Relative influence of inter- and intra-specific competition in an ungulate assemblage modified by introductions

Inter-specific competition from introduced and naturally-colonising species has potential to affect resident populations, but demographic consequences for vertebrates have rarely been tested. We tested hypotheses of inter- and intra-specific competition for density, body mass and fertility of adult female roe deer Capreolus capreolus across a heterogeneous forest (195 km 2 ) landscape also occupied by introduced Mediterranean fallow deer Dama dama and sub-tropical muntjac Muntiacus reevesi. Species-specific deer densities in buffers around culling locations of 492 adult female roe deer (sampled over seven years 2011-2017), were extracted from spatially-explicit Density Surface Models calibrated through extensive annual nocturnal thermal imaging distance sampling. Roe deer fertility and body mass were related to local species-specific deer 23 densities and local extent of arable lands (that provides nutritious food) using Piecewise Structural Equation Models. Muntjac density (mean=15.1 individuals.km-2 SD=7.6) was lower at higher fallow deer densities (inter-quartile effect size, IQ=-2.4 individuals.km-2), suggesting inter-specific avoidance (interference), but was greater when buffers included more arable (IQ=+1.01 individuals.km-2). Roe deer body mass (13.7kg, SD=1.52) was marginally greater when buffers included more arable (IQ=0.32kg) and was independent of deer densities. However, roe deer fertility was unrelated to body mass, suggesting fertility benefits of condition exceeded an asymptotic threshold in this low-density population. Consistent with this, roe deer fertility was slightly greater (not reduced) in areas with greater local roe deer density (IQ=+0.9% probability of two instead of one or zero embryos), again indicating negligible intra-specific competition. In contrast, roe deer were less fertile in areas with greater muntjac densities (IQ=-14%), thus inter-specific exceeded intra-specific competition in this assemblage. In contrast, we found no support for any effects of fallow deer density on roe deer density, body mass or fertility. Complex networks of inter-specific competition operate in this deer assemblage. For muntjac, interspecific interference from fallow deer exceeded habitat effects. For native roe deer, inter-specific competition from introduced, smaller sedentary muntjac reduced fertility, unlike intra-specific or potential competition with larger, more mobile, fallow deer. Mechanisms may include behavioural interference or stress; resource depletion is considered less likely as roe deer fertility was independent of body mass. Findings emphasise the importance of ensuring appropriate management strategies for controlling invasive species.