It is currently based on un-loading and re-loading the map.
This is very stupid.
Respawning rate should not be dependent on that, animals and plants should respawn regardless of whether the map is loaded or not.
Of course, maybe not appear into the screen from nothingness, if there is a player nearby, but if its outside of view distance it should be enough.
Not sure about technical details, but something like spawning new things every time when a new part of map is sent to the player view distance sounds good.