Does water vole diet meet the prerequisites of the “plant hypothesis” for explaining population cycles?

Abstract Rodent population cycles are observed in highly seasonal environments. As most rodents are herbivorous, the availability and the quality of their food resources vary greatly across seasons. Furthermore, it is well documented that herbivore densities have a measurable effect on vegetation and, conversely, that vegetation dynamics can influence herbivore population dynamics. Many previous studies have investigated whether rodent population cycles could be induced by bottom‐up regulation. A recent review summarized several sub‐hypotheses regarding 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), or to changes in quality of food (decrease in quality of preferred food or switch towards lower quality food) in response to rodent grazing (e.g., plant defenses). While 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. The fossorial water vole shows contrasted population dynamics depending on its geographical location and exhibits large outbreaks in grasslands in highly seasonal climate. It is thus a good model species to investigate plant hypotheses. In this study, the diet of water vole was investigated in and out of an outbreak area with a combination of approaches in the field, in different sampling sites and across seasons. We demonstrated that voles have a very large fundamental trophic niche, but strong behavioral selection, inducing a narrower realized niche, especially during winter. Using an experimental device based on camera traps and cafeteria tests, we observed a strong preference for dandelion ( Taraxacum officinale ) in wild water voles, resulting in exclusive selection during winter for food stores. These preferences were constant across seasons, in control and outbreak areas and independent of vole density on plots. When dandelions were overabundant on plots, this preference was less marked, with voles favoring white clover ( Trifolium repens ). We concluded 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.