The ecosystems in which hunter-gatherers exist are generally sustainable because nutrients are recycled efficiently: they are drawn from the soil by vegetation which is consumed by animals (including people) and then excreted back into the same environment. 

As soon as humans start deliberately growing food but consuming it elsewhere, the balance can be lost with nutrients excreted away from the environment that produced them. This not only begins to deplete the soil of its nutrients but can also create a problem of human ‘waste’ (excrement) at other locations.
 

Above image developed at the University Of Nottingham