Visualizing Environmental Science

The Flow of Energy Through Ecosystems 105

Once an organism uses energy, it is lost as heat (recall

the second law of thermodynamics) and is unavailable

for any other organism in the ecosystem.

Organisms at each step of a food chain use a large

amount of the potential energy available to them. Be-

cause this energy is ultimately lost into the environment

as heat, the number of steps in any food chain is limited.

The longer the food chain, the less energy is available for

organisms at the higher trophic levels.

An important feature of energy flow is that most of

the energy going from one trophic level to the next in

a food chain or food web dissipates into the environ-

ment as a result of the second law of thermodynamics.

Ecological pyramids often graphically represent the

relative energy values of each trophic level. A pyramid

of numbers shows the number of organisms at each tro-

phic level in a given ecosystem, with greater numbers

illustrated by a larger area for that section of the pyra-

mid (Figure 5.7). In most pyramids of numbers, the

organisms at the base of the food chain are the most

abundant, and fewer organisms occupy each successive

NUMBER OF TROPHIC LEVEL

INDIVIDUALS

Secondary

consumer

(bird of prey)

Primary consumer

(field mouse)

1

10

10,000 Producers(grass)

From Raven, Hassenzahl, Berg:

Environment

, 8e, copyright 2012.

John Wiley and Sons, Inc. This material is reproduced withpermission of John Wiley and Sons, Inc.

Pyramid of numbersÊUÊFigure 5.7

This pyramid is for a hypothetical area of temperate grassland;

in this example, 10,000 grass plants support 10 mice, which

support one bird of prey. (Note that decomposers are not shown.)

trophic level. Another type of ecological pyramid, pyra-

mids of energy, are shaped similarly, illustrating how en-

ergy dissipates into the environment as it moves from

one trophic level to another.

Ecosystem Productivity

The original source of energy in all ecosystems is the sun.

The gross primary productivity (GPP) of an ecosystem is

the rate at which energy is captured during photosynthe-

sis. (Gross and net primary productivities are referred to as

primary because plants occupy the first trophic level in food

webs.) Of course, plants respire to provide energy for their

own use, and this acts as a drain on productivity. Energy in

plant tissues after cellular respiration has occurred is net

primary productivity (NPP). That is, NPP is the amount of

biomass found in excess of that broken down by a plant’s

cellular respiration. Only the energy represented by NPP

is available as food for an ecosystem’s consumers.

Ecosystems differ strikingly in their productivities

(Figure 5.8). On land, tropical rain forests have the

Estimated annual net primary productivities

­ **®ÊvœÀÊÃiiVÌi`ÊiVœÃÞÃÌi“ÃÊUÊFigure 5.8

NPP is expressed as grams of dry matter produced per square

meter per year.

Ecosystem

Algal beds and reefs

Tropical rain forest

Swamp and marsh

Estuaries

Temperate evergreen forest

Temperate deciduous forest

Savanna

Boreal (northern) forest

Woodland and shrubland

Agricultural land

Temperate grassland

Lake and stream

Arctic and alpine tundra

Ocean

Desert and semidesert scrub

Extreme desert (rock, sand, ice)

500

Estimated net primary productivity

(grams/square meter/year)

1000 1500 2000 2500

From Raven, Hassenzahl, Berg:

Environment

, 8e, copyright 2012. John Wiley and Sons,

Inc. This material is reproduced with permission of John Wiley and Sons, Inc.

Use what you know about the

movement of energy in a food web to explain why

there are so many more organisms at the bottom of this

pyramid than at the top.

Think Critically