Within‐sex density dependence and population dynamics of red squirrels Sciurus vulgaris

Abstract

Summary1. Social organization and dispersal of red squirrels ( Sciurus vulgaris L.) differ between sexes, and intrasexual competition is intense. Therefore, we predicted that demographic parameters should be gender-specific: that is density-dependent factors will be more strongly related to density of the same sex than to density of the opposite sex. We studied the relative importance of within-and between-sex density-dependent factors and of density-independent factors (habitat type, food abundance, winter temperature) on different demographic parameters, in two populations in northern Belgium. 2. Spring density of males was positively correlated with tree-seed abundance in the previous year, but this was not the case for females. None of the population parameters we measured differed between habitats, indicating that the same density-dependent and density-independent mechanisms prevailed in coniferous and deciduous habitat. 3. Within each sex, we found several demographic parameters that were dependent on the densities of the same sex; however, none of these parameters was found to be dependent on the density of the opposite sex. 4. Reproductive rate increased with food abundance and decreased with female density. Adult survival of females decreased with female density in autumn-winter, while survival of adult males in spring-summer increased with the size of the previous year's seed crop. 5. Immigration rate of males was higher in spring than in autumn, and spring immigration increased with food abundance. Male recruitment rate, in both seasons, increased with food abundance, but was male density dependent. However, springsummer loss rates also increased when food supplies were good, suggesting that despite high food availability, emigration of juvenile and subadult males increased when intrasexual competition was intense. Recruitment rate of females decreased with increasing female density. After a good seed crop, more subadult females dispersed, but their settlement success (recruitment) was lower at high female density. 6. Seed crop size positively affected red squirrel densities through increased reproduction, immigration and adult survival of males, but density-dependent reproduction and withinsex density-dependent recruitment of locally born juveniles and dispersing subadults limit the fluctuations in numbers and regulate densities in winter-early spring, as well as in summer.

Dispersal and habitat cuing of Eurasian red squirrels in fragmented habitats

“…Pregnant and/or lactating females were radio-collared using SR-1 and TW-4 transmitter collars (±12 g, 9 and 18 months lifetime, respectively; Biotrack, Wareham, UK) to locate the nursing nest . We used the minimum number of animals known to be alive (MNA), from trapping, radio-tracking or observations, as our estimate of population density (Verbeylen et al 2003(Verbeylen et al , 2009Wauters et al 2004). This allowed us to determine densities encountered before and after dispersal for those squirrels that left the natal woodland in site SKW.…”

“…Food abundance, expressed as seed-crop size of food-producing trees (see Wauters et al 2004 for seed count methods), varied between sites and over years (Table 1). In CON, conifers produced a mast-crop in 1991 and a medium seed-crop in 1992, while in SKW food abundance varied over years with a mast crop (oak and beech; G. Verbeylen, unpublished data) in 1996 and medium seed-crops in 1997-1999 (Table 1).…”

Anticipatory Reproduction and Population Growth in Seed Predators

“…During years of low seed production, competition among juveniles for available resources is intense, and, although litter augmentation experiments in American reds show that females are capable of supporting larger litters (30), they refrain from doing so because offspring recruitment is low (30). However, when mast years occur, competition among juveniles is reduced, and females produce more offspring, which successfully recruit into the population (31,32). Further, the increased production of young by females in mast years does not come with any obvious cost to the female, because overwinter survival is not reduced after years of increased reproductive investment (for American reds, offspring production in the previous year versus proportion of adult females surviving to spring has slope = -0.023 ± 0.034, t 15 = -0.7, and P = 0.51; for Eurasian reds, proportion of estrous females in the previous year versus adult female survival to spring has slope = 0.37 ± 0.27, t 15 = 1.4, and P = 0.19).…”

The use of species‐specific cone remains of sympatric arboreal rodents to monitor their distribution

“…(Council Directive 92/43/EEC). Finally, the Eurasian red squirrel, S. vulgaris Linnaeus, 1758, is being replaced in parts of its range, in the British Isles and areas in northern Italy, by the alien grey squirrel, S. carolinensis, a species introduced from North America that, in Europe, occupies a similar niche as the native congener (Gurnell & Pepper 1993;Lurz et al 2001;Wauters et al 2001Wauters et al , 2002aWauters et al , 2002bWauters et al , 2004. Recently the Finlayson's squirrel Callosciurus finlaysonii (Horsfield, 1824) has been found colonising an area along the Tyrrhenian coast in southern Italy (Aloise & Bertolino 2005;Amori & Aloise 2005).…”

“…In areas where conifers are common, cone line feeding transects have been used to study squirrels in Britain, Belgium and Italy (Moller 1986;Wauters & Dhondt 1987Wauters & Lens 1995;Lurz et al 1998Lurz et al , 2000Gurnell et al 2004;Wauters et al 2004). In areas where both red and grey squirrels occur, this technique does not allow differentiation between the two species since the two congeners leave similar-shaped stripped cones after feeding on the seeds (Gurnell et al 2004).…”