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.

Targeted gene flow is an emerging conservation approach which involves introducing a cohort of individuals with particular traits to locations where they can effect a conservation benefit. This technique is being proposed to adapt recipient populations to a known threat, but questions remain surrounding how best to maximize conservation outcomes during periods of continuous directional environmental change. Here we introduce a new management objective — to keep the recipient population extant and with maximum diversity of local alleles — and we explore how varying the timing and size of a given introduction can maximise this objective. Our results reveal a trade-off between keeping a population extant and maintaining a high level of genetic diversity, but management levers can often optimize this so that nearly 100% of the allelic diversity is preserved. These optimum outcomes sets are highly sensitive to the predicted rate of environmental shift, as well as the level of outbreeding depression in the system.