Fox predation on cyclic field vole populations in Britain

The diet of the red fox Vulpes vulpes L. was studied during three winter pe riods in spruce plantations in Britain, during which time the cyclic field vole Microtus agrestis L. populations varied in abundance. Field voles and roe deer Capreolus capreolus L. were the two main prey species in the diet of the red fox. The contribution of lagomorphs to fox diet never exceeded 3 5% and species of small mammal other than field voles were of minor importa nce. The contribution of held voles was dependent on vole density. The non- linear density dependent relationship with a rather abrupt increase of fiel d voles in fox diet when vole density exceeded ca 100 voles ha(-1) was cons istent with a prey-switching response. The contribution of field voles to f ox diet during the low phase of population cycles was lower in Kielder Fore st than in other ecosystems with cyclic vole populations. The number of fox es killed annually by forestry rangers was consistent with the evidence fro m other studies that foxes preying on cyclic small rodents might show a del ayed numerical response to changes in vole abundance. Estimates of the maxi mum predation rate of the fox alone (200-290 voles ha(-1) L of vole habitat year(-1)) was well above a previously predicted value for the whole genera list predator community in Kielder Forest. Our data on the functional respo nse of red foxes and estimates of their predation rates suggest that foxes should have a strong stabilising impact on vole populations, yet voles show characteristic 3-4 yr cycles.