Ecology

While ecology is 10% of the IBO test and National olympiad tests, most questions are fairly easy and involve material right out of Campbell's. Things not covered in Campbell that shows up on occassion are life tables, game theory, and statistics. Life tables and basic statistics show up often in semifinals tests, though game theory is often more reserved for National Finals tests.

Sparknotes gives a good summary of Behavorial Ecology, a mix between ethology and ecology, that isn't too well covered by Campbells.

The Wikipedia page on Evolutionary Game Theory sums up most of what you'd need to know at the semifinals or finals level of USABO.

This PDF covers Population Tables. These have shown up a lot of USABO semifinals tests, especially in the FRQ section.