Page 103 - Environment: The Science Behind the Stories

Keystone Figure 4.12 Sea otters are a
absent keystone species. A keystone (a)
is the wedge-shaped stone at the
top of an arch that holds its struc-
ture together. A keystone species
(b) is a species that exerts great
Keystone influence on a community’s com-
position and structure. Sea otters
consume sea urchins that eat kelp in
coastal waters of the Pacific. Otters
(a) A keystone
keep urchin numbers down, allow-
ing lush underwater forests of kelp
to grow, providing habitat for many
Sea otter absent species. When otters are absent,
urchins increase and devour the kelp,
destroying habitat and depressing
Sea otter species diversity.
(keystone
species) Kelp

Overgrazed
kelp

Explosion of
Sea sea urchin
urchin population

(b) A keystone species

The removal of top predators in the United States was redistribute nutrients, and selectively protect or destroy differ-
an uncontrolled large-scale experiment with unintended con- ent insects and plants within the radius of their colonies. And
sequences, but ecologists have verified the keystone species zebra and quagga mussels alter the communities they invade
concept in controlled scientific experiments. Classic research by filtering plankton out of the water.
by marine biologist Robert Paine established that the preda- Less conspicuous organisms toward the bottoms of
tory sea star Pisaster ochraceus shapes the community com- food chains may exert still more impact. Remove the fungi
position of intertidal organisms (p. 445) on North America’s that decompose dead matter, or the insects that control plant
Pacific coast. When Pisaster is present in this community, growth, or the phytoplankton that are the base of the marine
species diversity is high, with various types of barnacles, food chain, and a community may change very rapidly indeed.
mussels, and algae. When Pisaster is removed, the mussels However, because there are usually more species at lower
it preys on become numerous and displace other species, sup- trophic levels, it is less likely that any single one of them alone
pressing species diversity. has wide influence. Often if one species is removed, other spe-
Research off the U.S. Atlantic coast published in 2007 cies that remain may be able to perform many of its functions.
suggests that the reduction of shark populations by commer-
cial fishing has allowed populations of certain skates and rays
to increase, which in turn has depressed numbers of bay scal- Communities respond to disturbance
lops and other bivalves they eat. in various ways
Animals at high trophic levels—such as wolves, sea stars,
sharks, and sea otters (see Figure 4.12)—are often viewed as The removal of a keystone species is just one type of distur-
keystone species that can trigger trophic cascades. However, bance that can modify the composition, structure, or function
other types of organisms also exert strong community-wide of an ecological community. In ecological terms, a disturbance
effects. “Ecosystem engineers” physically modify the envi- is an event that affects environmental conditions rapidly and
ronment shared by community members. Beavers build dams drastically, resulting in changes to the community and ecosys-
across streams, creating ponds and swamps by flooding large tem. A disturbance can be as localized as a tree falling in a
areas of land. Prairie dogs dig burrows that aerate the soil forest, creating a gap in the canopy that lets in light and alters
102 and serve as homes for other animals. Ants disperse seeds, conditions for plants and animals in the gap. Or it can be as

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