Fertilisation, cattle grazing and voles: collapse of meadow vole populations in young forests?

Context A puzzling aspect of microtine population fluctuations is the damping out or collapsing of cycles over the last three decades, particularly in northern Europe. Occasional population fluctuations of Microtus may also have been damped out in temperate and boreal forests of the Pacific North-west of North America. One cause might be the presence of cattle (Bos taurus) grazing in forest habitats that offer summer forage. Aims We tested hypotheses (H) that abundance, population fluctuations, and demographic parameters of reproduction, recruitment and survival, of Microtus pennsylvanicus would be driven by understory plant productivity. Two predictions follow from this hypothesis: (H1) enhanced abundance and demography in fertilised stands, and (H2) reduced abundance and demography in stands with cattle grazing. Methods Study areas were located in ‘grazed’ and ‘ungrazed’ young forests in south-central British Columbia, Canada. Each study area had four replicate units of unfertilised and repeatedly fertilised stands. Herbaceous vegetation and meadow vole (M. pennsylvanicus) populations were sampled from 1993 to 2002. Key results Mean abundance of total herbs, grasses, and fireweed (Epilobium angustifolium) increased dramatically (8 to 34 times higher) with fertilisation in the ungrazed stands. Mean abundance of meadow voles was 3.1 to 8.5 times higher in the nutrient-enriched herbaceous vegetation in the ungrazed fertilised than grazed fertilised stands. Demographic variables also followed this pattern of abundance. Except for two years, mean abundance of meadow voles was similar between fertilised and unfertilised stands at the grazed area, with some degree of fluctuations generated in grazed stands. Thus, H1 and H2 were at least partly supported for M. pennsylvanicus in fertilised stands. Conclusions In high-quality habitats where cover and other attributes of vegetation are substantial enough to generate population increases and fluctuations of Microtus, grazing of vegetation by cattle or other livestock may indeed lead to potential collapse of fluctuations. Degree of grazing pressure would be crucial, but considering the widespread nature of grazing in the continuum of post-harvest forested sites in the Pacific North-west of North America, moderate to heavy grazing pressure is common. Implications Reductions in populations of microtines have serious consequences for predator communities and other ecological functions.