind of food. This is called a food web and shows the feeding relationships in an
ecosystem.
 Each step in the transfer of energy through an ecosystem is known as a trophic level.
Food Chains and Food Webs
Trophic levels
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In this example, grass is at the bottom of the trophic level and is the most prolific. Each time energy is
transferred, less of it is available to the organisms at the next level.
Why? Some energy is lost during the process of converting food to energy. The remaining energy is
used to carry out life functions (making new cells, moving, breathing, etc.)
90% of the energy is lost at each level, only 10% passes on to the next level
How energy loss affects an Ecosystem
 Decreased amount of energy at each level results in fewer organisms at the higher trophic levels.
 ex: zebras and other herbivores outnumber lions on the African savanna by about 1000 to 1.
 The loss of energy from trophic level to trophic level may place a limit on the number of trophic levels
in an ecosystem. Therefore, ecosystems rarely have more than four or five trophic levels.