~90% of the calories a species takes in get lost going up the food chain. You need to grow ~1000 calories of plants to make 100 calories of beef.
>Not to mention plants are nutrient deficient and as a result people have to eat far more of it
Source? Also, what specific nutrients are you referring to? Where do you think the energy and minerals an animal has come from?
>The most environmentally friendly method for humans would be the "hunter gatherer" diet ( wild vegetation and wild animals ). But we might be too populous for that to work.
Modern agriculture is hands down more efficient. You know this, because you know that it can support more people. By extension, modern agriculture with a small population is more environmentally friendly than "hunter gatherer" with a small population.
>You essentially have to farm one plant to feed livestock for your burger. How many different types of plants have to be planted to produce a single burger?
More variety but far far far less total quantity. And monoculture farming is much more destructive than farming a variety of plants. There's a reason that farmers rotate corn and soy, even when growing animal feed.