Effects of Food Availability and Density on Red Squirrel (Sciurus Vulgaris) Reproduction

Density—dependent reproductive rate may be the result of all individuals in a population suffering reduced fecundity when density increases (Lack 1954), or it may be the result of a lower proportion of individuals living in favorable places at high density (Andrewartha and Birch 1954). We used a key—factor analysis to investigate whether reproductive rate in two red squirrel (Sciurus vulgaris) populations was density dependent. Four k values were determined, indicating offspring mortality caused by: adult females not entering oestrus (k₁), oestrous females not giving birth (k₂), lactating females failing to wean offspring (k₃), and mortality in litters of successful females (k₄). We also investigated the effect of food availability on the different k values and tested predictions of the two contrasting hypotheses (Lack vs. Andrewartha and Birch) by examining the mechanisms of density dependence. The reproductive performance of female squirrels was monitored over 18 breeding seasons in a coniferous forest and over 16 breeding seasons in a deciduous forest. In the coniferous forest, annual variation in pine seed abundance only affected k₄, which was only weakly related to total mortality of offspring (K). In the deciduous woodland, however, the key factors k₂ and k₃, which were strongly related with total K, were inversely correlated with food abundance. In the coniferous forest, only k₂ was female density dependent, and breeding failure before parturition occurred primarily among floaters, which were more numerous at high density. In the deciduous forest, annual and seasonal variation in the number of floaters caused density dependence in fecundity (k₁). Increased breeding failure of lactating females (k₃) with high female density was positively correlated with the number of floaters and with the proportion of females occupying poor quality home ranges. We concluded that density—dependent reproduction in red squirrels can be caused by two mechanisms: a direct density effect of more floaters at high female density, and an effect of habitat heterogeneity, with more females occupying poor territories at high densities, resulting in low breeding success. That reduced fecundity or breeding failure at high densities occurred mainly in female squirrels not occupying good territories, and that annual and seasonal variation in litter size of successful females was not density dependent, supported the predictions of the Andrewartha and Birch (1954) hypothesis.