Page 109 - One Million Things: Animal Life - The Incredible Visual Guide

HOW A FOOD WEB WORKS
A food web shows at a glance what eats what in an
ecosystem. In each of the food chains that make up
the web, arrows show the direction in which energy
flows when one organism consumes another. At each
step in a chain, energy is lost. Therefore, less energy is
passed on to build and run an animal in the next level.

PRODUCERS
All food webs start with producers—living things that
1
use sunlight energy to make food by a process called
4 photosynthesis. In the case of this Antarctic food web,
the producers are microscopic phytoplankton—
Elephant seal
plantlike organisms that float in the well-lit surface
waters. Producers provide the energy that supports all
the other species in the food web.

PRIMARY CONSUMERS
Unlike producers, primary consumers cannot make
2
their own food. Instead, they survive by eating the
producers, in this case the phytoplankton. Antarctic
primary consumers include zooplankton (masses of

tiny animals) and krill.
3
Squid
SECONDARY CONSUMERS
Crabeater seals, despite their name, feed almost
3
exclusively on krill that they filter from the water using
their unusually shaped teeth. Along with penguins,
ice-tolerant fish, and squid they are secondary
consumers—animals that feed on primary consumers.
However, the categories in a food web are only for
guidance: consumers often belong to more than one
food chain, and occupy different levels in each.

TERTIARY CONSUMERS
In each food chain, an animal that is eaten passes on
4
only around 10 percent of the energy it received from
the animals it ate. The rest is used for movement and
maintaining its body or it is lost as heat. Each level,
therefore, supports fewer individuals than the one
Zooplankton Copepods are small before. At tertiary consumer level these are the

form a key part of
2 crustaceans that elephant seals.
the zooplankton
5 TOP PREDATORS
Leopard seals and killer whales (orcas) are the top
predators and consumers in this Antarctic food
chain. They are the equivalent of lions in the
African savanna, hunting a wide range of prey.
They have no natural predators—although
orcas will eat leopard seals—so this marks
the upper limit of the food web.

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