Interactions Among Organisms 119A parasite, usually much smaller than its host, obtains
nourishment from its host, but although a parasite may
weaken its host, it rarely kills it quickly. (A parasite would
have a difficult life if it kept killing off its hosts!) Some
parasites, such as ticks, live outside the host’s body; other
parasites, such as tapeworms, live within the host.
Predation
Predators kill and feed on other organisms. Preda-
tion includes both animals eating other animals (for
example, herbivore–carnivore
interactions) and animals eat-
ing plants (producer– herbivore
interactions). Predation has re-
sulted in an “arms race,” with
the coevolution of predator
strategies—more efficient ways to catch prey—and
prey strategies— better ways to escape the predator. An
efficient predator exerts a strong selective force on its
prey, and over time the prey species may evolve some
sort of countermeasure that reduces the probability of
its being captured. The countermeasure that the prey
acquires in turn may act as a strong selective force on
the predator.
Adaptations related to predator–prey interactions in-
clude predator strategies (pursuit and ambush) and prey
strategies (animal defenses and plant defenses). Keep
in mind that such strategies are not “chosen” by the re-
spective predators or prey. New traits arise randomly in a
population as a result of mutation and natural selection.
(See Chapter 6 to learn more about how traits evolve.)
The cheetah is the world’s fastest animal and can sprint
at 110 km (68 mi) per hour for short distances ( Figure
5.18a). Orcas (commonly known as killer whales) hunt in
packs and often herd salmon or tuna into a cove so that
they are easier to catch. Any trait that increases hunting ef-
ficiency, such as the speed of a cheetah or the intelligence
of orcas, favors predators that pursue their prey.
Ambush is another effective way to catch prey. The
goldenrod spider is the same color as the white or yel-
low flowers in which it hides ( Figure 5.18b). This cam-
ouflage prevents unwary insects that visit the flower for
nectar from noticing the spider until it is too late.*Ài`>ÌÊUÊ}ÕÀiÊx°£nÊÊÊ
predation The
consumption of one
species (the prey)
by another (the
predator).Chris Johns/NG Image Collection Rich Reid/NG Image Collectiona. The cheetah sprints at high speed to catch prey. Photographed
in the Okavango Delta, Botswana, Africa.
b. The goldenrod spider employs camouflage to ambush
its prey.