Biology 12

THINKING LAB

Biological Magnification

Background

Certain toxic compounds are not easily broken down by
decomposers, so they remain in the water or soil for long
periods of time. This increases the probability that these
compounds will be ingested by small organisms — which
are then eaten by increasingly larger organisms and passed
through food webs. Since each animal on a trophic level
tends to eat many organisms from the level below, the
amount of toxins taken in becomes magnified with each step
up the food chain. These compounds, which accumulate in
the fatty tissues of animals, have been observed to have
diverse (and often harmful) effects on these organisms.
These effects include damage to the nervous and
reproductive systems as well as the production of genetic
mutations leading to various forms of cancer.

One of the first cases in which the phenomenon of
biological magnification was observed involved DDT. DDT is
a powerful insecticide that was used widely during World
War II to control mosquitoes and other insects that
transmitted diseases (such as malaria) to humans. Signs of
damage caused by this chemical started to turn up about
10 years after it was first used. Mounting evidence of its
deleterious effects caused the Canadian government to
restrict the use of DDT after 1969. Study the the table on
the right and the diagram on the following page, and
answer the following questions.

You Try It

1.What is the relationship between an organism’s trophic

level and the concentration of DDT in its body?

2.How might an animal that lives a long distance from an

area sprayed with DDT accumulate the chemical in its

body?

3.Describe the general patterns you find in the data in the

table. Speculate on the possible reasons for the

differences in concentrations of DDT measured among

species and localities and over time.

4.Research the specific types of prey consumed by one

of the bird species listed in the table. Draw a biomass

pyramid involving this species, incorporating trophic

levels. Include the correct quantity of DDT (in ppb) at

each level of your chart.

DDT concentration in the eggs of three species of sea birds

breeding along Canada’s east coast

DDT concentration in eggs (ppb)

Species Year Bay of Fundy Atlantic Ocean

Leach’s storm-petrel

(Oceanodroma

leucorhoa)

(feeds on small

organisms near the

water surface)

Atlantic puffin

(Fratercula arctica)

(feeds on small fish)

double-crested

cormorant

(Phalacrocorax

auritus)

(feeds on larger fish)

1968

1972

1976

1980

1984

1968

1972

1976

1980

1984

1972

1976

1980

1984

no data

6810

1750

1130

1050

no data

2570

1270

1030

740

6510

1490

1910

1070

1460

2480

750

460

400

890

760

590

550

300

2850

2180

1340

1880

Chapter 13 Ecological Principles • MHR 449

Figure 13.23(A) Many ecosystems have a trophic structure
that produces a pyramid of numbers with a broad base.
(B) In some ecosystems, however, there can be fewer
producers than primary consumers.

The pyramid of productivity shows that primary consumers

harvest more of the energy trapped during the process of

photosynthesis (by eating photosynthetic organisms

directly) than secondary consumers (who eat the organisms

who ate the photosynthetic organisms). For humans, it is

far more efficient in terms of obtaining energy to eat grain

directly rather than to eat grain-fed beef. The pyramid of

numbers shows that the biosphere could successfully feed

far more humans if they were herbivores.

BIO FACT

B

A

producer

primary consumer

secondary consumer

tertiary consumer

producer

secondary consumer

tertiary consumer

Deciduous forest

Grassland

top level consumer

primary consumer