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“As extinction spreads, some of the lost forms prove to be keystone species, whose disappearance brings down other species and triggers a ripple effect through the demographics of the survivors. The loss of a keystone species is like a drill accidentally striking a powerline. It causes lights to go out all over.” 

Edward O. Wilson The Diversity of Life ~
“Organisms possessing common ancestry rise to dominance, expand their geographic ranges, and split into multiple species. Some of the species acquire novel life cycles and ways of life. The groups they replace retreat to relict status, being diminished in scattershot fashion by competition, disease, shifts in climate, or any other environmental change that serves to clear the way for the newcomers. In time the ascendant group itself stalls and begins to fall back. Its species vanish one by one until they are all gone. Once in a while, in a minority of groups, a lucky species hits upon a new biological trait that allows it to expand and radiate again, reanimating the cycle of dominance on behalf of its phylogenetic kin.”