Modifications to the idea
Decided to change the concept a little...I decided the 'farm' theme was way too boring. So now you are a new employee of a heartless multi-national corporation. You are testing out their new automated critter-killing drones to clear out critters trying to consume a new alien and highly profitable mineral growing around the earth. Unfortunately the minerals effect on the critters causes them to get stronger once enough of them are killed, willing them to demand retribution against the killer (that's you).