Population substructure, local density, and calf winter survival in Red deer (Cervus elaphus)

Population substructure and the effects of scale have recently received much theoretical attention, but few studies have examined these factors in free-living populations of vertebrates. We used > 200000 sightings of recognized females recorded over a continuous 20-yr period to explore population substructure and spatial heterogeneity in red deer on the Isle of Rum, Scotland. We used hierarchical cluster analysis to group individuals together by their proximities in space, and we explored the influence of scale, considering scales ranging between the whole population and groups of one or two individuals. Intermediate scales were isolated as being the best at describing calf winter survival, the key factor in determining future population density. The most statistically explanatory scale isolated a population substructure related to vegetation, with higher local densities occurring around herb-rich Festuca-Agrostis grassland. Calves at high local density were most likely to die. Patterns of local population density varied between seasons in relation to food availability. High-resolution scales were the best descriptors of calf winter survival in summer; coarser scales were better in winter. In both summer and winter, local population density was more important than total population density in influencing calf winter survival. The effects on calf survival of local population density during the summer interacted significantly with calf sex and the mother's reproductive status. In this study, the technique of grouping animals by their proximity in space was more realistic and informative than discrete spatial divisions of the study area.