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