Biology Now, 2e

Of Wolves and Trees ■ 365

In predation, one species benefits and the

other is harmed, and predators are defined as

consumers that eat part or all of other organ-

isms. A parasite is a kind of predator that lives

in or on the organism it harms, its host. An

important group of parasites is pathogens, which

cause disease in their hosts. The bacteria that

cause strep throat, tuberculosis, and pneumonia

are pathogens, for example. Many organisms

have evolved mechanisms to avoid being hosts,

such as immune systems to help fight off para-

sitic diseases and infections.

Other types of predators are distinguished

by what they eat. Elk are herbivores, animals

that eat plants. Yellowstone elk feed on the

shoots, saplings, and new branches of woody

plants like aspen, cottonwood, and willow,

especially in winter, when other plants are

scarce. Wolves are carnivores, animals (and,

in rare cases, plants) that kill other animals

for food. Yellowstone wolves predominantly eat

elk, especially in winter, but they also eat deer

and any small mammals they can catch, nota-

bly beaver. Other animals, such as raccoons and

coyotes, eat both animals and plants, so we call

them omnivores. Both raccoons and coyotes

are also scavengers, animals that eat dead or

dying plants and animals. Of the species that

are scavengers, decomposers, such as fungi,

dissolve their food; and detritivores, includ-

ing worms and millipedes, mechanically break

apart and consume their food.

The clownfish gains protection from predators by hiding

within the sea anemone’s stinging tentacles. The clownfish is

not harmed, because a thick mucus covers its body.

The anemone is protected

from grazing predators by the

clownfish and absorbs the

fish’s nutrient-rich excrement.

Figure 20.8

Mutualism: friends in need

Both the clownfish and the sea anemone benefit

from their relationship.

Q1: What might happen to an anemone

without a resident clownfish?

Q2: What would happen to a clownfish that

did not produce a mucous coating?

Q3: In the example of mutualism given in

the text, what would happen if ticks were no

longer able to feed on bison?

The barnacles on a whale’s

snout enjoy a continuous stream

of nutrient-rich water flowing

across them from which they

can feed. The whale is neither

harmed nor helped by these

stowaways.

Figure 20.9

Commensalism: a whale of a ride!

This gray whale’s snout is covered in barnacles.

Q1: How do barnacles benefit from living on a whale?

Q2: Do you think a whale could avoid being colonized by barnacles?

Why or why not?

Q3: Explain why detritivores are considered commensal to the

organisms they consume.