Fertilizer and tractors both contributed enormously to devastaing the biodiversity of the planet. The question is not whether we can sustain a larger population, but whether we can sustain a population as large as currently exists - while maintaining any semblance of a natural world. Over a long enough time line both questions almost certainly do in fact merge. But it's equally possible to conceive of a medium term future with nature utterly devastated and humanity clinging on in increasingly baroque ways to a dying rock.