Of Wolves and Trees ■ 373driving many mammal species to extinc-
tion, including 113 species in Southeast
Asia. Because of all the species interac-
tions discussed here, it is clear that those
changes will have major effects on ecolog-
ical communities. “Humans are affecting
predators around the globe in a major way,”
says Ripple. “It’s a worldwide issue.”REVIEWING THE SCIENCE
● (^) An ecological community can be characterized
by its species composition, or diversity. This
diversity has two components: relative species
abundance (the number of individuals of each
species that exist in the community) and
species richness (the total number of different
species that live in the community).
● (^) Keystone species have a disproportionately large
effect, relative to their own abundance, on the
richness and abundances of the other species
in a community. The removal or disappearance
of these keystone species results in dramatic
changes to the rest of the community.
● (^) A food chain is a single direct line of who eats
whom among species in a community. A food
web depicts how overlapping food chains of a
community are connected.
● (^) Producers, organisms found at the bottom of
a food chain that use light energy to produce
their own food, are eaten by consumers.
Consumers are classified as carnivores,
herbivores, or omnivores, depending on
whether they eat animals or plants or both,
respectively.
● (^) Dead and dying organisms are prey for many
kinds of consumers. Scavengers are omnivores
or carnivores that hunt for dead and dying
prey. Decomposers dissolve their food, and
detritivores mechanically break apart and
consume their food.
● (^) Species interactions in a community can be
beneficial, harmful, or without benefit or harm to
each of the interacting species.
● (^) In mutualism, both species benefit.
● (^) In commensalism, one species benefits at no
cost to the other.
● (^) In predation, one species benefits and the
other is harmed. Parasites are predators that
live in or on their hosts.
Figure 20.16
Secondary succession: from disturbance to climax community
Lodgepole-pine forest has been slowly but steadily regrowing in Yellowstone National Park.
Q1: What other types of disturbances could you imagine destroying a forest?
Q2: How is secondary succession different from primary succession?
Q3: What is a climax community?
In 1988, a large fire
destroyed a portion of the
mature lodgepole-pine forest
in Yellowstone National Park.
By 1992, the lodgepole-pine
forest regrowth was gaining
momentum.
An example of a
mature, climax
community
lodgepole-pine forest.