Anyway, our practice problem deals with the snakehead fish. If you don't know what that is, it is an invasive species from Asia & Africa that is pretty much a frankenfish. It eats almost everything in its spare time when it's not reproducing.
In other words, it's one of the most student-engaging invasive species to ever untangle a food web.
You can check out our plan here. Last year I used the snakehead fish to hook students into some of our Ecology objectives; all we did was revise that original plan a bit (since students did find it so interesting last year) and turned it into a problem to solve instead of something to read and watch to prepare them to do more reading and watching. We wanted them talking and discussing and thinking and solving after the reading and watching.
Judging by all the excited and on-task chatter in the room (I had to veto a solution involving assault rifles, it was getting that "engaged"), I'd say our practice problem is giving students the thinking and problem-solving practice they need. One key, I think, was choosing the right organism with the perfect amount of freak-out factor--an advantage of teaching biology is that "gross" usually translates into engagement.
The other key, of course, is choosing the right problem for the students in front of you.