Biology 12

13.3 Energy Flow in Ecosystems

444 MHR • Unit 5 Population Dynamics

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.

Figure 13.17Very little of the total amount of radiation

leaving the Sun is converted to chemical energy during

photosynthesis.

absorbed by the

atmosphere and

the surface of

Earth

1 to 2% captured

by photosynthesis

1% drives air

and water

currents

drives the water

cycle

23%

34%

42%

reflected back by

clouds, dust, and

Earth’s surface

100% of solar

radiation reaching

Earth

EXPECTATIONS

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.