Empirical evaluation of the strength of interspecific competition in shaping small mammal communities in fragmented landscapes

Theory predicts that habitat loss and fragmentation may have drastic consequences on species' interactions. To date, however, little empirical evidence exists on the strength of interspecific competition in shaping animal communities in fragmented landscapes. Our aim was to measure the degree of ongoing competitive interference between species in fragmented landscapes. Our model system was the community of ground-dwelling rodents in deciduous woodlands in central Italy, composed of a habitat generalist species (Apodemus sylvaticus) and two forest specialists (Apodemus flavicollis and Myodes glareolus). Our objectives were to test whether species were segregated among forest patches and whether spatial segregation was determined by interspecific competition or habitat and resource availability. We surveyed the populations inhabiting 29 woodland patches in a highly fragmented landscape using a capture-mark-recapture protocol, capturing > 4500 individuals. First we modelled species' distribution as a function of habitat, resource availability and landscape variables. The second stage of our analyses involved measuring the response of vital rate parameters (body mass, reproduction, survival, recruitment, population density) to competitor density. The relative distribution of species reflected a spatial segregation of habitat generalists and specialists according to habitat quality, cover and connectivity. Interspecific competition mainly affected individual level vital rates, whereas we found no substantial effects at the population level. Competitive exclusion of specialist species by generalist species was occurring. However, when compared to other factors such as habitat connectivity and resource availability, interspecific competition played a relatively minor role in shaping the studied community.