New Guinea is really unusual because it doesn’t have any large native carnivores, unless you count crocodiles, which aren’t present in the highlands. When humans arrived about 60,000 years ago we became, in effect, the large carnivores. It’s possible for an exosystem to evolve without significant carnivores. The Ediacaran biome seems to have been on, as was Mauritius before 1600. So an alien ecology without carnivores could be interesting. This doesn’t mean there’s not competition. It’s just along different lines. Also most modern ecosystems pretty much depend on carnivores to stay stable. This is a thought experiment.