What is the best way to deal with being undermined?

Go to the person first. This may have been a common practice with the guy previous to you and has now become a habit they don't even realize they are doing. Always stay calm and don't ever sound over bearing. Make it seem as if you just noticed it and that you were wanting to work out some kind of system to let you know how much man power you have to complete your tasks. Then when the time comes, if you can't spare someone be honest with them.