That is a very good question. I think collecting frogs from an area where this an abundance of frog won't destroy the population if you are the only one doing it. There is a local woodland pond where they are so many Green and leapord frogs you almost step on them walking along the edge of the pond. I see atl east 4-5 garter snakes with a casual look. I read that frogs are a good marker as to the cleanlyness of a pond as far as pesticides and this pond has a ton. I saw in a 2 foot area near the shore at least a hundred newly morphed frogs and toads (spade foots!) etc. So there is plenty to eat. As far as parasite load goes? I guess if it's a clean area the load would less? but not nil and I think all live stuff even pet store unless it's frozen dead mice are going to have bacteria and probably parasites because do you think they really care?. If you have a good healthy specimen of snake with a nice clean healthy enviroment I don't think they would overwhelmed but the occasional frog. My garters are wild caught and they look healthy clean and active they came form this pond area I spoke of but I feed them worms and guppies although one ate a frog on the way home because I did have a seperate container for the snake and frog ooops
Dwight