ECOLOGICAL PROCESSES 65
Reintroducing
beavers to the UKBeavers were wiped out in the UK
400 years ago, but the beneficial
role of this keystone mammal is
now better understood. Beavers
are ecological engineers, building
dams and canals, and their
presence increases biodiversity.
In 2009, 11 beavers were
reintroduced to Knapdale Forest,
Scotland, and in 2011, the Devon
Wildlife Trust introduced a pair to
a fenced enclosure. Both projects
have been monitored to test their
impact on the environment. In
Knapdale Forest, the beavers’
dams changed the water level of
a loch, and Devon’s beavers builtseveral dams on the headwaters
of the Tamar River, creating 13
new freshwater pools and making
surrounding areas wetter.
In Devon, the damp areas
created by beavers led to an
increase in the number of
bryophyte species (mosses and
liverworts), and the range of
aquatic invertebrates has risen
from 14 to 41 species. Increased
numbers of flying insects have
also improved bat diversity, with
two nationally rare bat species
drawn into the area. More
beaver reintroduction projects
are now planned in the UK.from the seabed, causing it to drift
away and die. If the kelp disappears,
however, so do the many other
marine invertebrates that graze on
it. “Forests” of kelp also absorb large
amounts of atmospheric carbon
dioxide and, by slowing water
currents, help protect coastlines
from storm surges. The protection
that sea otters offer kelp along
stretches of open coast is therefore
particularly significant.
Unlike the sea otter, some
keystone species are also “apex”
predators at the top of the food
chain, such as the gray wolf. Before
1995, there had been no gray wolves
in Yellowstone National Park for at
least 70 years. American elk were
common in the park, but there was
just a single colony of beavers. That
year, 31 wolves were introduced to
the park and by 2001 their numbers
had increased to more than 100,
largely due to the abundance of
elk for food.
The presence of wolves in the
park forced the elk to become more
mobile. Rather than over-grazing
willow, aspen, and cottonwood trees
in favored locations, the elk moved
on, allowing plants to regenerate
and provide a food source for otherherbivores, such as beavers. Within
10 years, the number of beaver
colonies increased from one to nine.
Beaver dams helped revive wetlands,
and wetland wildlife flourished. The
increase in elk carcasses also
benefited carrion-eaters—especially
coyotes, red foxes, grizzly bears,
Golden Eagles, Ravens, and Black-
billed Magpies—as well as several
smaller scavengers.
Jaguars are apex predators in
South and Central American forests,
preying on more than 85 species.
Although there are very few jaguars
in any given area, their impact on
the numbers of other predators—
such as caimans, snakes, large
fish, and large birds—as well as
herbivores, such as capybaras and
deer, has a significant ripple-down
effect on their ecosystem. Left
unchecked, the herbivores could
devour most of the plants and
destroy the habitat on which so
many other species depend.Keystone plants
Not all keystone species are
animals. One example is the fig
tree, of which there are about 750
species, mostly found in tropical
forests. In this habitat, most fleshy-fruited plant species share one or
two peaks of ripening each year.
Fig trees bear fruit throughout the
year, supporting many animals
when other trees are fruitless.
More than 10 percent of the world’s
bird species and 6 percent of
mammals (a total of 1,274 species)
are known to eat figs, as do a small
number of reptiles and even fish.
Fig trees therefore provide a vital
support mechanism for fruit-eating
species. Without them, fruit bats,
birds, and other creatures would
decline or disappear. ■By protecting a keystone
species such as the prairie
dog, the public could be
educated about the value of
ecosystem conservation.
Brian Miller
American ecologistUS_060-065_Keystone_species.indd 65 12/11/18 6:24 PM