"Elephants play a really critical role in the balance between woody and savanna ecosystems" Wittemyer says. Elephant dung contains a tremendous number and variety of plant seeds, which the pachyderms can deposit up to 35 miles away from where they ate fruit. And after fire (and humans), elephants are one of the most powerful forces of destruction, tearing down trees and allowing grassland to move in.
As harbingers of both plant life and death, elephants create what Wittemyer calls "habitat mosaics." Instead of a uniform blanket of woody bush or old forest, elephants weave environments into a patchwork quilt that moves from forest to grassland and back. (Black rhinos, likewise, are particularly good at chomping down thorned plants other animals can't stomach.) This ecological tide allows for a much richer diversity of species than would be possible otherwise. A 2009 study found many more lizard species, for example, in areas where elephants had damaged trees.
Biologists have a phrase - "a trophic cascade" - for what happens after the populations of elephants and other keystone species are altered within an ecological community. If the term evokes a waterfall, or a series of falling dominoes, it's an apt metaphor. "Remove a cog in the network," Wittemyer says, "and the network is going to shift around." Without vultures or other scavengers to eat meat festering in the sun, for instance, it's possible that the incidence of disease could increase.