Explain why worldwide agriculture could feed more people if all humans consumed only plant material.

This is because the crowing of crops requires much less water and land than raising the energy equivalent in livestock. To explain, consider that most livestock are primary consumers, and are one level above primary producers, in this case, food crops. So there is a large loss in energy between primary producers and primary consumers. So if people chose instead to have a plant-based diet, satisfying their dietary needs could require much less water and land resources. Most livestock, particularly in developed countries, are grain fed. Therefore, grain is cultivated specifically for animal feed, and then people consume the animal protein after a large proportion of the original energy captured by the grain is lost. 

A- a reduction in biodiversity