keystone

Last year, some 1,000 U.S. troops withdrew from neighboring Niger, ending a nearly decadelong partnership that was the keystone of U.S. counterterrorism operations in the region.

They "play a central role in the food web, consuming fish and invertebrates and recycling nutrients" and are one of the "keystone species" of their ecosystem.

Some scientists trying to recover sunflower sea stars see the finding as a strong guide for future research — and efforts to boost the decimated keystone species.

Despite the pile-up of particulars, Metcalfe knows he must find the lost poem, that it is the keystone without which the story crumbles into insignificance.

The orangutan is a keystone species and plays a vital role in maintaining the health of the ecosystem.