A comparison of Eurasian red squirrel distribution in different fragmented landscapes.
Simple logistic regression models empirically derived in other fragmented landscapes (in the Netherlands and Italy) were explored for their ability to predict the distribution of red squirrels (Sciurus vulgaris) in fragmented woodlands in Sweden. Comparisons between predictions led to identifying thresholds in fragment size (> 10 ha) and distance to a source (<600 m) where the probability of squirrel occupancy was at least 0.9 in all landscapes. These values may reflect squirrel minimum habitat requirements for home range and dispersal in the worst study area. For fragments <10 ha (outside shared thresholds), models developed in one landscape could predict squirrel occupancy elsewhere only in 17% of cases. The predictive ability for small fragments also improved when the range of fragment sizes in the area of observation fell within the range of sizes in the area where the model was developed. Some models gave correct between-year predictions of squirrel distribution in the same landscape despite noticeable changes in regional squirrel population density. When size and distance thresholds were met, the models could be used successfully elsewhere. In addition, threshold values indicated how large forest fragments should be and how they should be arranged to favour squirrel occurrence in a landscape.