Context . Since the introduction of fallow deer (Dama dama) to Tasmania in the early 1830s, the management of the species has been conflicted; the species is partially protected as a recreational hunting resource, yet simultaneously recognised as an invasive species because of its environmental impact and the biosecurity risk that it poses. The range and abundance of fallow deer in Tasmania has evidently increased over the past three decades. In the 1970s, it was estimated that ∼7000–8000 deer were distributed in three distinct subpopulations occupying a region of ∼400 000 ha (generally centred around the original introduction sites). By the early 2000s, the estimated population size had more than tripled to ∼20 000–30 000 deer occupying 2.1 million ha. No study has attempted to predict what further growth in this population is likely.

Aims . The purpose of our study was to provide a preliminary estimate of the future population range and abundance of fallow deer in Tasmania under different management scenarios.

Methods . We developed a spatially explicit, deterministic population model for fallow deer in Tasmania, based on estimates of demographic parameters linked to a species distribution model. Spatial variation in abundance was incorporated into the model by setting carrying capacity as a function of climate suitability.

Key results . On the basis of a conservative estimate of population growth for the species, and without active management beyond the current policy of hunting and crop protection permits, abundance of fallow deer is estimated to increase substantially in the next 10 years. Uncontrolled, the population could exceed 1 million animals by the middle of the 21st century. This potential increase is a function both of local increase in abundance and extension of range.

Conclusions . Our results identify areas at high risk of impact from fallow deer in the near future, including ecologically sensitive areas of Tasmania (e.g. the Tasmanian Wilderness World Heritage Area).

Implications . The research approach and results are presented as a contribution to debate and decisions about the management of fallow deer in Tasmania. In particular, they provide a considered basis for anticipating future impacts of deer in Tasmania and prioritising management to mitigate impact in ecologically sensitive areas.