Mid-continent snow geese historically wintered on coastal marshes with limited food. The spread of intensive agriculture across their migration corridor gave them waste grain and rice on a scale marshes never offered, improving survival and body condition at the moment when nutrition most affects breeding.
The increase played out on the breeding grounds. Snow geese feed by grubbing, pulling roots and rhizomes out of the ground rather than clipping the tops, so heavy use kills the plants outright. Around long-used colonies on Hudson Bay, extensive areas of salt marsh were converted to bare sediment that evaporates into a salt crust and resists regrowth for decades. Wildlife agencies responded with special conservation measures liberalising hunting to reduce the population.
The complication is that the response has been mixed. Harvest increased but did not clearly reverse the growth, and researchers continue to argue about how much of the damage is now self-perpetuating and whether hunting alone can be the remedy.