propose a hypothesis that goes beyond the simple (but acceptable) one above. Something that takes advantage of the various competition and predation connections.
Suppose both organisms A and B eat organism C and organism B also eats organism D. A creative hypothesis might be, “Increasing the population size of A will result in organism B eating more of organism D.” (Rationale: As A increases, it eats more C, which is also a food resource for B so B will have to eat more D.) You can test this by changing the population size of A and measuring the gut contents of B.
Or, maybe come up with a hypothesis that involves moving organisms. Maybe A eats B and you’re curious to see if the distribution of A (i.e., clustered versus spread out) changes the impact on B. Of course, you’ll need a rationale to propose this.