Biology 12

448 MHR • Unit 5 Population Dynamics

occurs because the phytoplankton are eaten so

quickly that there is no time for a large population

to develop. The population of zooplankton can

only exist because the phytoplankton have an

extremely high reproductive rate — new organisms

appear as fast as others are eaten. This means that

the productivity of phytoplankton is very high, and

the pyramid of productivity for this ecosystem is

therefore wide at the top and narrow at the bottom.

In every ecosystem, the biomass of carnivores at

the highest trophic level is very limited. Only a

tiny fraction of the chemical energy captured by

photosynthesis flows all the way through a food

web to a tertiary or higher-level consumer. Thus,

most food webs are limited to five or fewer trophic

levels — there is just not enough energy left to

support more levels.

Figure 13.22The biomass pyramid in (A) is based on data

collected from a Florida bog. In the English Channel

ecosystem, the pyramid of biomass is inverted (B).

Animals that make up the highest trophic level

in an ecosystem tend to be large, predatory species

such as lions, whales, hawks, and eagles. Since

biomass is limited at the top of the pyramid, there

can only be a few of these large animals in any

ecosystem at one time. In fact, when you compare

the number of individual organisms at each trophic

level, you will find that the same pyramidal shape

appears. The pyramid of numbersin Figure 13.23A

shows the effect of the decreasing energy supply on

the number of individuals at each level. Although

the supply of solar energy is almost limitless,

almost all of the energy is eventually lost from

ecosystems as a result of inefficient transfers

between trophic levels.

Not all pyramids of numbers have this shape.

In a forest, for example, a few individual primary

producers (trees) have enough biomass to support

a large population of herbivores. As is true for the

pyramid of biomass in some aquatic ecosystems,

this could result in a pyramid of a different shape

(see Figure 13.23B).

Completing the Thinking Lab on the following

page will demonstrate that it is not only energy

that can be passed through a food web. Certain

toxic compounds can cause serious damage to

species in an ecosystem when those compounds

are passed from one trophic level to another.

In the next section, you will learn that nutrients

are essential for proper growth and the repair of

body tissue, and that they also cycle in ecosystems.

Trophic level

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

Primary consumers

(zooplankton)

Primary producers

(phytoplankton)

1.5

11

37

809

21

4

Dry weight

(g/ )m^2

B

A Florida bog

English Channel

Tertiary

consumers

Secondary

consumers

Primary

consumers

Primary

producers

1 000 000 J of sunlight

10 000 J

1000 J

100 J

10 J

Figure 13.21This figure is drawn

to show a 10 percent efficiency of

energy transfer from one trophic

level to the next. Although the rate

of efficiency varies (from 5 to

20 percent), 10 percent is a

commonly used average figure.

This gives rise to what is

sometimes called “the rule of 10”

when describing the shape of this

pyramid.