Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
442
degradation as a result of soil erosion once the vegetation
is removed.
Although agriculture provides most of our food, we
still rely on taking wild fish from the environment. It is
very difficult to know whether fish stocks are sustainable,
but the history of the fishing industry suggests that many
species have been driven to near extinction by overfishing.
Many fisheries, such as those for cod on the Grand Banks
in the North Atlantic, herring in the North Sea and a
variety of species in the East China Sea have declined or
collapsed. The response to the steep decrease in large,
predatory species is to fish further down the food chain
taking smaller fish that other animals, such as marine
mammals and sea birds, depend upon. Fishing is just one
example of the overexploitation of resources. Another
example is the removal by logging companies of valuable
trees, such as teak and mahogany, at a rate faster than they
can regenerate.
The loss of a single species can have devastating
effects on the rest of its community. The Pacific sea
otter, Enhydra lutris, is a predator of sea urchins in kelp
forests (Figures 18.27 and 18.28). In the 19th century sea
otters were hunted for their fur and there was a striking
change to the whole of the food web as urchins exploded
in numbers and ate their way through the stipes (‘stems’)
of the kelp forests (Figure 18.28). The loss of one species,
the sea otter, led to catastrophic loss of many others.
Organisms like the sea otter that play a central role in an
ecosystem are known as keystone species. Sea otters are
now protected and their numbers increased in the latter
half of the 20th century but now they are being predated
Figure 18.27 A sea otter, Enhydra lutris.
Figure 18.28 A kelp forest in the Pacific Ocean off the coast
of California. The giant seaweeds provide habitats for many
species including sea urchins, fish and sea otters.
by killer whales that may have less prey to hunt because
of overfishing.
The African bush elephant, Loxodonta africana, is a
keystone species of the savannah grasslands of East and
Southern Africa. Bush elephants are very destructive of
vegetation as they push over and eat many tree species.
This extreme form of grazing helps to maintain this
ecosystem, which is renowned for its diversity of large
mammals as well as many other species. Elephant dung
provides a very rich habitat – in fact almost an ecosystem
in itself – for many organisms including fungi and dung
beetles. Elephants were once hunted widely for their
ivory and their populations decreased considerably. Now
protected by international agreements, they are still at risk
of poaching to supply the illegal trade in ivory
(Figure 18.29).