The arrival of novel predators can trigger trophic cascades driven by shifts in prey numbers. Predators also elicit behavioural change in prey populations, and this may also contribute to trophic cascades. We document rapid demographic and behavioural changes in rodent populations (grassland melomys) following the introduction of an ecologically novel predator (northern quoll). Within months, melomys from quoll-invaded populations suffered reduced survival relative to quoll-free populations. They also exhibited increased shyness which became fine-tuned to more threat-specific antipredator behaviour. These behavioural shifts were associated with lower per-capita seed take, and avoidance of quoll-scented seeds. These behavioural shifts could reflect phenotypic plasticity or may be adaptive responses to selection imposed by predation. Overall, our study reveals rapid numerical and behavioural shifts in response to a novel predator and shows that both behavioural and numerical responses can drive trophic cascades.