364 ■ CHAPTER 20 Communities of Organisms
ECOLOGY
including cottonwoods). But he needed a way to
quantify that change. “It’s a scientifically difficult
task to connect a wolf to a plant. Obviously wolves
don’t consume plants, so instead we had to connect
the dots. There are data that wolves affect elk, and
other data that elk affect plants,” says Ripple.
Ripple and Beschta went into the field in 2006
and again in 2010 to take measurements. In
addition to recording the ages of the trees, they
looked for and documented signs of elk feed-
ing on aspens—such as scars where branches
or buds had been bitten off—and measured the
heights of young aspens. Beschta did the same
with the cottonwoods. They were eager to see
which species interactions would occur now that
wolves were back.
The first type of species interaction, mutu-
alism, occurs when two species interact and
both benefit. For example, Yellowstone is home
to 4,600 bison, the largest land mammals in
North America. Bison have a mutualistic rela-
tionship with the black-billed magpie. Pests
such as ticks burrow into a bison’s short, dense
hair to suck the beast’s blood, but hungry
little magpies perch on top of the bison and
eat those ticks. Thus, both the bison and the
magpie benefit from close interaction with one
another. Mutualism is common and import-
ant in ecosystems all over Earth: many species
receive benefits from, and provide benefits to,
other species, as the clownfish-anemone part-
nership in Figure 20.8 illustrates. These bene-
fits increase the survival and reproduction of
both interacting species.
When they aren’t perched atop bison, black-
billed magpies can be found in large nests atop
deciduous or evergreen trees, where they repro-
duce once a year. These trees, another member
of the community, share a commensal relation-
ship with the magpies. Commensalism happens
when one partner benefits while the other is
neither helped nor harmed—in this case, the
magpie benefits from having a safe place to lay
eggs, and the interaction has no effect on the
tree. Another example of a commensal relation-
ship is barnacles living on a whale (Figure 20.9).
As you might have guessed, not all species
interactions are as pleasant as those among
bison, birds, and trees. In two other types of
interactions, at least one of the two species is
harmed: competition (which we will return to in
a moment) and predation.The classification is based on whether the inter-
action is beneficial, harmful, or neutral to each
species involved. These interactions affect where
organisms live and how large their populations
grow. Species interactions also drive natural
selection and evolution, thereby changing the
composition of communities over short and long
periods of time.
With wolves back in the park, Ripple suddenly
had a way to test his hypothesis about aspen
decline. If the loss of wolves was responsible for
aspen loss, then the return of the wolves should
incite a revival of aspens (and other woody plants,19951997199920012003200520072009201120132015Number of wolvesYear050100150200 Yellowstone National Park wolf populationFigure 20.7
Wolves today in Yellowstone
Q1: When were wolves first reintroduced to
Yellowstone?Q2: What was the highest number of
wolves observed in Yellowstone? In what
year did that occur?Q3: Why did it take a few years after the
reintroduction of wolves for the aspen
population to increase, as well as the
beavers and bears?