The effect of extra food and cover on declining populations of Microtus townsendii

Abstract

Microtus townsendii populations were supplied with extra food and extra cover during a spring decline in numbers in 1979. Compared with the control population, late-winter addition of food resulted in lower rates of decline of males and females. On the area with extra cover, the rate of decline was significantly lower in males, and females actually increased in number. Breeding was earlier in the population with extra cover, and four times as many females became pregnant during the spring. This resulted in twice as many young voles entering the population in early summer. Survival was higher in both experimental populations during the spring. Females with extra food and males with extra cover had the smallest home ranges for their sex group.The experiments were repeated in summer 1979, except that cover was reduced on one grid. Demography during the summer experiments was similar in all populations. This period coincided with a botfly (Cuterebra) larva infection which reduced survival of voles in all populations.The addition of cover reduced bird predation. If this was the only effect of added cover, the demography of the covered population suggests that predation may be important in the spring decline of M. townsendii. Further, we suggest that the severity of the spring decline, particularly in female voles, may determine when the population "cycles."