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

Abstract

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

The effects of seed availability on habitat use by a specialist seed predator

“…Densities of red squirrels in montane spruce-fir forests are low and comparable with other types of "low quality" conifer habitats in Europe (Gurnell 1987;Lurz et al 1995;Wauters et al 2008). However, space-use patterns in lowdensity populations are rarely investigated (Lurz et al 2000;Bryce et al 2002;Dahle and Swenson 2003).…”

Estimating offspring production using capture‐mark‐recapture and genetic methods in red squirrels

“…Two sites in the Cogne (COG) and Rhemes (RHE) Valleys of the Gran Paradiso National Park (western Alps), and four sites in the Valtellina Valley, Central Alps: Cedrasco (CED), OGA, Valfurva (VAL), and Bormio (BOR). Details of the study populations, squirrel density, landscape, tree species composition and specific location and distances between study sites have been reported previously (e.g., Trizio et al 2005;Wauters et al 2008). The social system of this species is based on, predominantly intra-sexual, hierarchical relationships formed through agonistic encounters between both resident and immigrant individuals, and intrasexual territoriality among adult, reproducing females Dhondt 1989, 1992;Wauters et al 1990).…”

“…Hence, not all juveniles were marked before reaching age at which dispersal starts (4-5 months old, Dhondt 1993, 1995) and might have been classified incorrectly as immigrants. To estimate whether and to what extend classification of individuals as locally born offspring, or as immigrants, using CMR (e.g., Wauters et al 2004Wauters et al , 2008 was correct, we compared their assignment by CMR with genetic offspring assignment using CERVUS.…”

Density of red squirrels and their use of non-native tree species in the Rogów Arboretum

“…its diet depends on the environment where squirrel is found and also changes throughout the year depending on food availability (Krauze-gryz, gryz 2015). however, red squirrel more highly prefers seeds of coniferous and broadleaved trees, whilst squirrel population density fluctuates depending on the seed availability (lurz et al 1995;Wauters et al 2008). red squirrel population densities are higher in places where food availability is steady throughout the year (Wauters et al 2008).…”

“…however, red squirrel more highly prefers seeds of coniferous and broadleaved trees, whilst squirrel population density fluctuates depending on the seed availability (lurz et al 1995;Wauters et al 2008). red squirrel population densities are higher in places where food availability is steady throughout the year (Wauters et al 2008). During the periods when main food supply of tree seeds is low, red squirrels try to locate the more productive tree groups within their forest, increase their home ranges or migrate (lurz et al 2000;Wauters et al 2005).…”