Biology Now, 2e

380 ■ CHAPTER 21 Ecosystems

ECOLOGY

energy from the sun and transform it into fuel

energy. That fuel is passed up the food chain as

one organism eats another. An energy pyramid

represents the amount of energy available to organ-

isms in an ecosystem. Each level of the pyramid

corresponds to a step in a food chain and is called

a trophic level. In the ocean, for instance, phyto-

plankton are on the first trophic level. Zooplank-

ton, larger, multicellular plankton that feed on

phytoplankton, are the second trophic level. Small

fishes such as herring are the third level, and large

fishes such as tuna are the fourth (Figure 21.3).

At each trophic level, a portion of the energy

captured by producers is lost as metabolic heat,

the heat released as a by-product of chemical

reactions within a cell, especially during cellu-

lar respiration. Organisms lose a lot of energy as

metabolic heat, as revealed by the fact that a small

room crowded with people rapidly becomes hot;

that warmth is the result of metabolic heat leav-

ing our bodies. On average, roughly 10 percent of

the energy at one trophic level is transferred to

the next trophic level. The remaining 90 percent

of the energy that is not transferred is either not

consumed (for example, when we eat an apple,

we eat only a small part of the apple tree), is not

taken up by the consumer’s body (for example,

we cannot digest the cellulose contained in the

apple), or is lost as metabolic heat.

Because of this steady loss of heat, energy

flows in only one direction through ecosystems. It

enters Earth’s ecosystems from the sun (in most

cases) and leaves them as metabolic heat. Every

unit of energy captured by producers is eventu-

ally lost from the biotic world as heat. Therefore,

energy cannot be recycled within an ecosystem.

It travels up an energ y pyramid, never down.

In contrast, nutrients—chemical elements

required by living organisms—are recycled and

reused within and across ecosystems. Although

Earth receives a constant stream of light energy

from the sun, our planet does not acquire more

nutrients on a daily basis; rather, a constant

and finite pool of nutrients cycles through the

land, water, and air. If nutrients were not cycled

between organisms and the physical environ-

ment, life on Earth would not exist.

Nutrients pass through the abiotic world, from

rocks and mineral deposits into soil, water, and

air, and then on to the biotic world via absorp-

tion by producers. Once in the biotic world, they

are cycled among consumers for varying lengths

of photosynthesis. Because they are photosyn-

thetic, these water-living organisms inhabit the

top layer of water in the ocean (and almost every

body of freshwater as well), a location that gives

them easy access to sunlight. Thanks to their

ability to photosynthesize using sunlight, phyto-

plankton are primary producers and the central

means through which energy enters the ocean

ecosystem. If overfishing was affecting phyto-

plankton levels, Boyce worried, the whole ocean

ecosystem could be in danger.

Bottom of the Pyramid

Energy and nutrients flow through ecosystems in

distinctive patterns. First, let’s consider the path

of energy. Producers, like phytoplankton, capture

Trophic level Energy (Kilocalories)

Tertiary consumer =

fourth trophic level

10

100

1,000

10,000 Kcal

Secondary consumer =

third trophic level

Primary consumer =

second trophic level

Producer =

first trophic level

Of each 10,000 Kilocalories

of energy captured from

the sun by producers,

primary consumers capture

and store only about 10%.

Only 10% of the energy

captured and stored at

each trophic level is

transferred to the next

trophic level. The

remainder is lost through

metabolic heat and

inefficient consumption.

Figure 21.3

Energy pyramid

The levels of the energy pyramid correspond to steps in a food chain. M

Q1: What percentage of the original 10,000 Kilocalories is available

to a shark that might eat the tuna in this figure?

Q2: What trophic level and term would describe a predator of tuna?

Q3: Give an example of a primary consumer in a terrestrial

environment.