How Ecosystems
Work
Chapter 3
“You could cover the whole world with asphalt, but
sooner or later green grass would break through.”
Ilya Ehrenburg, Russian Writer
Section 3.1 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
• 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:
ex: rabbits, cows, sheep, deer, grasshoppers, etc.
• Carnivores (flesh-eaters) are consumers that eat only other
consumers:
ex: lions, hawks
• Omnivores (eaters of all) are consumers that eat both producers and
other consumers:
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:
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.
C6H12O6 +6 O2  6 CO2 + 6 H2O + ENERGY
• 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.
ex: algae  krill  cod  leopard seal  killer whale
• 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
Food chain
Food Web
Trophic levels
• 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.
Section 3.2
The Cycling of Materials
• Materials in the ecosystem are used again and again; otherwise, they
would be gone, and life could no longer exist.
• This section covers three different cycles that allow materials to be
reused.
THE WATER CYCLE
THE CARBON CYCLE
THE NITROGEN CYCLE
The Water Cycle
• Water is essential to life.
• The sun provides the energy that drives the water cycle.
ex: Heat from the sun evaporates water from the Earth.
As the water vapor cools in the atmosphere, it condenses,
forms droplets in the clouds. When the clouds meet cold
air, the water returns to Earth as precipitation (rain, sleet,
snow.)
• Some of the water goes through the cycle again, other
seeps into the ground or flows back to the ocean via rivers
and streams.
Water
Cycle
The Carbon Cycle
• Carbon is the essential component of the proteins, fats, and
carbohydrates necessary for life.
• Carbon enters an ecosystems when producers take in CO2
from the atmosphere during photosynthesis.
• Consumers eat the producers, obtaining carbon.
• Consumers break down the food molecules during cellular
respiration and release carbon back into the atmosphere as
CO2.
• Photosynthetic organisms also release CO2 during cellular
respiration.
Carbon
Cycle
How Humans are affecting the
Carbon Cycle
• Fossil fuels (coal, oil, natural gas) are essentially stored
carbon.
• They are left over from the bodies of plants and animals
that died millions of years ago and were trapped
underground.
• When we burn fossil fuels, we release carbon into the
atmosphere as CO2.
• Problem: We burn such large quantities of fossil fuels; we
are increasing the CO2 levels in the atmosphere.
The Nitrogen Cycle
• All organisms need nitrogen to build proteins.
• There are vast quantities around us; nitrogen gas makes up 78% of the
atmosphere.
• Nitrogen-fixing bacteria are the only organism that can use nitrogen
gas directly from the atmosphere. All other organisms depend upon
these bacteria for their nitrogen.
• These bacteria take nitrogen gas from the air and transform, or “fix,” it
into a form that ecosystems can use.
• Nitrogen-fixing bacteria live within the roots of a few plants (beans,
peas, clover, alder trees) and in the soil. They both release nitrogen
into the soil.
• Animals obtain nitrogen through consumption of plants or other
animals.
Nitrogen
Cycle
Closing the Nitrogen Cycle
• In the nitrogen cycle, nitrogen moves back and forth between the
atmosphere and living things.
• Decomposers, such as fungi and bacteria, return nitrogen to the soil
by breaking down dead organisms and waste products (urine, dung,
leaves, and other plant parts.)
• After decomposers return the nitrogen to the soil, some nitrogen is
converted into a usable form of nitrogen for plants and other bacteria
transform a small amount into nitrogen gas, returning it back into the
atmosphere, completing the nitrogen cycle.
• Once nitrogen enters an ecosystem, most of it stays within the
ecosystem, cycling between organisms and the soil in an endless
loop.
Section 3.3
How Ecosystems Change
• Scientists refer to ecological change as
Succession.
• ex: As you walk through a forested area, you see many
different kinds of plants and animals. Your grandparents
and great-grandparents may have walked through the same
forested area and saw many of the same plants and
animals; however, that area may not have always been a
forest. It may have been a meadow or even a shallow lake.
Succession
• Succession is a regular pattern of changes over time in the
types of species in a community.
• May take hundreds or thousands of years.
• As each new community arises, the previous one dies.
ex: Tall pines trees shade the ground. Pine seedlings need
light but the taller trees block the sunlight. Maples
and oaks can grow in less light, so they replace the
pine trees.
• The community that eventually forms if the land is left
undisturbed is called the climax community.
Secondary Succession
•
•
•
In 1980, Mount St. Helens
(Washington State) erupted,
burning and flattening
18,000 hectares (about 44,
460 acres) of land.
Today, if you were to visit,
the forest has already begun
to regenerate.
Succession that occurs on a
surface where an ecosystem
has previously existed is
call secondary succession.
Primary Succession
•
•
•
•
Succession that occurs on surfaces
where no ecosystem existed before
is called primary succession.
Occurs on new islands created by
volcanic eruptions and in areas
exposed when a glacier retreats.
Primary succession is much slower
to progress than secondary
succession because there is no soil.
It takes several hundred to several
thousand years to produce fertile
soil naturally.
After soil forms, seeds of small
plants are able to germinate and
grow.