Fantasy setting. Several generations ago-- I've not settled on an exact number of years-- there was a great magical war in which one side sought to corrupt the other side's land, turning all that lived on it into monsters. The spell went out of control, and the corrupting energy spread to cover all the world except the city-states that managed to raise wards before the energy crept over their land. Even with the wards, most of the population of the world died-- from the war itself, disease, starvation, riots, and countless other factors-- and humanity entered a long dark age from which it is just starting to recover. In recent years, as the old magics begin to be rediscovered and the spell of corruption starts to reach the end of its long duration and fade, the new mages have started to expand the wards and purify land, allowing for more population growth.
(Google search one: how much farmland does it take to feed a person? This, I got an answer to.)
The problem is, these spaces are relatively small-- I've yet to do calculations on how many people make up a sustainable population as a baseline so I can work out the current size of any city-states that survived the dark age, but I doubt it would be more than a day or so of travel from the core of a city-state to the borders of the ward. There is a small amount of trade between city-states, but this is difficult and dangerous-- it usually involves caravans with guards to fight off corruption-maddened monsters and mages casting wards over the vehicles to keep the traders from being mutated.
My concern is that any animals that strayed outside the ward would, in theory, be corrupted, meaning that roaming or migrating species wouldn't stand a chance. Assuming that I don't just handwave and say magic did it or start making up animals, how would I start to find out what animals and plants could survive in such conditions? Would losing some species-- such as bees, as I've often heard since the recent crisis with hives dying like... well, flies-- have a devastating effect on the food supply (possibly corrected by magic, possibly devastating?)
I have no idea what to research to even try to answer this one. Thank you for any help you can provide.