120. Read the following passage, then answer the question below.

In architecture, the keystone is the wedge-shaped stone at the top of an arch; remove it, and the whole structure collapses. In 1969, the ecologist Robert Paine borrowed the term for species that hold an ecosystem together out of proportion to their numbers. His evidence came from a rocky shore in Washington State, where he removed a single predator, the purple sea star, from a stretch of coast. Without it, mussels spread rapidly, crowding out other animals and algae. Within a few years the number of species in the cleared area fell by about half, and a varied community had become a bed of mussels.

Another well-known case involves sea otters along the Pacific coast. Otters eat sea urchins, which in turn graze on kelp. When fur traders nearly eliminated the otters, urchin populations boomed and kelp forests shrank to bare rock, which in turn affected the fish and seabirds that depend on the kelp for shelter. After otters were protected, kelp forests recovered in many areas. Such chains of cause and effect, in which a change at the top of a food web ripples downward, are called trophic cascades.

The concept is useful, but ecologists warn against applying it too neatly. Whether a species acts as a keystone often depends on the place and the time. The sea star that dominates one coast may have little influence on another where wave action, rather than predation, limits the mussels. Reintroduced wolves in Yellowstone National Park are often cited as an example, yet scientists still disagree about how much of the park's recovery they caused, because other factors, such as climate and changes in elk management, were also at work. Conservation planners therefore treat the idea as a guide for where to look, not as a rule that settles what to protect.

According to the final paragraph, how do conservation planners use the keystone idea?