As fertile as the land around Athera might be, there are certain limitations to how crops grow. Potatoes might only grow in the colder climates of the mountains, whereas melons and pumpkins only flourish in the dense jungles of Zarakam.
The Atheran people have adapted to this, however, and through several tricks of both science and magic, have created a universal wheat seed that can grow no matter the climate. The one draw back to their experiments, however, are that all crops require direct sunlight to grow.
Furthermore, much like the crops, certain animals only find natural homes in certain biomes, such as cows in the Plains of An'She and pigs in the Dunsmoor Hills. Much like the Atheran people, chickens also adapted to the universal wheat seed. With only a small need of living space and having plentiful food sources, chickens can be found and bred all around the world.
It would seem, however, that animals in close proximity fall ill as disease spreads quickly amongst them. The most experienced of farmers know to let their animals graze in large fields to avoid this epidemic.