Greening the Desert - Feral: Rewilding the Land, the Sea, and Human Life - page 87

horses that grazed the steppes.* (When the glaciers blocking their pas-
sage into the rest of the Americas melted, they went on to wreak even
greater havoc in the New World.) The result, it appears, was that they
helped turn the steppe grasslands into mossy tundra. Much of this
land has remained that way ever since.
As the Russian scientist Sergey Zimov has shown, grasslands, espe-
cially in the far north, are sustained by the animals that feed on them.
By grazing, they make the grass more productive (in the steppes it
grows five times faster than it does when it is not mowed). They
recycle the soil's nutrients through their dung. The grass dries out the
soil and smothers moss and lichens. 49 When the animals disappear, the
self-reinforcing process goes into reverse. The dead grass, flopping
over the soil, insulates it, ensuring that it stays cold, reducing the fur-
ther growth of grass and encouraging moss to take over. As the moss
begins to dominate, the soil becomes wetter and colder - still more
hostile to grass. If the animals return, their trampling quickly breaks
up the fragile layer of moss and lichens, allowing the grass to domin-
ate again within one or two years. 50 The grazers in this habitat, in
other words, are keystone species, flipping the entire ecosystem from
one state to another.
This suggests, incidentally, that large-scale rewilding of the tundra,
which Zimov and others promote, while a fascinating prospect, could
have a damaging consequence. Moss is such a good insulator that it
prevents even the top layer of soil from thawing. 51 It helps to stabilize
the permafrost, locking up the methane it contains. If the moss layer is
broken up and grasses return, while this might greatly increase the prod-
uctivity and trophic diversity of the region, it could accelerate the melting
which threatens to release large quantities of a powerful greenhouse gas.
This is a reminder that rewilding, like any change we contemplate, has
costs. In some cases the costs may outweigh the benefits.
Hunting by humans might also have transformed the environment
of Australia. Before people arrived on that continent, it teemed with
* Mammoths might have been made more susceptible to extinction through hunting
by the simultaneous shrinkage of their habitat. One paper suggests that this caused a
90 per cent reduction in their geographical range between 42,000 and 6,000 years
ago. 48