5.3

While the most abundant and dominant species
within a particular ecosystem is often crucial in
perpetuating the ecosystem, a "keystone" species,
here defined as one whose effects are much larger
than would be predicted from its abundance, can
also play a vital role. But because complex species
interactions may be involved, identifying a keystone
species by removing the species and observing
changes in the ecosystem is problematic. It might
seem that certain traits would clearly define a
species as a keystone species; for example,
Pisaster ochraceus is often a keystone predator
because it consumes and suppresses mussel
populations, which in the absence of this starfish
can be a dominant species. But such predation on a
dominant or potentially dominant species occurs in
systems that do as well as in systems that do not
have species that play keystone roles. Moreover,
whereas P. ochraceus occupies an unambiguous
keystone role on wave-exposed rocky headlands, in
more wave-sheltered habitats the impact of
P. ochraceus predation is weak or nonexistent, and
at certain sites sand burial is responsible for
eliminating mussels. Keystone status appears to
depend on context, whether of particular
geography or of such factors as community
diversity (for example, a reduction in species
diversity may thrust more of the remaining species
into keystone roles) and length of species
interaction (since newly arrived species in particular
may dramatically affect ecosystems).

111. The passage mentions which of the following as a
factor that affects the role of P. ochraceus as a
keystone species within different habitats?
(A) The degree to which the habitat is sheltered
from waves
(B) The degree to which other animals within a
habitat prey on mussels
(C) The fact that mussel populations are often not
dominant within some habitats occupied by
P. ochraceus
(D) The size of the P. ochraceus population within
the habitat
(E) The fact that there is great species diversity
within some habitats occupied by P. ochraceus


112. Which of the following hypothetical experiments most
clearly exemplifies the method of identifying species'
roles that the author considers problematic?
(A) A population of seals in an Arctic habitat is
counted in order to determine whether it is the
dominant species in that ecosystem.
(B) A species of fish that is a keystone species in
one marine ecosystem is introduced into
another marine ecosystem to see whether the
species will come to occupy a keystone role.
(C) In order to determine whether a species of
monkey is a keystone species within a particular
ecosystem, the monkeys are removed from that
ecosystem and the ecosystem is then studied.
(D) Different mountain ecosystems are compared
to determine how geography affects a particular
species' ability to dominate its ecosystem.
(E) In a grassland experiencing a changing climate,
patterns of species extinction are traced in
order to evaluate the effect of climate changes
on keystone species in that grassland.


113. Which of the following, if true, would most clearly
support the argument about keystone status advanced
in the last sentence of the passage (lines 24–31)?
(A) A species of bat is primarily responsible for
keeping insect populations within an ecosystem
low, and the size of the insect population in turn
affects bird species within that ecosystem.
(B) A species of iguana occupies a keystone role on
certain tropical islands, but does not play that
role on adjacent tropical islands that are
inhabited by a greater number of animal species.
(C) Close observation of a savannah ecosystem
reveals that more species occupy keystone
roles within that ecosystem than biologists had
previously believed.
(D) As a keystone species of bee becomes more
abundant, it has a larger effect on the
ecosystem it inhabits.
(E) A species of moth that occupies a keystone role
in a prairie habitat develops coloration patterns
that camouflage it from potential predators.


114. The passage suggests which of the following about
the identification of a species as a keystone species?
(A) Such an identification depends primarily on the
species' relationship to the dominant species.
(B) Such an identification can best be made by
removing the species from a particular
ecosystem and observing changes that occur in
the ecosystem.
(C) Such an identification is likely to be less reliable
as an ecosystem becomes less diverse.
(D) Such an identification seems to depend on
various factors within the ecosystem.
(E) Such an identification can best be made by
observing predation behavior.