HUMAN BIOLOGY

462 Chapter 24

consumers The organisms
in an ecosystem that eat
producers (plants and other
photosynthesizers).

food chain Linear
sequence of who eats
whom in an ecosystem.

food web Network of inter-
linked food chains.

producers Photosynthetic
organisms that capture
energy and use it to make
organic compounds.

n Although there are many different types of ecosystems, they

are all alike in many aspects of their structure and function.

Nearly every ecosystem runs on

energy from the sun. Plants and

other photosynthetic organisms are

producers (or autotrophs, which

means “self-feeders”). They capture

sunlight energy and use it to build

organic compounds from inorganic

raw materials (Figure 24.4).

All other organisms in an eco-

system are consumers (or hetero‑

trophs, “other-feeders”). One way or

another, consumers take in energy

that has been stored in the tissues

of producers. For the most part, consumers fall into

four categories:

Herbivores, such as grazers and insects, eat plants; they

are primary consumers.

Carnivores, such as lions, eat animals. Carnivores are

secondary or tertiary (third-level) consumers.

Omnivores, such as humans, dogs, and grizzly bears,

feed on a variety of foods, either plant or animal.

Decomposers, such as fungi, bacteria, and worms, get

energy from the remains or products of organisms.

Producers obtain an ecosystem’s nutrients and its initial

pool of energy. As they grow, they take up water and

carbon dioxide (which provide oxygen, carbon, and hydro-

gen), as well as minerals such as nitrogen. These materials,

recall, are the building blocks for biological molecules.

When decomposers get their turn at this organic matter,

they can break it down to inorganic bits. If those bits are

not washed away or otherwise removed from the system,

producers can reuse them as nutrients.

It is important to remember that ecosystems must have

an ongoing input of energy; in most cases, this energy

source is the sun. Ecosystems often depend on outside

sources of nutrients as well, as when erosion carries miner-

als into a lake. Ecosystems also lose energy and nutrients.

Most of the energy that producers capture eventually is

lost to the environment in the form of metabolic heat.

Though nutrients generally are recycled, some also are

lost, as when minerals are leached out of soil by water

seeping down through it.

energy moves through

a series of ecosystem

feeding levels

Each species in an ecosystem has its

own position in a hierarchy of feeding

levels (also called trophic levels; troph‑

means “nourishment”). A key factor

in how any ecosystem functions is the

transfer of energy from one of its feed-

ing levels to another.

Primary producers, which gain

energy directly from sunlight, make

up the first feeding level. Corn plants

in a field or waterlilies in a pond

are examples. Snails and other herbi-

vores that feed on the producers are

at the next feeding level. Birds and

other primary carnivores that prey

on the herbivores form a third level.

A hawk that eats a snake is a second-

ary carnivore. Decomposers, humans,

and many other organisms can obtain

energy from more than one source.

For this reason they can’t be assigned

to a single feeding level.

24.2 Feeding Levels and Food Webs

Upland

Sandpiper

Garter

Snake

Cutworm

Plants

Marsh

Hawk

A simple food chain

Figure 24.5 Animated!

A food chain is a simple

hierarchy of feeding levels.

(© Cengage Learning)

Figure 24.4 Animated! In ecosystems, there is a flow

of energy and cycling of materials. A Energy from the

environment flows through producers, then consumers.

All energy that entered this ecosystem eventually flows

out of it, mainly as heat. B Producers and consumers

concentrate nutrients in their tissues. Some nutrients

released by decomposition get cycled back to producers.

Producers

plants and other

self-feeding organisms

BNutrients that

become incorporated

into the cells of producers

and consumers are

eventually released by

decomposition. Some

cycle back to

producers.

AProducers harvest energy from

the environment. Some of that energy

flows from producers to consumers.

CAll of the energy that enters the world of life

eventually flows out of it, mainly as heat released

back to the environment.

sunlight

energy

Consumers

animals, most fungi,

many protists, bacteria

© Cengage Learning

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).