I second the Daphnia suggestion. I'm not sure if you want your water predator to be eating things like guppies and shrimp. It could make for a messy dinner, and a "denser" bioload in terms of waste. I'd personally try to find mayfly nymphs. They make a great food source for lots of fish, and the ones that escape turn into short-lived flies that can be eaten by your land predator. Red cherry shrimp are also good cleaners, and their fry are readily consumed. As far as the terrestrial portion, you may want to include insects like springtails and/or isopods (I know that tropical fish love springtails). Be careful of which animal species you use. If you have herps in there, you don't want them infecting any amphibians that can visit dry land (or vice versa).
In my (very humble) opinion, it's almost impossible to make a fully self-sufficient enclosure that includes significant amounts of water. Your aquatic animals may survive, but I still argue that PWC's are essential to healthy aquatics. One of the main reasons why Mother Nature doesn't need water changes is because said water is continually being recycled, rained down, or otherwise circulated in new ways around the world. You never step into the same stream, so to speak. Your tank water, on the other hand, is "stuck with itself". Now, you may be thinking that many successful, self-sustaining ecosystems like dart frog tanks have water features and get along fine. I'd like to point out that most people who have small ponds, etc., in their terrariums keep them that way for ease of water removal. At least, that's what I've seen in the majority of pond-inclusive builds that I have witnessed. I also don't mean to imply that you need to change water all the time; it could very well be occasional or infrequent, depending on how well you filter your water.
I guess what I'm trying to say is that if you plan on doing a land/water tank, you should try to design it assuming that you will need to do PWC's, even if infrequently. I don't know your experience level, so if you happen to be a long-time hobbyist, then I could very well be missing part of the equation. Just in case you're relatively new, though, I wanted to offer my $0.02.