That might also have to do with the pig spawners people set up. That might be taking up all the passive animal slots.
Spawners activate on a player getting within a range of 16 blocks of it.
From what I know, the game populates mobs until limits are met. (within a certain range of chunks, that is) Passive mobs usually stop at around 20, whereas hostile mobs get close to 200. That's how it is for singleplayer, at least...
I had to stick to the Sphere world to farm my sheep. They were much more abundant there due to the smaller "legal" spawning surface area.
--Warning- while you were... blah blah--
There's less people than normal in the server. Is there something wrong in the server?
Server is fine now. Spawn rates are somewhat normal too.
Just now finished the UFO for the most part. I'm taking a break for a bit before I tidy up other things around the area. As for others, not sure... Sphere world is pretty popular with them.