Living in the mountains or in the plain: what's “best” for red squirrels?

The Eurasian red squirrel (Sciurus vulgaris) has the largest distribution of all tree squirrels: from the British Isles across the whole of Eurasia to China, and northern Japan. In northern Italy, the species occurs from fragmented and relatively isolated mixed deciduous woods of the Po plain up to the continuous subalpine conifer forests, including high-elevation marginal habitats at the timberline. Differences in habitat quality (elevation, forest structure, food availability, degree of fragmentation) can affect red squirrels at individual level (body size, body mass, presence of parasites) and at population level (sex ratio, density, parasite prevalence). In Italy, the long-term survival of the red squirrel has become a major conservation issue since range expansion of introduced invasive grey squirrels (Sciurus carolinensis) is causing extinction of local red squirrel populations. Having co-evolved with conifer species, it has been suggested that the native species may have an advantage over the alien congener in extensive pine or spruce forests, being better adapted to feed on the small conifer seeds. Here we try to answer the following question: is there any evidence, at the individual or population level, that red squirrels are doing better in conifer forests in the Alps than in mixed deciduous woods of the Upper Po-plain? We studied red squirrels in 6 study areas that follow a gradient from high elevation subalpine conifer forests to lowland mixed deciduous woods, including an urban park. Of the subalpine forest, two are “edge” habitats with regard to the vertical distribution of the species. In all areas squirrels were studied by capture-mark-recapture. Each trapped squirrels was individually marked and information on sex and body condition were taken. To determine whether a squirrel was infected by the most common intestinal helminth (Tripanoxyuris sciuri), a tape-test was done at each capture to look for parasite eggs. We had 277 trapping events of 174 different squirrels. Body size and body mass differed between areas: squirrels were smaller and weighed less in the subalpine “edge” habitats than in the two lowland mixed woods. There were no differences between the sexes in either body size or mass. Thus, in subalpine conifer forests near the timberline red squirrels are smaller and weigh less than in lower elevation conifer forests dominated by Norway spruce (Picea abies) and than animals occurring in lowland mixed deciduous woods and parks. There was no effect of an individual’s body mass on the probability of being infected by T. sciuri, but male squirrels were more likely to be infected than females. At the population level, parasite prevalence did not differ significantly between study areas. However, in the urban park (lowest prevalence) fewer squirrels were infected than in the lowland area with the highest prevalence. Population densities varied over time and between areas: squirrels lived at lower densities in the two “edge” habitats than in the Norway spruce forest and in the lowland mixed deciduous habitats. Our results support earlier work on alpine populations which claims that differential selection for smaller or larger animals in different habitats is linked to seed-size and to temporal variability in seed production. This implies that loosing red squirrel populations in lowland deciduous woods due to replacement competition by alien grey squirrels