Greening the Desert - Feral: Rewilding the Land, the Sea, and Human Life - page 84

greatest extent possible ecology's dynamic interactions. In other
words, the scientific principle behind rewilding is restoring what ecol-
ogists call trophic diversity. Trophic means relating to food and
feeding. Restoring trophic diversity means enhancing the number of
opportunities for animals, plants and other creatures to feed on each
other; to rebuild the broken strands in the web of life. It means
expanding the web both vertically and horizontally, increasing the
number of trophic levels (top predators, middle predators, plant eat-
ers, plants, carrion and detritus feeders) and creating opportunities
for the number and complexity of relationships at every level to rise.
One of the most fascinating discoveries in modern ecology is an
abundance of trophic cascades. A trophic cascade occurs when the
animals at the top of the food chain - the top predators - change the
numbers not just of their prey, but also of species with which they
have no direct connection. Their impacts cascade down the food
chain, in some cases radically changing the ecosystem, the landscape
and even the chemical composition of the soil and the atmosphere.
The best-known example is the dramatic change that followed the
reintroduction of wolves to the Yellowstone National Park in the
United States. Seventy years after they had been exterminated, wolves
were released into the park in 1995. When they arrived, many of
the streamsides and riversides were almost bare, closely cropped by the
high population of red deer (which in North America, confusingly, are
called elk*). But as soon as the wolves arrived, this began to change. It
was not just that they sharply reduced the number of deer, but they also
altered their prey's behaviour. The deer avoided the places - particularly
the valleys and gorges - where they could be caught most easily. 32
In some places, trees on the riverbanks, until then constantly sup-
pressed by browsing, quintupled in height in just six years. 33 The trees
shaded and cooled the water and provided cover for fish and other
animals, 34 changing the wildlife community which lived there. More
seedlings and saplings survived. The bare valleys began reverting to
aspen, willow and cottonwood forest. One apparent result is that the
* The European red deer, Cervus elaphus , and the North American elk, Cervus
canadensis , are so closely related that until 2004 they were believed to be the same
species.