Giant herds of herbivores are not the natural state of things
Those who use animal manure argue that the way they farm is how nature works: animals excrete on the ground, plants suck it up, and the cycle sustains itself. But there are few natural systems that look anything like agricultural ones. The vast herds of wild herbivores Europeans encountered when they first arrived in Africa and the Americas are likely to have been an artefact of the suppression of predators by the people who already lived there. Palaeontological evidence suggests that, before humans began competing with them and killing them, large carnivores existed in far greater concentrations than they do in any ecosystem today. Rarely, if ever, would dung have been deposited at agricultural rates.