Inspire Biology

Kangaroo rat

(seed eater)

Pronghorn

Gambel’s quail

Jackrabbit

Long-tailed weasel

Ridgenose

rattlesnake

Tortoise

Chihuahuan

raven

Prickly pear cactus

Roadrunner

Mexican whiptail

lizard

Coyote

(top carnivore)

Food webs

Feeding relationships usually are more complex than a single food chain because most

organisms feed on more than one species. Birds, for instance, eat a variety of seeds, fruits,

and insects. The model most often used to represent the feeding relationships in an ecosys-

tem is a food web. A

food web

is a model representing the many interconnected food chains

and pathways in which energy flows through a group of organisms.

Figure 15

shows a food

web illustrating the feeding relationships in a desert community.

Ecological pyramids

Another model that ecologists use to show how energy flows through ecosystems is the

ecological pyramid. An ecological pyramid is a diagram that can show the relative

amounts of energy, biomass, or numbers of organisms at each trophic level in an

ecosystem.

Notice in

Figure 16,

on the next page,

that in a pyramid of energy, only 10 percent of all

energy is transferred to the level above it. This occurs because most of the energy

contained in the organisms at each level is consumed by cellular processes or released

to the environment as heat. Usually, the amount of

biomass

—the total mass of living

matter at each trophic level—decreases at each trophic level.

Figure 15

A food web is a model of the many ways in which energy flows through organisms.

Lesson 2 • Flow of Energy in an Ecosystem