Animals in nature move nutrients uphill, against the natural gravitational flow from high ground to low ground. This is why low lands and valleys are fertile and the uplands are less so. Animals are the only mechanism nature has to defy this natural downward flow. Fortunately, predators make the prey animals want to lounge on high ground (where they can see their enemies), which ensures that manure will concentrate on high look-out spots rather than in the valleys.
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