The dramatic cycling of vole populations with time may be driven by grasses responding to the furry creatures' herbivory. So say Fergus Massey of the University of Sussex in Brighton, UK, and his co-workers, who studied interactions between the vole Microtus agrestis and its main winter food source, Deschampsia caespitosa, at four sites in a forest in northern England.
Grasses contained the most silica where the density of voles had been high in the previous spring but was declining during the study period. Meanwhile, little silica was found where the vole population had been low in spring the year before but had since begun to rise.
The team proposes that munching voles prompt grasses to store more silica, which reduces the ease with which voles can digest the plants. So vole growth and reproduction rates fall, and the population tumbles. Grasses then reduce their silica content and the cycle begins again.