The pigs in most cases are eating food raised on other farms. Meat is incredibly taxing of natural resources.It depends. In our climate where we have 6 month of winter all these huge prairies are a wasteland for half the year anyway. You can't grow anything there but raise cattle. So no crops would out compete free ranging cattle when it comes to calorie output per square ft in Alberta anytime soon. And no matter the climate no crops or vegetables would produce as much calories per SQ ft as pig farm. It would be different if human population would be declining but with numbers going up to maintain enough food humanity will depend on animal farming for very long time unfortunately.
If you wanna make an argument about pigs (and/or goats) eating waste, that is a different conversation. But the overwhelming majority of meat is sustained from ranching (at odds with natural flora/fauna) or commercial feed.
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