Is water vole diet consistent with the plant hypothesis for explaining population fluctuations?
Rodent population cycles are observed in highly seasonal environments. As most rodents are herbivorous, the availability and the quality of their food resources varies greatly across seasons. Furthermore, it is well documented that herbivore densities have a measurable effect on vegetation and conversely. So, many studies investigated whether rodent population cycles could be induced by bottom-up regulation. A recent review summarized several sub-hypotheses leading to rodent population cycles: cycles may be due to inherent inter-annual variations of plant quantity, to overshoot of carrying capacity by overgrazing (i.e. lack of quantity), to changes in quality of food (decrease of quality of preferred food or switch towards less quality food) in response to rodent grazing (e.g. plant defences). If some sub-hypothesis seems to be more important than others, there is currently a prerequisite to construct scientific consensus: dietary description is still overlooked in many systems and should be more investigated. This study focuses on fossorial water vole. It shows contrasted population dynamics depending on its geographical locations. It is known to be able to exhibit large outbreaks in grasslands in highly seasonal climate. It is thus a good model species to investigate plant hypotheses, first beginning by diet description. The diet of water vole was investigated in and out of the outbreak area with a combination of approaches in the field, in different sampling sites and considering seasonality. We demonstrated that voles have a very large fundamental trophic niche, but strong behavioural selection, inducing a narrower realised niche, especially during winter. We created an experimental device based on camera trap and cafeteria tests. We observed a strong preference for dandelion (Taraxacum officinale) in wild water voles, that results in exclusive selection during winter for food stores. These preferences were constant across seasons, altitudes and grassland productivity gradients, despite the scarcity of this species in some experimental sites. First, we conclude on the importance of using different methods to fully describe the diet of rodents Second, we assess that dandelion is a winter key resource for water vole. It thus might be interesting to investigate the role of dandelion in vole population dynamics.