Visualizing Environmental Science

The Flow of Energy Through Ecosystems 103

Simple food chains rarely occur in nature because

few organisms eat just one kind of organism. Plants,

for example, are eaten by a variety of insects, birds, and

mammals; and most of these herbivores are consumed

by several different predators. Thus, the flow of energy

through an ecosystem typically takes place in accordance

with a range of food choices for each organism involved.

In an ecosystem of average complexity, numerous alter-

native pathways are possible. An owl eating a rabbit is

a different energy pathway than an owl eating a snake.

A food web, a complex of interconnected food

chains in an ecosystem, is a more realistic model of

the flow of energy and materials through ecosystems

(Figure 5.6). A food web helps us visualize feeding

relationships that indicate how a community is organized.

The most important thing to remember about energy

flow in ecosystems is that it is linear, or one way. Energy

moves along a food chain or food web from one organ-

ism to the next, as long as it isn’t used for biological work.

The Path of Energy Flow

in Ecosystems

In an ecosystem, energy flow occurs in food chains, in

which energy from food passes from one organism to the

next in a sequence. When a food

chain is diagrammed, it consists of

a series of arrows, each of which

points from the species that is

consumed to the species that uses

it as food ( Figure 5.5). Each level, or “link,” in a food

chain is a trophic level. (The Greek tropho means “nour-

ishment.”) An organism is assigned a trophic level based

on the number of energy transfer steps to that level.

Producers form the first trophic level, primary

consumers form the second trophic level, secondary

consumers form the third trophic level, and so on. At

every step in a food chain are decomposers, which respire

organic molecules in the carcasses and body wastes of all

members of the food chain.

energy flow The

passage of energy in

a one-way direction

through an ecosystem.

PROCESS DIAGRAM

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✓✓THE PLANNER

From which trophic level will

decomposers gain the highest percentage of the sun’s

original energy input? Why?

Think Critically

Energy enters ecosystem

from the sun.

1

Energy exits as

heat loss.

3

Energy flows linearly—in a one-way

direction—through ecosystems.

Decomposers gain energy

from all other trophic levels.

2

Heat Heat Heat Heat Heat

First trophic level:

Producers

Second trophic level:

Primary consumers

Third trophic level:

Secondary consumers

Fourth trophic level:

Energy Tertiary consumers Decomposers

from

sun

Much of the energy acquired by a given level of a

food chain is used and escapes into the surrounding

environment as heat. This energy, as the second law

of thermodynamics stipulates, is unavailable to the

next level of the food chain.