WHAT WOULD HAPPEN IF EARTH BECAME A PLANET OF VEGANS?

Numbers

The number of animals would have been influenced solely by climate. Rapid reproduction in a crop year would provoke the rapid destruction of the flora, as only they are capable of converting inorganic matter into organic matter.

The mass of plants required to feed the increased population of those who feed on them must increase rapidly. But this is impossible, hence the fauna would starve to death en masse.

Slowness

Evolution would be very slow. And organisms would also tend to simplify their own structure.