Widely held unsupported assumption based on lies of omission
"Dominionism" including the belief that all land in sight must be conquered and "used" by humans, no matter how little that use gives us in return
The land used to grow crops for people to consume directly (vegan food) supplies more calories and protein for the global population than the almost 4-times larger area used for animal agriculture (Our World in Data). Yet there remains an allegiance to the idea of using marginal lands to graze ruminants for consumption. This doesn't hold up, because the paltry yields don't justify the negative impacts. A vegan shift would free up such land for conservation and other purposes, including growing hardy human-edible plants.
Land use > yields
To graze farmed ruminants, staggering amounts of land is used for incredibly little return. For example, about 60% of the world's agricultural land is used for "beef," which only accounts for less than 5% of protein and 2% of calories worldwide (Union of Concerned Scientists). A portion of that land is considered marginal.
Much ecological damage occurs in the process, including the release of greenhouse gases like methane. Of grazing livestock, George Monbiot (Guardian journalist & UN Global 500 Award winner for outstanding environmental achievement) says, "The damage caused is out of all proportion to the meat produced."
So if we removed more than half of ALL agricultural land on the entire planet, which is being used JUST for "beef," we would only lose a very small amount of protein and calories from the global food supply.
Ranching proponents would likely be outraged that more than half of all agricultural land was being "wasted." But in reality, the yield of all that land was ridiculously low anyway and not worth it. We'd be better off leaving nature to do its thing on that land, allowing it to support free-living animals and rich ecosystems that include native flora and fauna and robust trophic cascades including large predators (rather than mere remnants of those living systems) and are therefore truly thriving.
To insist that we instead monopolize such land by exploiting domesticated breeds of non-native species when it could be far better and more ethically used otherwise – including, for example, by native bison which are instead routinely shot because they “compete for resources” with cattle ranchers – is just illogical.
Can marginal land really not be used for vegan food?
To make up the small amount of protein and calories that grazing ruminants on marginal lands contribute to the global food supply, it wouldn't be hard to use some marginal lands to grow vegan crops. The idea that poor quality soils can only support animal production is a widely held yet non-supported assumption. Examples of hardy, human-edible plants that grow in difficult conditions include leafy greens, fruit, roots, buckwheat, rye, barley, quinoa, amaranth, and several leguminous plants that grow in difficult conditions. (Detailed by a soil scientist here.)
This includes places that appear to be non-fertile sand. After all, the number one use of irrigation water in the dry and arid West is for growing field sand fields of alfalfa – used mostly as feed for dairy cows!
In “Tree Crops: A Neglected Source of Food and Forage from Marginal Lands,” by L. H. MacDaniels and Arthur S. Lieberman, it's noted that:
Although seldom discussed in relation to world food production, tree crops on rough and marginal lands have a potentially valuable role to play in supplementing traditional cereal and grain crop agriculture. Perennial tree crops, once established, offer the prospect of controlling erosion and a sustained yield of food and forage obtained without excessive mechanization. There are very large areas now producing very little that would, if properly managed, support tree crops.
What about other uses?
The rest of that land could be returned to wild habitats performing invaluable ecosystem services. The study "Grazed and Confused?" written by an international team of experts and published by Food Climate Research Network, stated, "While ‘grassfed’ animals may not be dependent on arable-based feeds, the supposition that they are using spare land that could not be used for something else is mistaken... land that is used to graze animals could potentially be used for something else – for food, for nature conservation, for forests, or for bioenergy."
Less land needed by vegan diets
The idea that land currently being used for agriculture supposedly can't be used for plant-based agriculture becomes irrelevant anyway when we acknowledge that a vegan shift would require astronimcally less land for food production.
The most comprehensive analysis to date of the damage farming does to the planet, published in the journal Science in 2018, found that without meat and dairy consumption, global farmland use could be reduced by more than 75% – an area equivalent to the US, China, European Union and Australia combined – and still feed the world.
Research published in Climactic Change found that substituting beans for beef alone would free up 42% of US cropland.
A plant-based shift would negate the false sense of need to use every square inch of land possible for food production, a false premise the marginal lands argument hinges on.
Esteemed biologist and National Geographic Hubbard Award winner E.O. Wilson has outlined a plan to set aside half the planet for conservation, which he says would conserve 80 to 90 percent of all species on Earth. He says this can only happen "if people eat significantly less meat and livestock products, a trend that is currently going in the opposite direction globally." Remember, animal agriculture currently occupies nearly half the global surface area, mostly for incredibly small yields that a vegan shift could put to shame.