GWD-31-Q32

Whitney

 To find the answer quick, try to summarize the structure of passage.
The simplest structures are: 1) an opion, then followed by one or more detailed explanations;
                    2) Providing an example, then draw the conclusion.

 Just look at line 22-26.   P occupies keystone roles in somewhere, in other place, the impact of P ... is weak or nonexistent...

20)     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,

25)        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.