To preserve Mojave Desert tortoises, it is essential to control native ravens, whose numbers have been boosted by distant landfill sites.

WSJ: Ecology and Change: Nature Has No 'Balance' for Us to Keep | Mind & Matter

In the face of unconstrained and ever more competent Soviet defenses, large numbers of ALCMs have become essential to the continued viability of the B-52 fleet.

CENTERFORSECURITYPOLICY: Center for Security Policy | A Schizophrenic Policy Toward The USSR: Hard Line On Defense Cuts; Soft Line In Arms Talks

In the yeast cell, they found only about a quarter of the genes and proteins appeared to be essential, in that they connected to large numbers of other proteins and substances.

WSJ: Genes May Link Disparate Diseases

Jennifer Stewart, head of the Heritage Lottery Fund in Wales, said bats were essential to the ecosystem and their dramatic decline in numbers clearly demonstrated the urgent need to protect and understand them.

BBC: Powys lesser horseshoe bat 'stronghold' in ?100k boost

Staffing levels are so stretched that the essential services that the nurses were ordered to maintain during the strike actually increased the numbers on duty in some hospitals.

ECONOMIST: Quebec

"You do not want to go after large numbers (of missiles) with the very expensive missiles unless it's absolutely essential, " he said.

CNN: Obama scraps Bush-era missile defense for new plan

His regular-season greatness is exceeded by only his remarkable postseason numbers, where his 42 saves, 0.70 ERA, and 0.759 WHIP were essential in carrying the Yankees to five World Series titles.

WSJ: Yankees' Rivera Announces Retirement