13.3
Energy Flow in Ecosystems
E X P E C TAT I O N S
Explain how autotrophs sustain ecosystems by supporting higher trophic
levels.
Describe the concept of primary productivity and explain why it varies
among ecosystems.
Use the energy pyramid to explain production, distribution, and use of food
resources.
Using the ecological hierarchy of living things, evaluate how a change in one
population can affect the entire hierarchy.
Relate the pyramid of primary productivity to the pyramids of biomass
and numbers.
All organisms require energy for growth, body
maintenance (such as repairing damage to body
parts), and reproduction, and many species require
energy for locomotion. Energy to support these
activities is released from large, energy-rich organic
molecules during the process of cellular respiration
(or, in a few species, fermentation) and stored in
the form of ATP.
Most primary producers use energy from the Sun
to drive the production of energy-rich molecules in
their own bodies. Consumers subsequently obtain
these molecules by eating either the bodies of
primary producers or the bodies of other consumers.
Therefore, the amount of energy available to an
ecosystem is determined by the amount of energy
captured by the autotrophs.
Every day the Sun bombards Earth with about
1022 joules of solar radiation. The majority of this
radiation is absorbed, scattered, or reflected by the
atmosphere. Much of the radiation that does reach
the biosphere hits bare ground or water, with only a
small fraction landing on photosynthetic organisms.
Of this, only a portion is of a wavelength suitable
for photosynthesis. The result is that only one to
two percent of the total radiation emitted by the
Sun is converted into chemical energy. The amount
varies somewhat from place to place, depending on
the type of organisms found in each region, the
intensity of the light (recall Figure 13.9 on page 434),
and many other factors (see Figure 13.17).
Even though the fraction of solar radiation
actually captured by primary producers is very
small, these organisms produce between 150 and
200 billion tonnes of organic material each year.
This amount supports the majority of life on Earth.
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100% of solar
radiation reaching
Earth
reflected back by
clouds, dust, and
Earth’s surface
34%
23%
drives the water
cycle
42%
absorbed by the
atmosphere and
the surface of
Earth
1% drives air
and water
currents
1 to 2% captured
by photosynthesis
Figure 13.17 Very little of the total amount of radiation
leaving the Sun is converted to chemical energy during
photosynthesis.
Primary Productivity
of rainfall the system receives (since water is a
raw material of photosynthesis). In addition,
photosynthetic organisms need certain nutrients,
such as nitrogen, potassium, and phosphorus, to
grow. Even though many of these nutrients are not
directly involved in photosynthesis, they contribute
to limiting the rate of primary productivity by
affecting plant growth.
Primary productivity is the amount of light energy
that autotrophs in an ecosystem convert to chemical
energy (and store in organic compounds) during a
specific period of time. It is commonly measured in
terms of energy per area, per year (J/m2/a). It can
also be expressed as biomass (mass) of vegetation
added to an ecosystem per area, per year (g/m2/a). It
is important to remember that primary productivity
is the rate at which organisms produce new
biomass, which is not the same as the total mass of
all photosynthetic autotrophs present in an area at
one time. For example, a forest has a very large
biomass — the mass of its vegetation is greater
than that of a grassland of equal size. But primary
productivity of the grassland may actually be
higher because animals are constantly eating the
plants and new ones are being produced. Thus,
new mass is being accumulated in the grassland at
a higher rate than in the forest.
The amount of primary productivity (as shown
in Figure 13.18) can vary significantly, both among
ecosystems and within an ecosystem over time.
The rate of productivity depends on many factors,
including the number of autotrophs present in the
ecosystem, the amount of light and heat present
(the process slows during winter), and the amount
Not all solar energy captured by primary producers
is passed on to higher trophic levels. A substantial
portion of the energy captured by producers is used
in their own cellular respiration reactions. Another
large portion is simply never eaten by consumers
— which is why many ecosystems look green.
Only some of the total biomass eaten by
consumers is converted into the body tissues of the
organism that ate it. Figure 13.19 on the following
page shows what happens to the energy a herbivore
(caterpillar) obtains from the plant material it eats.
Approximately half the plant tissue is indigestible,
and the energy from this portion is expelled with
the caterpillar’s feces. Although some of this energy
will be consumed by decomposers and will
continue to be part of the ecosystem, it will not
3
4
5
6
A Percentage (%) of Earth’s
surface area
Figure 13.18 Why do the oceans contribute such a high
proportion of Earth’s total primary productivity when their
average productivity is low compared with that of algal beds
and reefs? Net primary productivity is the total amount of
0
B Average net primary
productivity (g/m2 /a)
2500
2
2000
1
1500
0
1000
65.0
5.2
4.7
3.5
3.3
2.9
2.7
2.4
1.8
1.7
1.6
1.5
1.3
1.0
0.4
0.4
0.3
0.1
0.1
500
open ocean
continental shelf
extreme desert, rock, sand, or ice
desert and semi-desert scrub
tropical rain forest
savanna
cultivated land
boreal forest (taiga)
temperate grassland
woodland and shrubland
tundra
tropical seasonal forest
temperate deciduous forest
temperate evergreen forest
swamp and marsh
land and stream
estuary
algal beds and reefs
upwelling zones
Energy Transfer at
Higher Trophic Levels
0
5
10
15
20
25
C Percentage (%) of Earth’s
net primary productivity
solar energy transformed to chemical energy by autotrophs
(called gross primary productivity), minus the amount used
by the autotrophs during cellular respiration.
Chapter 13 Ecological Principles • MHR
445
be available to the secondary consumers that eat
caterpillars. Approximately one third of the energy
the caterpillar obtains is used in its own cellular
respiration (providing energy for locomotion,
maintaining body temperature, and other body
processes), and therefore lost to the ecosystem.
In fact, only about one sixth of the energy is
incorporated into new caterpillar tissue — tissue
that can be eaten by secondary consumers.
This energy loss (and its related unusable heat)
occurs between all trophic levels in a food web (see
Figure 13.20). Although the efficiency with which
energy is transferred from one level to the next
varies among different types of organisms, it
usually ranges between 5 and 20 percent. In other
words, roughly 80 to 95 percent of the potential
energy available at one trophic level is not
transferred to the next one. This pattern of energy
loss is often illustrated as a pyramid of
productivity (see Figure 13.21 on page 448).
100 J feces
200 J
plant material eaten
by caterpillar
33 J
cellular
respiration
67 J
growth
Figure 13.19 Why is so much energy lost as waste from a
caterpillar?
The next MiniLab will help you understand
pyramids of productivity by examining three
typical food chains.
heat (energy) loss to environment
decomposers
100% energy to environment
loss = 23.8%
loss = 76.2%
5.5%
loss = 23.5%
loss = 71%
carnivores
11.4%
loss =
21.4%
respiratory heat loss
organic wastes/dead tissue
top carnivores
loss =
67.2%
herbivores
loss =
20.4%
loss = 63.4%
16.2%
producers
photosynthesis
Figure 13.20 Why is the
respiratory heat loss of
consumers higher than
that of producers? Note
that it is highest for
top-level carnivores.
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MHR • Unit 5 Population Dynamics
1–2% solar energy
Earth
Sun
Since progressively less energy is transferred from
lower to higher levels in a food web, less biomass
can be produced at the higher trophic levels. This
concept can be represented in a biomass pyramid,
in which each tier represents the biomass of that
trophic level (see Figure 13.22A on the following
page). Typically, the shape of a biomass pyramid is
similar to that of a pyramid of productivity. However,
in some aquatic ecosystems, a relatively low biomass
of primary producers (called phytoplankton)
supports a higher biomass of primary consumers
(zooplankton), as shown in Figure 13.22B. This
BIO FACT
Meat is more easily digested than most plant materials, so
carnivores are slightly more efficient at converting food into
biomass. However, carnivores typically use up more of this
biomass during their own cellular respiration because their
energy needs are higher than the energy needs of herbivores.
Carnivores tend to move around more to find food, and
many of them are endothermic.
MINI
LAB
Where Do You Fit in
the Food Chain?
2. How might diet influence the number of humans Earth
can ultimately support?
How might your knowledge of pyramids of productivity
influence your decision about the type of foods you include
in your diet? Recall that only a very small fraction of the
energy released by the Sun is assimilated into plant material
(see Figure 13.17 on page 444). For ease of calculation,
assume that the amount of energy captured by plants and
contained in their tissues is two percent of the total energy
available from sunlight. Additionally (although it is a
simplification), assume that 10 percent of the energy at one
trophic level is transferred to the next level. Study the
diagram and determine the percentage of the Sun’s energy
available to humans (as shown at the top of each of the
three food chains).
3. A square metre of land planted with rice produces
about 5200 kJ of energy per year. A chicken farm
produces about 800 kJ/m2 of potential food energy
per year. Assume that a human must consume 2400 kJ
per day to survive. Although it is an oversimplification to
imply that a person could survive by eating only one
type of food, calculate the total area of land needed to
support the student population of your school for one
year on a diet of:
Analyze
1. About 80 percent of the world’s population eat mostly
grain-based foods. Why do you think this is the case?
(a) rice
(b) chicken
4. Research the differences between the food used to
feed chickens and other poultry in small family-run
farms and the type of feed used in large, commercial
agribusiness operations. Which do you think is more
environmentally friendly?
2% of
original
energy
Eighty percent of humans have a diet mostly of grain…
Sun
grain
humans
2%
…but many Canadians, like Americans
and Europeans, also consume much
meat and fish.
0.2%
Sun
grain
beef
2%
0.02%
0.2%
Sun
phytoplankton
humans
zooplankton
copepods
0.002%
herring
0.0002%
tuna
humans
Three typical food chains for humans with different diets.
Chapter 13 Ecological Principles • MHR
447
Tertiary
consumers
10 J
Secondary
consumers
100 J
Primary
consumers
1000 J
Primary
producers
10 000 J
1 000 000 J of sunlight
occurs because the phytoplankton are eaten so
quickly that there is no time for a large population
to develop. The population of zooplankton can
only exist because the phytoplankton have an
extremely high reproductive rate — new organisms
appear as fast as others are eaten. This means that
the productivity of phytoplankton is very high, and
the pyramid of productivity for this ecosystem is
therefore wide at the top and narrow at the bottom.
In every ecosystem, the biomass of carnivores at
the highest trophic level is very limited. Only a
tiny fraction of the chemical energy captured by
photosynthesis flows all the way through a food
web to a tertiary or higher-level consumer. Thus,
most food webs are limited to five or fewer trophic
levels — there is just not enough energy left to
support more levels.
Trophic level
Dry weight
(g/m2 )
1.5
11
37
809
Tertiary consumers
Secondary consumers
Primary consumers
Primary producers
A Florida bog
21
4
Primary consumers
(zooplankton)
Primary producers
(phytoplankton)
B English Channel
Figure 13.22 The biomass pyramid in (A) is based on data
collected from a Florida bog. In the English Channel
ecosystem, the pyramid of biomass is inverted (B).
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MHR • Unit 5 Population Dynamics
Figure 13.21 This figure is drawn
to show a 10 percent efficiency of
energy transfer from one trophic
level to the next. Although the rate
of efficiency varies (from 5 to
20 percent), 10 percent is a
commonly used average figure.
This gives rise to what is
sometimes called “the rule of 10”
when describing the shape of this
pyramid.
Animals that make up the highest trophic level
in an ecosystem tend to be large, predatory species
such as lions, whales, hawks, and eagles. Since
biomass is limited at the top of the pyramid, there
can only be a few of these large animals in any
ecosystem at one time. In fact, when you compare
the number of individual organisms at each trophic
level, you will find that the same pyramidal shape
appears. The pyramid of numbers in Figure 13.23A
shows the effect of the decreasing energy supply on
the number of individuals at each level. Although
the supply of solar energy is almost limitless,
almost all of the energy is eventually lost from
ecosystems as a result of inefficient transfers
between trophic levels.
Not all pyramids of numbers have this shape.
In a forest, for example, a few individual primary
producers (trees) have enough biomass to support
a large population of herbivores. As is true for the
pyramid of biomass in some aquatic ecosystems,
this could result in a pyramid of a different shape
(see Figure 13.23B).
Completing the Thinking Lab on the following
page will demonstrate that it is not only energy
that can be passed through a food web. Certain
toxic compounds can cause serious damage to
species in an ecosystem when those compounds
are passed from one trophic level to another.
In the next section, you will learn that nutrients
are essential for proper growth and the repair of
body tissue, and that they also cycle in ecosystems.
Grassland
top level consumer
tertiary consumer
secondary consumer
primary consumer
producer
A
Deciduous forest
tertiary consumer
secondary consumer
primary consumer
producer
B
BIO FACT
The pyramid of productivity shows that primary consumers
harvest more of the energy trapped during the process of
photosynthesis (by eating photosynthetic organisms
directly) than secondary consumers (who eat the organisms
who ate the photosynthetic organisms). For humans, it is
far more efficient in terms of obtaining energy to eat grain
directly rather than to eat grain-fed beef. The pyramid of
numbers shows that the biosphere could successfully feed
far more humans if they were herbivores.
Figure 13.23 (A) Many ecosystems have a trophic structure
that produces a pyramid of numbers with a broad base.
(B) In some ecosystems, however, there can be fewer
producers than primary consumers.
THINKING
LAB
Biological Magnification
Background
Certain toxic compounds are not easily broken down by
decomposers, so they remain in the water or soil for long
periods of time. This increases the probability that these
compounds will be ingested by small organisms — which
are then eaten by increasingly larger organisms and passed
through food webs. Since each animal on a trophic level
tends to eat many organisms from the level below, the
amount of toxins taken in becomes magnified with each step
up the food chain. These compounds, which accumulate in
the fatty tissues of animals, have been observed to have
diverse (and often harmful) effects on these organisms.
These effects include damage to the nervous and
reproductive systems as well as the production of genetic
mutations leading to various forms of cancer.
One of the first cases in which the phenomenon of
biological magnification was observed involved DDT. DDT is
a powerful insecticide that was used widely during World
War II to control mosquitoes and other insects that
transmitted diseases (such as malaria) to humans. Signs of
damage caused by this chemical started to turn up about
10 years after it was first used. Mounting evidence of its
deleterious effects caused the Canadian government to
restrict the use of DDT after 1969. Study the the table on
the right and the diagram on the following page, and
answer the following questions.
You Try It
1. What is the relationship between an organism’s trophic
level and the concentration of DDT in its body?
2. How might an animal that lives a long distance from an
area sprayed with DDT accumulate the chemical in its
body?
3. Describe the general patterns you find in the data in the
table. Speculate on the possible reasons for the
differences in concentrations of DDT measured among
species and localities and over time.
4. Research the specific types of prey consumed by one
of the bird species listed in the table. Draw a biomass
pyramid involving this species, incorporating trophic
levels. Include the correct quantity of DDT (in ppb) at
each level of your chart.
DDT concentration in eggs (ppb)
Species
Year
Bay of Fundy
Atlantic Ocean
Leach’s storm-petrel
(Oceanodroma
leucorhoa)
(feeds on small
organisms near the
water surface)
1968
no data
1460
1972
6810
2480
1976
1750
750
1980
1130
460
1984
1050
400
1968
no data
890
1972
2570
760
1976
1270
590
1980
1030
550
1984
740
300
1972
6510
2850
1976
1490
2180
1980
1910
1340
1984
1070
1880
Atlantic puffin
(Fratercula arctica)
(feeds on small fish)
double-crested
cormorant
(Phalacrocorax
auritus)
(feeds on larger fish)
DDT concentration in the eggs of three species of sea birds
breeding along Canada’s east coast
Chapter 13 Ecological Principles • MHR
449
sea birds
2 800 ppb
fish
43 ppb
dolphins
5 200 ppb
squid
22 ppb
plankton
1.7 ppb
seawater
0.0001 ppb
Figure 13.24 An example of data collected on DDT in a food web of organisms in
the north Pacific Ocean. Concentration of DDT is measured in parts per billion
(ppb). The arrows indicate the flow of energy from one trophic level to another.
SECTION
450
REVIEW
1.
K/U Explain why the primary productivity of a
grassland usually exceeds the primary productivity of
a forest ecosystem covering an area of the same size.
2.
Identify the factors that might cause annual
fluctuations in the primary productivity of a specific
grassland ecosystem.
3.
On average, less than 20 percent of the food
energy consumed by a grasshopper is converted into
new grasshopper tissue. What happens to the rest of
the food energy that was contained in the tissues of
the producer species consumed by the grasshopper?
4.
Explain why many models showing the
relationships among trophic levels in an ecosystem
are shown in the shape of pyramids. Describe the
difference between the information depicted in a
biomass pyramid, a pyramid of productivity, and
a pyramid of numbers.
5.
K/U Explain why the primary productivity levels of
equatorial ecosystems generally exceed those of
ecosystems situated farther north.
6.
K/U Explain why the energy transfer from herbivores
to carnivores is more efficient than the energy transfer
from producers to herbivores within the same food
chain.
7.
K/U Suppose that the biomass for primary
producers is less than the biomass for primary
consumers. Can this ecosystem survive? Explain
your answer.
8.
Describe the factors that determine the shape
of a pyramid of numbers diagram for a specific type
of ecosystem. Explain why a grassland pyramid
would have a wider base than a forest pyramid.
9.
MC We have seen that there are fewer carnivores
than herbivores in ecosystems because of the
inefficiency of energy transfer between trophic levels,
and that the world could support more people if we
ate only plant material. Some people feel this means
that humans should switch to a vegetarian diet;
others disagree. There are, in fact, a variety of issues
to consider in addition to the relatively simple one of
energy transfer. Take a stand. Prepare your arguments
carefully and be prepared to debate the issue in
class. You might want to prepare a pamphlet that
could be used to educate the public (or the rest of
the class) about your point of view.
K/U
K/U
K/U
MHR • Unit 5 Population Dynamics
K/U