Demography and Dynamics of a Stoat Mustela erminea Population in a Diverse Community of Vertebrates

Abstract

Demography of a stoat population was studied for 6 yr by mark-recapture methods, and population dynamics were examined in relation to variations in prey density and intensity of interspecific competition. Mean density in autumn varied between 0.3 and 1.0 stoats/10 ha. Local and between-year differences in density followed variations in the stoats'' reproductive output and was related to local differences in food supply (water voles). Apart from 1 yr with a low reproductive output, young of the year constituted a predominant part of the population each autumn (59-76%). Recruitment rate was not correlated with stoat density in spring. Winter mortality rate was similar (.apprx. 50%) in all years, irrespective of stoat density in autumn. Mean further life expectancy of a stoat at the age of independence (3-4 mo. old) was 1.4 yr for males and 1.1 yr for females. The highest age recorded for males was 4.5 yr and for females 3.5 yr. Sex ratios for each year did not deviate statistically from 50:50, but males tended to outnumber females during years of population decline. Data strongly indicated that the stoat population was limited by a shortage of food caused by interspecific competition: .apprx. 90% of the stoats'' basic food (field voles and water voles) was exploited by competitors, especially some generalist predators; the calculated total consumption of field voles by predators equalled the calculated annual production, heavy losses due to the predators throughout winter resulted in low vole densities every spring, continued intense interspecific competition for voles in spring and early summer resulted in a food shortage for breeding female stoats. A marked decrease in rabbits, the food of primary importance for several generalist predators, caused a change in the generalists'' diet and increased competition for voles. This probably was the primary cause of a marked decrease in stoat numbers.