National Geographic, Dec. 2, 2015: The huge herbivores of the Ice Age were ecosystem engineers. Wherever they went, mastodons, sloths, bison, and their ilk changed the landscape by eating, defecating, trampling, and otherwise going about their plant-mashing business. But they were not isolated agents. Following out the engineer analogy, the megaherbivores of times past had managers. These were the sabercats, hyenas, wolves, and other predators past.