Rainbow is right about all those critters, but don't be deterred by them. They just do their thing in there and that's where the magic comes from.
At my office we compost all our coffee grounds, food scraps etc. The large pallet 'finishing' bin that gets the partially-composted stuff out of the black plastic bins twice a year, has a plastic tarp cover to manage the moisture. Sometimes, in spring, keeping warm under that tarp, we find a harmless but large black snake, waiting for a mouse lunch. We don't bother him and he doesn't bother us. I don't advertise that to *all* the volunteers, though. They don't use that particular bin and don't need to know.
Bottom line, all these guys are around your yard anyway, you just don't see them. I've seen a cat catch mice in the most manicured, Better Homes and Gardens suburban lawn you have ever seen. They're out there.