So thinking about this, we've a few places to start:
1) lack of predators - I'm not sure how we can solve that one without creating further problems down the line, but I'm open to suggestions.
2) abnormal aggression - I know there's a few people whose skillset runs to soothing / mollifying, though the order of magnitude here feels like it would be an issue?
3) multiplying quickly - we know the Natives are big on their cyclical animism and fertility rites. Is there a way to provoke the opposite effect? What terrible drawbacks does that have?