Introduction

The water vole (Arvicola terrestris; Arvicolinae) is a 250–300 g herbivorous rodent that inhabits the banks of freshwater–courses in Britain. In contrast, a fossorial form, Arvicola terrestris scherman, predominates in central Europe, co–existing with the aquatic form in the rest of Europe. The water vole is adapted to live in burrows, to swim and dive. In Britain it is largely associated with steep riverbanks, with abundant grass and layered vegetation (Lawton & Woodroffe, 1991). Woodall (1993) reported that in English Midland rivers such as the Thames, water vole presence was positively correlated with water depth and with the herbs Urtica sp. and Phragmites sp. and negatively correlated with bank height, bank depth and with herbs of the genera Polygonum., Phalaris, Sparganium and Juncus.

Several lines of evidence suggest that water vole populations have been declining in Britain at least since 1900. Jefferies et al. (1989) analysed the data contained in County Mammal Reports and literature from the beginning of the century onwards. These were supplemented by a questionnaire survey and by site descriptions from the Waterways Bird Survey organized by the British Trust for Ornithology (BTO). The literature search showed that, formerly, water voles had often been very abundant in all regions of Britain, except South Wales and Northern Scotland. However, by the late 1980s a general perception prevailed that water voles were becoming less numerous. The use of the word ‘common’ in descriptions of populations decreased from 1900 to 1985.