Greening the Desert - Feral: Rewilding the Land, the Sea, and Human Life - page 85

number of songbirds increased: among the resurgent trees a study has
found higher populations of species such as the song sparrow, war-
bling vireo, yellow warbler and willow flycatcher.35 35
The regrowth of the bankside forests also appears to have allowed
the populations of both beavers and bison to expand: beaver colonies
rose from one to twelve between 1996 and 2009. 36 The beavers then
trigger all the effects I have just mentioned, creating niches for otters,
muskrats, fish, frogs and reptiles. The returning trees have also stabi-
lized the banks of the streams, reducing the rate of erosion and the
movement of channels, narrowing the width of the streams and creat-
ing a greater diversity of pools and riffles.37 37 Similar effects have been
recorded in Zion National Park in Utah: where cougars are abundant,
the streamsides are stable and fish numbers are high, where they are
scarce, the rivers wander and fish numbers are three times lower. 38
The soil on the hillsides in Yellowstone, depleted through sheet ero-
sion after the wolves were all killed and deer numbers rose, may now
begin to build up again. 39 Conversely, on the grasslands where the
deer and pronghorn antelope grazed heavily when their predators
were absent, five years after the wolves returned, nitrogen in the soil
declined by between a quarter and a half. This is because less of it is
now recycled through dung. 40 This will change the species of plants
that grow there and their numbers.
By hunting coyotes, the wolves allow the populations of smaller
mammals - such as rabbits and mice - to rise, providing prey for
hawks, weasels, foxes and badgers. Scavenging animals such as bald
eagles and ravens feed on the remains of the deer the wolves kill. The
return of the wolf appears to have increased the number of bears. They
eat both the carrion abandoned by the wolves and the berries growing
on the shrubs that have sprung back as the deer declined. 41 The bears
also kill deer calves, reinforcing the impact of the wolves. The reintro-
duction of wolves to Yellowstone shows that a single species, allowed
to pursue its natural behaviour, transforms almost every aspect of the
ecosystem, and even alters the physical geography of the site, changing
the shape and flow of the rivers and the erosion rates of the land.
There is no substitute for these complex relationships. Through-
out the period in which wolves were absent from Yellowstone
National Park, its managers tried to control the deer and contain their