I don't know for sure, but off the top of my head I can come up with two reasons: 1. Bacteria can be dangerous (for the animals, for us) 2. Birds are beautifu

I don't know for sure, but off the top of my head I can come up with two reasons: 1. Bacteria can be dangerous (for the animals, for us) 2. Birds are beautiful and squirrels are adorable (esp. with their their fuzzy tails). A pool of sludge is repulsive. So, uhm, I like animal growth better for personal, subjective reasons :)