The Koala of Australia is perfectly adapted to one specific tree. the eucalyptus, and doesn't need anything else, not even water. It is one of the few land animals that do not need water to supplement their food. If the eucalyptus tree were to become extinct, the koala would become extinct, too, for it eats nothing but eucalyptus leaves.
If the zebra population were to vanish overnight, African lions would decrease drastically in numbers.
Bees and numerous species of plants that depend on bees for cross-pollination would be wiped out as well.