Inter-specific competition from fallow deer (Dama dama) reduces habitat quality for the Italian roe deer (Capreolus capreolus italicus)

The roe deer of Mediterranean habitats in the central and southern parts of Italyhas recently been recognised as a distinct subspecies, Capreolus capreolus italicus.A population of this endangered subspecies has been monitored in the Preserve ofCastelporziano, near Rome, since 1988. We observed an abrupt population decline in2000, which may severely threaten the sustainability of this population. We evaluatedthe 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 fallowdeer density (FDD) in the study area to show that 1) habitat quality for roe deer was aninverse function of FDD, 2) habitat apportionment between fallow and roe deerincreased as a function of FDD and by applying structural equation modelling 3)FDD was superior to habitat composition in explaining observed variations in homerange size and probably in habitat quality for roe deer. This analysis is the first todocument that inter-specific competition may influence the spatial behaviour of a deerspecies leading to poor phenotypic performance in the inferior competitor. Weconclude that the conservation of this relict population would benefit by reducingfallow deer numbers at Castelporziano and from other measures aimed to decrease thelevel of inter-specific competition.