Page 102 - Environment: The Science Behind the Stories

Caterpillar and other
insects on leaves
Spider

White
oak

Eastern
chipmunk

Cedar
Cedar
Beetles and
waxwing Beetles and
waxwing
other insects
other insects
Ticks
Blackberry
Blackberry
Red-bellied
Red-bellied
Eastern woodpecker
woodpecker
cottontail
White-tailed deer
Rat snake
Rat snake
Shelf fungus
Shelf fungus

Deer
mouse

Grasses
American toad

Earthworm
Soil bacteria

Figure 4.11 Food webs represent feeding relationships in a community. This food web shows organ-
isms on several trophic levels in eastern North America’s temperate deciduous forest. Arrows lead from one
organism to another to indicate the direction of energy flow as a result of predation, parasitism, or herbivory.
Like most food web diagrams, this one is a simplification, because the actual community contains many more
species and interactions than can be shown. CHAPTER 4 • S PEC i ES i n TERA CT i on S A nd Co mmuni T y E C ology

Some organisms play outsized roles Often, secondary or tertiary consumers near the tops of
in communities food chains are considered keystone species. Top predators
control populations of herbivores, which otherwise would
“Some animals are more equal than others,” George Orwell multiply and could greatly modify the plant community
wrote in his classic novel Animal Farm. Although Orwell was (Figure 4.12b). Thus, predators at high trophic levels can indi-
making wry sociopolitical commentary, his remark hints at a rectly promote populations of organisms at low trophic levels
truth in ecology. In communities, ecologists have found, some by keeping species at intermediate trophic levels in check, a
species exert greater influence than do others. A species that phenomenon ecologists refer to as a trophic cascade. In the
has strong or wide-reaching impact far out of proportion to its United States, for example, government bounties promoted
abundance is often called a keystone species. A keystone is the the hunting of wolves and mountain lions, which were largely
wedge-shaped stone at the top of an arch that is vital for holding exterminated by the middle of the 20th century. In the absence
the structure together; remove the keystone, and the arch will of these predators, deer populations grew unnaturally dense
collapse (Figure 4.12a). In an ecological community, removal and have overgrazed forest-floor vegetation and eliminated
of a keystone species will likewise have major consequences. tree seedlings, causing major changes in forest structure. 101

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