The main point of veganism is to boycott factory-farming, which causes
animals to lead miserable lives.

The animals who live on factory farms have to be fed with plant
products, the production of which will cause the death of wildlife.
Animal products are an inefficient use of land, so their production
will cause more death of wildlife than the production of plant products
to be fed directly to human beings.

As for the argument that ruminant-pasture food production causes fewer
deaths than some forms of plant food production, the following article
is worth a look:

http://courses.ats.rochester.edu/nob...least-harm.pdf