As I explained in my entry on opossums a while back, I don't know of any real negatives about them (other than their appetite for expensive bird seed), so I tend to let them be in my yard.
Now I've learned of a true benefit to having them around!
Debbie Roberts wrote an interesting blog article for Native Plants & Wildlife Gardens, "The Link Between Lyme Disease and Biodiversity." In it she detailed information she'd heard at a lecture by Dr. Richard S. Ostfeld of the Cary Institute of Ecosystem Studies. (She's got the links to both Dr. Ostfeld and the Cary Institute, so if you're interested in these topics, I'll let you link from her blog post.)
One of the tidbits of information that I learned in reading her post was that opossums are great at grooming off (and eating? killing?) ticks. On average, they groom off 96.5% of the ticks that attach to them, compared to white-footed mice, which leave 50% of ticks on their bodies. Extrapolating from this, opossums seems to act as "tick magnets" or "tick vacuums" in their home areas.
I've wondered why we don't seem to have much of a tick problem out here. Perhaps at least part of the puzzle is that we do have opossums. (Rereading my opossum post, I had read a suggestion of this, but the information from Dr. Ostfeld puts "legs" under that suggestion.)
Chasing this line of thought a little further.... According to Dr. Ostfeld, white-footed mice seem to act as a Lyme disease reservoir. So, logically, any animals that decrease white-footed mice population numbers will, by default, act to decrease the reservoir of Lyme disease in the area. Score a big win for owls, hawks, fox, snakes, coyotes, and other rodent predators!!!
Biodiversity is a good thing. Seen any snakes or coyotes in your yard recently???