Vegans eat plants, which cause some large-scale ecological problem X, therefore we should be against veganism. Examples include:
etc. These are typically some industrial scarcity that is caused by raising plants or some other problem. There are pages within this category and elsewhere dedicated to the specific claim but it's difficult to capture all of them due to the sheer volume of poorly-sourced claims here it's worth creating a catch-all claim.
It takes typically an order of magnitude more plant calories to create meat as an intermediate for human consumption. So if there's some large-scale problem with growing plants, then that problem is magnified by feeding them to animals first.
The only place for arguments to lean on is the idea that somehow plants are grown in just one small area and then do animal agriculture in other areas, which minimizes the impact because that land wasn't used for anything. Ultimately, is the non-arable land argument with treating some quantity of preserved land as non-arable. But this argument doesn't work either.