HUMAN BIOLOGY

464 Chapter 24

hoW does energy floW through ecosystems?

• Energy enters ecosystems from outside—usually the sun.
  Energy leaves ecosystems mainly by the loss of metabolic
  heat from organisms.


• Ecological pyramids depict the tiered feeding (trophic) structure
  of ecosystems.


• A biomass pyramid shows the combined weight of organisms
  in each tier. An energy pyramid shows the loss of usable energy
  from the base tier to higher levels.

taKe-Home message

ecological pyramid
Graphic that represents the
energy relationships in an
ecosystem.

primary productivity The
total energy an ecosystem’s
producers store in a given
amount of time.

pyramid has a large energy base at the bottom (Fig-

ure  24.8B) and is always “right-side up.” It gives a more

accurate picture of the ever-diminishing amounts of energy

flowing through the ecosystem’s feeding levels.

energy flow through ecosystems

Figure 24.8 Ecological pyramids show the tiered structure

of ecosystems. A A biomass pyramid. B An energy pyramid.

(© Cengage Learning)

top carnivores

carnivores

herbivores

decomposers = 5,060

383

3,368

20,810

21

producers

B

5

1.5

11

37

809

decomposers

(bacteria, crayfish)

third-level carnivores

(gar, large-mouth bass)

second-level consumers

(fishes, invertebrates)

first-level consumers

(herbivorous fishes,

turtles, invertebrates)

primary producers (algae,

eelgrass, rooted plants)

A

Figure 24.7 This image summarizes satellite data on Earth’s

primary productivity during 2002. Productivity is coded as red

(highest) down through orange, yellow, green, blue, and purple

(lowest). (Image by Reto Stöckli, based on data provided by the MODIS Science Team/

NASA Earth Observatory)

n Energy flows into food webs from an outside source, usually

the sun. Energy leaves mainly when organisms lose heat

that is generated by their metabolism.

n Link to Metabolism 3.13

Producers capture and store energy

In land ecosystems, the usual primary producers are

plants. The rate at which they take in and store energy in

their tissues during a given period of time is called the

ecosystem’s primary productivity. How much energy

actually gets stored in the tissues of plants depends on

how many individual plants live there, and on the balance

between energy the plants trap (by photosynthesis) and

energy they use for their life processes.

Other factors also affect the final amount of stored

energy in an ecosystem at any given time. For example, how

much energy ecosystems trap and store depends partly

on how large the producers are, the availability of min-

eral nutrients, how much sunlight

and rainfall are available during a

growing season, and the tempera-

ture range. The harsher the condi-

tions, the less new plant growth per

season—so the productivity will be

lower. As Figure 24.7 shows,

there are big differences in the

primary productivity in dif-

ferent ecosystems.

Consumers take energy from ecosystems

An ecological pyramid is a way to represent the energy

relationships of an ecosystem. In these pyramids, primary

producers form a base for tiers of consumers above them.

Biomass is the combined weight of an ecosystem’s organ-

isms at each tier. In Figure 24.8A you can see the biomass

pyramid (measured in grams per square meter) for one small

aquatic ecosystem. This kind of biomass pyramid is very

common in nature. There are lots of primary producers

(aquatic plants) and few top carnivores such as large fishes.

Some biomass pyramids have the smallest tier on the

bottom. A pond or the sea is like this. In those ecosystems

primary producers have less biomass (they collectively

weigh less) than consumers feeding on them. The produc-

ers consist of phytoplankton (tiny floating photosynthetic

organisms), which grow and reproduce fast enough to pro-

vide a steady supply of food for a much greater biomass of

zooplankton (small floating animals). Zooplankton in turn

become food for larger animals.

An energy pyramid also shows how usable energy

declines as it flows through an ecosystem. An energy

24.3

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