372 ■ CHAPTER 20 Communities of OrganismsECOLOGY
Luckily, communities can and do bounce
back from disturbances, but the time required
to regain a previous state varies from years to
decades to centuries. Yellowstone is currently
experiencing a secondary succession as wolf,
aspen, and beaver populations slowly return. It
will likely still be a while before the park returns
to its previous state, says Ripple. “We were
70 years without wolves, and now we’re less than
20 years since wolves have returned, so this is
going to take time.” But he and others are hope-
ful that Yellowstone will return to its status as
a climax community, a mature community
whose species composition remains stable over
long periods of time (Figure 20.16).
Yet even as Yellowstone’s recovery is under
way, ecological communities around the planet
are being threatened by the loss of other
keystone species, especially large carnivores. In
2013, Ripple and colleagues analyzed 31 carni-
vore species around the globe, including leop-
ards, lions, cougars, and sea otters. They found
that more than 75 percent of those 31 species are
declining, and that 17 of them now occupy less
than half of their former ranges. In 2016, they
found that bushmeat hunting, in particular, iscan broadly change by the slow loss or gain of
populations of species over long periods of time
through natural selection.
In nature, ecologists have observed two major
types of succession: primary and secondary.
Primary succession occurs in newly created
habitats, such as an island that just emerged
from the sea, the soil left behind a retreating
glacier, or new sand dunes (Figure 20.15). A
new habitat begins with no species. Often the
first species to colonize an area alters the habi-
tat in ways that enable later-arriving species to
thrive. A specific type of flowering plant may
grow on a new island, for example, and then a
species of bee that feeds on that flower subse-
quently joins the habitat.
Secondary succession occurs after a distur-
bance within a community—such as the loss of
a keystone species like the wolves in Yellowstone
or the plant loss caused by a forest fire—reduces
the number of species in that community. During
secondary succession, communities usually
regain the successional state that existed before
the disturbance. This type of succession does
not take as long as primary succession, because
some species still exist in the community.Older sand dunesLake
MichiganStage 4: Climax
communities of black
oak have lasted up to
12,000 years.Stage 1: Bare sand is first
colonized by dune-building
grasses, which spread rapidly
and stabilize the moving sand of
the dunes.Stage 2: Pines
invade 5 0 – 100
years after the
dunes are stabilized
by the grasses.Stage 3: The
dominant species in
the community, black
oak, usually appears
after 10 0 – 150 years.Figure 20.15
Primary succession: from nothing to climax community
Sand becomes woodland near Lake Michigan.
Q1: What species represents the first colonizers of the sand dunes?Q2: What species is the intermediate species, and how does it become the dominant species?Q3: What species is the mature, climax community species, and how does it become the dominant species?