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The roe deer of Mediterranean habitats in the central and southern parts of Italy has recently been recognised as a distinct subspecies, Capreolus capreolus italicus. A population of this endangered subspecies has been monitored in the Preserve of Castelporziano, near Rome, since 1988. We observed an abrupt population decline in 2000, which may severely threaten the sustainability of this population. We evaluated the hypothesis that competition by fallow deer may be a principal cause of this decline. By a new and innovative methodology, we modelled the spatial distribution of fallow deer density (FDD) in the study area to show that 1) habitat quality for roe deer was an inverse function of FDD, 2) habitat apportionment between fallow and roe deer increased as a function of FDD and – by applying structural equation modelling – 3) FDD was superior to habitat composition in explaining observed variations in home range size and probably in habitat quality for roe deer. This analysis is the first to document that inter-specific competition may influence the spatial behaviour of a deer species leading to poor phenotypic performance in the inferior competitor. We conclude that the conservation of this relict population would benefit by reducing fallow deer numbers at Castelporziano and from other measures aimed to decrease the level of inter-specific competition.