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Lesson 7: Ecology (2.1.1)
An ecosystem is the relationship of living organisms and the environment in
which they live. Biotic factors include all living things, such as birds,
insects, trees, flowers, fungus, and bacteria. Abiotic factors are those
components of the ecosystem that are not living such as soil, water,
temperature, the amount of light, and weather.
THE FLOW OF ENERGY THROUGH AN ECOSYSTEM
Matter within an ecosystem is constantly recycled. Elements, chemical compounds, and other
sources of matter pass from one state to another through an ecosystem. For example, as a rabbit
eats grass, the nutrients contained in the grass are broken down into the chemical components
and then rearranged to
become tissues in the
rabbit and necessary
components for life and its
survival. When a snake
eats the rabbit, the
nutrients in the rabbits are
broken down into
chemical components and
rearranged in the snake,
just as the rabbit rearranged the nutrients from the grass. Waste products in the animals are
released back into the ecosystem and broken down into simpler chemical compounds. Plants
growing nearby take up these nutrients and rearrange them back into plant tissues and
compounds needed for survival. Thus the energy cycle begins again.
FOOD CHAINS
A food chain in an ecosystem is made up of producers, consumers,
and decomposers. Producers are organisms, such as plants, that use
abiotic (not living) factors to obtain, store, and use energy. Consumers
cannot make their own food and must eat other organisms in order to get
energy. Each time an organism is eaten, energy is passed to the
consumer. Deer, foxes, rabbits, raccoons, owls, hawks, snakes, mice,
spiders, and insects are all examples of consumers in a forest ecosystem.
Decomposers break down dead organisms into simpler chemical
compounds. Saprophytes are fungi (such as mushrooms) and bacteria
that act as decomposers. In a food chain, energy flows from one
organism to another; it is a single step in the energy transfer process of
an ecosystem.
FOOD WEBS
Energy transfer in an ecosystem is not as simple as a food chain. Food webs are used to
illustrate competition amongst organisms and the interaction between food chains. Sometimes
paths cross directly, and sometimes they do not. A food web shows how the presence of any one
species nearly always affects other organisms. Look at the food web in Figure 7.4. What impact
would a disease in the rabbit population have on the other organisms in the community? A
disease in the rabbit population would probably cause a number of rabbits to die, decreasing their
population size. Because snakes and
owls eat the rabbits, competition between
the wolves, snakes and owls would
increase. More of the wolves, snakes and
owls would not be able to find food and
die of starvation, thus decreasing their
overall population size as well. Because
the foxes eat snakes, and the snake
population has decreased, there would be
less food for the foxes. The fox
population would also decrease. In
effect, a decrease in the rabbit population
would most likely cause the populations
of its predators to also decrease.
Assume the rabbit population decreases as in the above example. What would happen to the
grass in this ecosystem? If there are less rabbits, then less grass is being eaten. We would
expect to see the amount of grass increase in this ecosystem. As the grass increases, there is
more food available for the rabbits to eat. An increase in food supply would mean less rabbits
might die of starvation, and their population size would most likely increase. How would an
increase in the rabbit population affect the rest of the ecosystem? More rabbits equal more food
available to the snakes, owls, wolves, and foxes. More snakes, owls, wolves, and foxes will be
able to eat and have the energy needed for survival. Thus their population sizes will increase
too.
As you can see, population sizes in an ecosystem depend on the other organisms in that
ecosystem. Populations fluctuate in size in response to other populations. As the rabbit
population decreased, the snake, owl, wolf, and fox populations also decreased. As the rabbit
population increased, so did the snake, owl, wolf, and fox. This cycle of fluctuation helps keep
population sizes in check and prevent one population of organisms from reaching carrying
capacity. If a population of rabbits is too large and over carrying capacity, disease, lack of food,
water, and shelter, as well as predation will help to bring the population of rabbits back down to
a size the ecosystem can support.
ENERGY PYARMIDS
A trophic level is the position occupied by an organism in a food chain. Organisms that share a
trophic level get their energy from the same source. Producers are found at the base of the
energy pyramid and make up the first trophic level of
the food chain. Producers capture energy as sunlight and
convert it into usable forms. The second trophic level is
the primary consumers. These are organisms that eat
the producers. The third trophic level in an ecosystem is
the secondary consumers. Secondary consumers
generally eat the primary consumers. The final trophic
level is the tertiary consumers. Tertiary consumers
generally eat the secondary consumers. Sometimes an
animal will eat organisms from multiple trophic levels.
For example, humans are at the top of the food chain and
considered tertiary consumers. Humans may eat lettuce
(producers), cows (primary consumers), and large fish
(secondary consumers). The number of trophic levels is
limited by the amount of available energy for organisms
to survive and reproduce. Look at the energy pyramid in
Figure 7.5.
Notice as you move up the pyramid, the size of each level decreases. This is because the amount
of available energy decreases as you move up the pyramid. The Law of 10% states that only
10% of the energy from one trophic level is passed to the next trophic level. Because producers
make their own food by using the sun’s energy, we can assume the producers have 100% energy.
When the mouse eats the grass, only 10% of the energy from the producers is passed on to the
mouse. The mouse gets 10% of the energy for each grass it eats. What happens to the other 90%
of the energy? The grass uses it for metabolic processes, defense, and reproduction. When the
snake eats the mouse, only 10% of the energy from the mouse is passed onto the snake. The
snake would get 1% energy for every mouse it eats. Again, the other 90% of the energy is used
by the snake for metabolic processes, defense, predation, and reproduction. When the owl eats a
snake, it only gets 10% of the energy from the snake. Thus, the owl would only get 0.1% energy
for every snake it eats.
The amount of energy decreases from a lower trophic level to the next. Therefore, in order to get
enough energy to survive, organisms in higher trophic levels must spend more time hunting for
prey and eating than in lower trophic levels. The number of trophic levels in an ecosystem is
determined by the amount of available energy for predators.
Activity
Create a food chain and a food web using pictures you cut out of magazines or draw yourself.
Show how energy flows through your ecosystem. Disrupt one thing in the food chain and
discuss how this would affect your entire ecosystem.
**This activity is related to DPI’s “Field Study on School Grounds” Activity.
CYCLING OF MATTER
Matter is recycled through plants, animals, and nonliving things in biochemical cycles to
maintain the health and sustainability of an ecosystem. We will discuss the water cycle, carbon
cycle, nitrogen cycle and oxygen cycle to analyze the interdependence of organisms with their
environment.
THE WATER CYCLE
The water cycle circulates fresh water between the atmosphere and the earth. Three quarters of
the earth is covered in water but the overwhelming majority of the water is salt water. Most of
the earth’s fresh water is unusable because it is found in the form of ice in glaciers. Fresh water
is necessary for organisms to carry out essential metabolic processes, and the water cycle helps
keep usable fresh water on earth for organisms to use. Precipitation in the form of rain, sleet,
snow, hail, and dew falls from the atmosphere and ends up in lakes, rivers, streams, and oceans
through the precipitation itself, or runoff. Runoff is the movement of water from higher
elevations to lower elevations. Heat energy from the sun causes evaporation which brings water
into the atmosphere from bodies of water here on earth. Water is moved through wind and ocean
currents as well. Aerobic respiration (C6H12O6 + O2
CO2 + H2O) from many organisms
and transpiration (loss of water from leaves) from plants also puts water back into the
atmosphere in the form of water vapor. When water vapor cools, it forms clouds, which cool to
become saturated and form precipitation. Without the cycle of precipitation, runoff and
evaporation, there would not be a supply of fresh water available on earth. Figure 7.6 shows the
water cycle.
Figure 7.6 The Water Cycle
THE CARBON CYCLE
The carbon cycle is the cycling of carbon between carbon dioxide and organic molecules.
Recall that organic molecules contain carbon, with the exception of carbon dioxide, CO2, which
is inorganic. Carbon dioxide accounts for approximately 0.03% of the atmosphere. Plants use
carbon dioxide and energy from the sun to perform photosynthesis and make glucose, C6H12O6.
When animals eat plants and other animals, carbon passes into their tissues. Through food
chains and food webs, carbon is passed from one organism to the next. Carbon is returned to the
earth through respiration, excretion, and decomposition. Sometimes when an organism dies, it
is not eaten by other organisms and decomposed. Instead, the organism gets buried deep in the
earth and over long periods of time, heat and pressure from the Earth convert the organism into
fossil fuels such as coal, oil, and gas. During the burning of these fossil fuels, carbon dioxide is
released back into the atmosphere.
Figure7.7 The Carbon Cycle
NITROGEN CYCLE
Nitrogen is the largest quantity of gas found in the atmosphere, accounting for approximately
78% of the atmosphere. Nitrogen is an element found in both amino acids and nucleic acids.
However, nitrogen gas from the atmosphere is not in a form that most organisms can use when
making these necessary compounds. Through the nitrogen cycle, nitrogen gas is converted into a
usable form such as ammonia (NH3), nitrite (NO2-), and nitrate (NO3-). Figure 7.8 shows the
nitrogen cycle.
Figure 7.8 The Nitrogen Cycle
Nitrogen gas is converted into nitrate by some types of bacteria through the process of nitrogen
fixation. In nitrogen fixation, nitrogen gas in converted into ammonia by bacteria called
nitrogen fixers. Some plants can use ammonia, but most cannot, and must use nitrate.
Nitrifying bacteria further convert ammonia into nitrite, and nitrite into nitrate. These
nitrogen-fixing bacteria are found in the roots of legumes (pea and bean plants). Nitrogen
fixation increases the amount of usable nitrogen in the soil. Plants can use this nitrogen
(ammonia and nitrate) to make nucleic acids and proteins. Just as carbon is passed between
organisms in the ecosystem through the carbon cycle, nitrogen is also passed from one organism
to the next through food chains. As decomposers break down the remains of dead organisms,
ammonia is released into the soil and often converted into nitrate by nitrifying bacteria. Other
types of bacteria will convert the ammonia from decomposition back into nitrogen gas which is
released back into the atmosphere.
THE OXYGEN CYCLE
In the oxygen cycle, oxygen is recycled from the atmosphere to the earth, and back to the
atmosphere. This cycle is mainly initiated by photosynthesis. Photosynthesis is the process in
which plants use energy from the sun to make glucose (C6H12O6), their main source of food. In
photosynthesis, plants and other photosynthetic organisms, take carbon dioxide from the
atmosphere and combine it with water to make glucose. Oxygen is released a waste gas into the
atmosphere (CO2 + H2O
C6H12O6 + O2). It is thought that early earth’s atmosphere
contained no oxygen, but because of the evolution of photosynthetic organisms, oxygen has been
released in the atmosphere in large quantities. Today’s atmosphere contains approximately 21%
oxygen and is used by a large number of organisms for aerobic cellular respiration. Cellular
respiration is the process in which organisms break down glucose (food) in order to gain the
energy needed for life. The equation for aerobic (requires oxygen) cellular respiration is
C6H12O6 + O2
CO2 + H2O. Oxygen is recycled through the processes of photosynthesis and
cellular respiration. To recap, cellular respiration removes oxygen from the atmosphere while
photosynthesis returns it. Although the atmosphere has 21% oxygen, the majority of the oxygen
is found stored in the earth’s crust and in shells made of calcium carbonate (CaCO3) which are
found in the ocean. Oxygen is also recycled through the processes of erosion and decay.
Figure 7.9 The Oxygen Cycle
Activity
Identify each cycle and describe the benefits of each cycle to living things.
FIELD TECHNIQUES
Scientists often do research on an entire population of organisms or a large forest, for example.
It is not always feasible for the scientist to study every individual in a population of organisms.
For example, what if a population of insects had 100 thousand individual bees? A scientist
would be unable to observe and study every single bee. So, how do scientists study populations
of organisms? Sampling techniques are used to take a random sample of organisms to study. A
large sample size must still be collected to help support that variation occurred by random
chance. A scientist would randomly collect a large sample of bees, mark them, and release the
bees back into the wild. By marking the bee, the scientist will know if he had collected this same
bee during a previous collection day, and not collect data twice from the same organism. This
same concept is used when studying many different living organisms.
One way scientist collect random samples of living organisms is through quadrant sampling.
To determine species diversity, a square
(usually 4 by 4) is constructed and randomly
placed throughout the study area. A scientist
may throw the square, to make where it lands as
random as possible. The scientist would then
count all the organisms that are found in that
square, for example, the number of different
types of grasses. As long a large quadrant
sample is collected, the scientist can estimate
both the number of different grasses and the
percent in which they appear in a forest, without
walking through and searching every square
meter of ground in that forest.
Activity
Over time, an ecosystem goes through a
series of changes known as ecological
succession. There are two types of
succession: primary succession and
secondary succession. Study the diagrams
and describe each type of succession.
Make a poster showing what an
ecosystem would look like before primary
succession, after primary succession, and
after secondary succession.
Lesson 7 Review: The Flow of Energy
A. Define the following terms
food chain
decomposers
biotic
saprophyte
primary consumer
law of 10%
biochemical cycle
water cycle
oxygen cycle
precipitation
respiration
decomposition
nitrifying bacteria
producer
runoff
secondary consumer
food web
tertiary consumer
carbon cycle
transpiration
nitrogen fixation
excretion
trophic level
consumer
energy pyramid
nitrogen cycle
aerobic respiration
nitrogen fixers
abiotic
B. Choose the best answer
1. Which two organisms below share a trophic level?
A. elephants and lions
C. chipmunks and grass
B. cheetahs and giraffes
D. wolves and sparrows
2. The owl is a nocturnal hunter of small mammals, insects and other birds. An owl is an
example of a/an
A. producer
B. omnivore
C. carnivore
d. decomposer
3. Which food would an herbivore always avoid?
A. worms
B. clover
C. pine nuts
D. grass
4. Emperor penguins feed on crustaceans, such as krill. They are prey to orca whales and
leopard seals. What ecological role does the Emperor penguin play? (Hint: Krill are
small animals that feed on tiny plant drifters)
A. producer
C. secondary consumer
B. primary consumer
D. top (tertiary) consumer)
5. Which describes a biochemical cycle?
A. Matter is recycled through plants, animals, and nonliving things
B. Matter is recycled through decomposers, animals, and viruses
C. Matter is recycled through plants, bacteria, and fungi
D. Matter is recycled through protists, animals, and fungi
6. What process is responsible for breaking down organic matter which is no longer living?
A. decomposition
C. respiration
B. excretion
D. photosynthesis
7. What is the main component of organic molecules?
A. phosphorous
C. nitrogen
B. carbon
D. carbon dioxide
8. How do plants use carbon?
A. to make sugar
B. to attract pollinators
C. to make proteins and nucleic acids
D. to transport water to their leaves
9. How is carbon recycled back into the atmosphere?
A. evaporation
C. respiration
B. burning of fossil fuels
D. choices B and C
10. What is the purpose of nitrogen fixing bacteria?
A. to convert ammonia into nitrite and nitrate
B. to convert nitrate into nitrogen gas
C. to convert nitrite into nitrogen gas
D. to convert ammonia into nitrogen gas
11. What is the main goal of the water cycle?
A. to move salt water between the atmosphere and Earth
B. to move dirty water between the atmosphere and Earth
C. to move fresh water between the atmosphere and Earth
D. to move toxic water between the atmosphere and Earth
12. What process contributes to the oxygen cycle by adding oxygen into the atmosphere?
A. cellular respiration
C. nitrogen fixation
B. photosynthesis
D. transpiration
13. What process contributes to the oxygen cycle by removing oxygen from the atmosphere?
A. cellular respiration
C. nitrogen fixation
B. photosynthesis
D. transpiration
14. What processes contribute to moving water through the water cycle?
A. cellular respiration, transpiration, and nitrification
B. photosynthesis, transpiration, and nitrification
C. cellular respiration, transpiration, and photosynthesis
D. transportation, fermentation, weathering
15. Why must organisms higher on an energy pyramid eat more in order to have enough energy
to survive?
A. because they are the largest organisms
B. because of the law of 10%
C. because they get hungry more often
D. because they do the most moving
C. Complete the following exercises.
1. Organisms that obtain food from dead organisms or waste material are
called?____________
2. If there were no decomposers, what might happen to all the dead organic matter on Earth?
3. Describe a sampling technique used to study a large area.