Biology 12

MINI LAB

Where Do You Fit in

the Food Chain?

How might your knowledge of pyramids of productivity
influence your decision about the type of foods you include
in your diet? Recall that only a very small fraction of the
energy released by the Sun is assimilated into plant material
(see Figure 13.17 on page 444). For ease of calculation,
assume that the amount of energy captured by plants and
contained in their tissues is two percent of the total energy
available from sunlight. Additionally (although it is a
simplification), assume that 10 percent of the energy at one
trophic level is transferred to the next level. Study the
diagram and determine the percentage of the Sun’s energy
available to humans (as shown at the top of each of the
three food chains).

Analyze

1.About 80 percent of the world’s population eat mostly

grain-based foods. Why do you think this is the case?

2.How might diet influence the number of humans Earth

can ultimately support?

3.A square metre of land planted with rice produces

about 5200 kJ of energy per year. A chicken farm

produces about 800 kJ/m^2 of potential food energy

per year. Assume that a human must consume 2400 kJ

per day to survive. Although it is an oversimplification to

imply that a person could survive by eating only one

type of food, calculate the total area of land needed to

support the student population of your school for one

year on a diet of:

(a)rice (b)chicken

4.Research the differences between the food used to

feed chickens and other poultry in small family-run

farms and the type of feed used in large, commercial

agribusiness operations. Which do you think is more

environmentally friendly?

Chapter 13 Ecological Principles • MHR 447

Since progressively less energy is transferred from

lower to higher levels in a food web, less biomass

can be produced at the higher trophic levels. This

concept can be represented in a biomass pyramid,

in which each tier represents the biomass of that

trophic level (see Figure 13.22A on the following

page). Typically, the shape of a biomass pyramid is

similar to that of a pyramid of productivity. However,

in some aquatic ecosystems, a relatively low biomass

of primary producers (called phytoplankton)

supports a higher biomass of primary consumers

(zooplankton), as shown in Figure 13.22B. This

Meat is more easily digested than most plant materials, so

carnivores are slightly more efficient at converting food into

biomass. However, carnivores typically use up more of this

biomass during their own cellular respiration because their

energy needs are higher than the energy needs of herbivores.

Carnivores tend to move around more to find food, and

many of them are endothermic.

BIO FACT

phytoplankton zooplankton copepods herring tuna

grain beef

grain humans

Sun

Sun

Sun

2% 0.2%

2% 0.2%

2% of

original

energy

Eighty percent of humans have a diet mostly of grain...

...but many Canadians, like Americans

and Europeans, also consume much

meat and fish.

0.02% 0.002% 0.0002%

humans

humans

Three typical food chains for humans with different diets.