The effects of food availability and winter weather on the dynamics of a grey squirrel population in southern England

1. The population ecology of grey squirrels Sciurus carolinensis living in a 9-ha oak Quercus robur wood in southern England was studied between 1976 and 1987 using live-trapping techniques. Trapping was carried out in winter, spring and summer. The availability of tree seeds during the autumn of each year, and the severity of cold weather over each winter were also measured to examine their effects on squirrel population dynamics. 2. Capture probabilities of squirrels in winter, and to a lesser extent in spring, were inversely related to food availability and data from these two seasons were not considered dependable. The analyses concentrated on the summer populations. 3. The long-term average summer density of squirrels was high at 8.8 ha(-1) (SE 3.41 ha(-1)) demonstrating that the oak wood was high quality habitat for grey squirrels. Over 10 of the 12 years, summer densities were remarkably similar, ranging between 7 ha(-1) and 10 ha(-1) (mean 8.5 ha(-1), SE 0.95 ha(-1)). However, numbers were driven upwards in 1977 to a density approaching 18 ha(-1) and downwards in 1982 to a density of about 3 ha(-1): a 6-fold difference. 4. In good seed years, breeding starts in December, in poor seed years the start of breeding is deferred until the spring. There was no or very little spring breeding in 5 years when food supplies were poor. Female reproductive success was positively associated with food availability. Partial correlation analysis showed that the level of association was not improved when the effects of winter weather were taken into account. The number of new adult females in the summer population was positively associated with food availability but there was no association between new males and food. This suggests that food availability is more important to breeding females than breeding males. 5. Persistence from summer to winter was positively associated with food availability, but persistence from summer to spring and to the following summer were not. Partial correlation analyses showed that the severity of winter weather tended to mask the effects of food availability on persistence and the partial correlation coefficients were higher when the effects of weather were held constant. Adult females had the highest persistence between summers (52%, n = 11, SE 6%), followed by juvenile females (38%, n = 7, SE 8%), adult males (36%, n = 11, SE 4%), and juvenile males (21%, n = 7, SE 7%). The persistence of adult males but not females was inversely related to the initial number of males present. 6. In males, there was no significant difference in winter body mass between years and winter body mass was not associated with food availability, although there were very few data for winters when the food supply was good. In 7 years when the food supply was poor to moderate, there was an increase in mean body mass between summer and winter in 4 years, and a decrease in 3 years. 7. The data were explored using ordination techniques; first a standardized principal component analysis and then the canonical form of principal component analysis or redundancy analysis. The analysis was carried out on years derived from MNA of males and females and various combinations of the environmental variables: food availability (FOOD) and the severity of winter weather (TEMP). The ordination biplots clearly showed the high correlation between MNA males and MNA females and that FOOD was the most important environmental variable. TEMP on its own had no effect but FOOD x TEMP was important. 8. Following on the exploratory data analysis and for predictive purposes, a general linear model between the numbers of squirrels in the summer populations and sex, FOOD and FOOD x TEMP as explanatory variables accounted for 77% the variance in squirrel numbers among years. 9. This study shows that tree seed availability is the most important factor limiting grey squirrel densities, but this factor both positively and negatively interacts with the severity of winter weather to affect grey squirrel population dynamics.