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The arrival of novel predators can trigger trophic cascades driven by shifts in prey numbers. Predators also elicit behavioral change in prey populations, via phenotypic plasticity and/or rapid evolution, and such changes may also contribute to trophic cascades. Here, we document rapid demographic and behavioral changes in populations of a prey species (grassland melomys Melomys burtoni, a granivorous rodent) following the introduction of a novel marsupial predator (northern quoll Dasyurus hallucatus). Within months of quolls appearing, populations of melomys exhibited reduced survival and population declines relative to control populations. Quoll‐invaded populations were also significantly shyer than nearby, quoll‐free populations of conspecifics. This rapid but generalized response to a novel threat was replaced over the following 2 yr with more threat‐specific antipredator behaviors (i.e., predator‐scent …