Of Wolves and Trees ■ 369Then there are species that, though not
poisonous, evolved coloration to make them
look as if they were. Through mimicry, the vice-
roy butterfly, which is not poisonous, imitates
the color and pattern of the monarch butter-
fly (Figure 20.11, middle). That “borrowed”
coloration scares away blue jays and other
birds that may have felt sick the last time they
ate a monarch. Another mechanism to avoid
being eaten is camouflage, any type of color-
ation or appearance that makes an organism
hard to find or hard to catch (Figure 20.11,
bottom). Finally, many prey, from musk oxen
to wood pigeons, have evolved a different strat-
egy to avoid becoming dinner: living together.
By group living, these animals are able to act
together to warn each other when a predator is
about to attack and even to repel attacks as a
united front (Figure 20.12).
In predation, the predator benefits. And each
of the other two types of species interactions
Although a single musk ox may
be vulnerable to predators such
as wolves, a group forming a
circle makes a difficult target.The success of
goshawk attacks on
wood pigeons
decreases greatly
when there are many
pigeons in a flock.1 2–10 11–50 >50
Number of pigeons in flock0
Goshawk attack success (%)20
40
60
80
100
Figure 20.12
Safety in numbers
Animals that live in groups are better able to warn each other and sometimes fend off attacking predators.
Q1: What percentage of pigeons are caught when they are alone and not in a flock?Q2: For wood pigeons, what is the minimum number of individuals that provides protection from goshawks?Q3: Why do you think a group of musk oxen versus a lone musk ox would be safer from a pack of wolves?we’ve discussed—mutualism and commensal-
ism—benefits at least one of the species involved.
But in one final type of species interaction,
competition, no one benefits. Instead, both
interacting species are negatively affected.
Competition most often occurs when
two species share an important but limited
resource, such as food or space. In Yellow-
stone, both beavers and elk eat woody plants.
For both species, woody plants are part of the
ecological niche, the set of conditions and
resources that a population needs in order to
survive and reproduce in its habitat. Because
the niches of beavers and elk overlap, these
species compete. When two (or three or more)
species compete, each has a negative effect on
the other because one is using resources that
the other then cannot access. (If resources
are abundant, however, there may be no
competition between species, even if their
niches overlap.)