Biology 12

442 MHR • Unit 5 Population Dynamics

Food Chains and Webs

The trophic structure determines the route taken by

the energy and matter (chemical elements) contained

in food as it moves through an ecosystem. Food is

transferred from primary producers to primary

consumers and then to secondary consumers along a

pathway referred to as a food chain(see Figure 13.15).

In reality, few ecosystems are so simple that they

consist of only a single, unbranched food chain.

More commonly, many species may feed on a

single species below themselves, while at the same

time a single upper-level consumer species may eat

many different species below itself. In addition,

organisms may eat individuals from two or more

levels. The hawk shown in Figure 13.15 may eat

mice, grasshoppers, and snakes, while the mouse

may eat plant parts (fruits and seeds) and

grasshoppers. In fact, many organisms, including

humans, are referred to as omnivores because they

eat plants, animals, and other types of organisms.

In other words, they are both primary and higher-

level consumers. The result is that the feeding

relationships in most ecosystems form complex

food webs, rather than simple food chains.

Figure 13.16 illustrates a food web.

In the Thinking Lab on page 441, you explored

how communities are structured and the specific

roles individuals can take within a community.

The next section will show you how energy flows

through ecosystems.

Figure 13.15Terrestrial and aquatic communities contain

different species, but can have the same overall trophic

structure. “Plankton” is a general term referring to small

aquatic animals and protists. What distinguishes the two

types of plankton?

Quaternary

consumers

Tertiary

consumers

Secondary

consumers

Primary

consumers

Primary

producers

Terrestrial food chain Marine food chain

carnivore

plant

herbivore

carnivore

carnivore

carnivore

phytoplankton

zooplankton

carnivore

carnivore

Your Electronic Learning Partner has animation clips

that will enhance your understanding of deep-sea vent

communities.

ELECTRONIC LEARNING PARTNER

Some types of bacteria use energy derived from breaking

the chemical bonds in hydrogen sulfide molecules to form

the organic molecules they use as food. This type of

autotrophic food production is called chemosynthesis. It is

quite rare and occurs in some very unusual environments,

including ocean depths of 2500 m. At these depths there

is no light and very little oxygen. In addition, hot magma

from Earth’s molten core escapes to superheat the

surrounding water.

Some unusual marine organisms live near undersea

vents off the coast of British Columbia. These organisms

include tube worms, limpets, and palm worms.

BIO FACT