Bill Mollison has argued in his Permaculture Design Course that vegetarianism exacerbates soil erosion. This is because removing a plant from a field removes all the nutrients it obtained from the soil, while removing an animal leaves the field intact. On US farmland, much less soil erosion is associated with pastureland used for livestock grazing than with land used for production of crops. Robert Hart has also developed forest gardening, which has since been adopted as a common permaculture design element, as a sustainable plant-based food production system.