Demography of alpine red squirrel populations in relation to fluctuations in seed crop size

Vertebrate population dynamics, social organisation and space use often are closely associated with the distribution of critical resources, such as food. Tree squirrels are ideal models to study these relationships, since both key demographic parameters (reproduction, survival and dispersal) and spatio-temporal variation in food supplies (measured as seed-crop size) can be reliably estimated. In this paper we test the following two predictions underlying the association between annual food abundance and demography in six alpine red squirrel populations, both with and without time-lag effects: 1) between-season and between-year fluctuations in survival rate, population density and increase parallel those in food availability; and 2) individuals follow a resource tracking strategy and increase in density mainly the year after a rich seed-crop. Red squirrels occurred at higher densities in Scots pine forest, characterised by stable seed-crops, than in Norway spruce with more abundant but more variable seed crops. Fluctuations in numbers were positively correlated with food availability, measured as annual conifer seed-crop sizes. Overall, adult survival rates were higher than those of subadults, and survival substantially fluctuated between seasons and years. Autumn densities and rates of population increase (summer-autumn) were strongly correlated with the same year's autumn seed-crop, while correlations with the previous year's seed-crop (time-lag models) were either weak (population density) or absent (population increase). Results of this paper show that fluctuations in red squirrel densities in habitats with strong temporal variation in seed production are more closely linked with food availability than in more stable habitats. In addition, in the Alpine conifer forests squirrel population sizes, in autumn, increase in synchrony with food resources, eliminating the population lag normally present when resources are produced in pulses.