Keystone Species
A species so critical to an ecosystem that its removal could potentially destroy the entire system. A good example of this are Blue Jays, which plant acorns that give rise to oak forests. Without Blue Jays, oak forests are not naturally replenished and without the forests, all other species in the ecosystem cannot survive. It is important to consider keystone species when making any decisions that could affect the natural environment. A small impact on keystone species could cause major disruption for the whole ecosystem.
What about honey bees and bats? Each are in trouble, but are bees, for example, too abundant to be worthy of the designation? http://www.triplepundit.com/mt/mt-tb.cgi/1504
What would you call an organism that does the direct opposite? Like sea urchins, where an abundance of these organisms can reduce kelp forest ecosystem to nothing but barren grounds. A reverse key-stone species??