374 ■ CHAPTER 20 Communities of Organisms
Challenge Yourself
(^6) Wolves are considered a keystone species in Yellowstone because
(a) their removal in the early twentieth century caused many
changes to the Yellowstone ecological community.
(b) their reintroduction in the late twentieth century caused many
changes to the Yellowstone ecological community.
(c) when they were removed, elk populations increased, leading to
increased competition with beavers and bears, which then declined.
(d) when they were reintroduced, aspen populations began to
increase because of decreased predation from elk.
(e) all of the above
(^7) Link each species interaction with an example of the
interaction.
MUTUALISM 1. Elk graze on aspen.
COMMENSALISM 2. Bison allow magpies to perch on them;
the birds eat ticks they find on the
bison’s bodies.
PREDATION 3. Birds nest in aspen trees.
COMPETITION 4. Beavers and elk eat the same trees.
(^8) Place the following elements of the scientists’ study of the
relationship between aspen populations and wolf populations in the
correct order from earliest to latest by numbering them from 1 to 5.
a. Beschta hypothesized that the decimation of wolf popula-
tions in the early twentieth century allowed elk populations
to grow, thus increasing grazing of the aspen by elk.
b. Beschta observed there were few trees in the river valley.
c. Ripple heard Beschta describe his observations in a talk.
d. Ripple and his graduate student collected data showing
that no new trees had grown in the valley since the 1920s.
e. With the reintroduction of wolves into the park, elk popula-
tions declined and aspen populations rebounded.
(^9) Which ecological community would be more diverse: one with
high relative species abundance or one with low relative species
abundance? Which community would be more diverse: one with
high species diversity or one with low species diversity?
Tr y Something New
(^10) When a female cat comes into heat and is ready to mate, she
urinates more frequently and in a large number of places. Male cats
from the neighborhood congregate near urine deposits and fight
with each other for the female’s attention and breeding rights. In
what type of interaction are the male cats engaging?
(a) commensalism
(b) predation
(c) interference competition
(d) exploitative competition
(e) mutualism
● (^) In competition, both species
are harmed. Competition
occurs when ecological
niches overlap and includes
exploitative competition and
interference competition.
The competitive exclusion
principle predicts that
different species that use
the same resource can
coexist only if one of the
species adapts to using
other resources.
● (^) Succession establishes
new communities (primary
succession) and replaces
disturbed communities
(secondary succession).
In stable environments
without disturbances,
called mature communities
or climax communities,
species composition
remains stable over long
periods of time.
THE QUESTIONS
The Basics
(^1) A single sequence of feeding relationships describing who eats
whom in a community is a
(a) life history.
(b) keystone relationship.
(c) food web.
(d) food chain.
(^2) The process of species replacement over time in a community
is called
(a) global climate change.
(b) succession.
(c) competition.
(d) community change.
(^3) Organisms that can produce their own food from an external
source of energy without having to eat other organisms are called
(a) suppliers.
(b) consumers.
(c) producers.
(d) keystone species.
(^4) A low-abundance species that has a large effect on the
composition of an ecological community, especially when removed
from that community, is called a
(a) predator.
(b) herbivore.
(c) keystone species.
(d) dominant species.
(^5) Select the correct terms:
A cheetah eats an antelope that ate some grass. The cheetah is a
(secondary consumer / primary consumer), while the antelope is a
(secondary consumer / primary consumer). The grass is a (tertiary
consumer / producer).