Depends - are you using FFH, or one of the modmods?
FFH, mushrooms just appear, and if you try to build something on the tile, they go away. As long as they are there, they provide +2 food. Orbis made them an improvement that you have to build mines on, and will then give the mushrooms resource. Wild mana (and FF IIRC) follow the FFH scheme, while RIFE follws the orbis model here.
-Colin
