Question 2 – 4:
While the most abundant and dominant species within a particular ecosystem 1
is often crucial in perpetuating the ecosystem, a “keystone” species, here defined 2
as one whose effects are much larger than would be predicted from its appearance, 3
also play a vital role. But because complex species interactions may be involved, 4
identifying a keystone species by removing the species and observing changes in 5
the ecosystem is problematic. 6
It might seem that certain traits would clearly define a species as a keystone 7
species; for example, Pisaster ochraceus is often a keystone predator because it 8
consumes and suppresses mussel populations, which in the absence of this starfish 9
can be a dominant species. But such predation on a dominant or potentially dominant 10
species occurs in systems that do as well as in systems that do not have species that 11
play keystone roles. Moreover, whereas P. ochraceus occupies an unambiguous 12
keystone role on wave-exposed rocky headlands, in more wave-sheltered habitats 13
the impact of P. ochraceus predation is weak or nonexistent, and at certain sites sand 14
burial is responsible for eliminating mussels. Keystone status appears to depend on 15
context, whether of particular geography or of such factors as community diversity 16
(for example, a reduction in species diversity may thrust more of the remaining species 17
into keystone roles) and length of species interaction (since newly arrived species 18
in particular may dramatically affect ecosystem). 19
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Question 3:
Which of the following, if true, would most clearly support the argument about keystone
status advanced in the last sentence of the passage (lines 15 – 19)?
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 habits.
E. A species of mouse that occupies a keystone role in a prairie habitat develops
coloration patterns that camouflage it from potential predators.
The answer is B, why it is not D | 
