Population Fluctuations in Mountain Hares: A Role for Parasites?

Abstract

Throughout their circumpolar distribution mountain hares Lepus timidus show unstable population dynamics characterised by regular and sometimes dramatic changes in abundance. The periodicity, amplitude and degree of cyclicity are different in different regions. The reasons for these fluctuations and geographic differences are not fully understood.
In Fennoscandia there is experimental and correlative evidence that some mountain hare populations are limited by predators, but the experiments needed to conclusively demonstrate the role of predators, or of other potential factors have not been undertaken. In Scotland the rigorous control of predators means that the role of predators is largely dismissed, but this has not been experimentally tested. The most promising line of enquiry suggests that intestinal parasites have the potential to destabilise some mountain hare populations. There is paucity of literature from Asia and central Europe and no firm conclusions could be drawn.
Time-series analysis of hunting bag records from Scotland largely confirmed the dominance of weak cycles with a mean periodicity of around 9 years found in earlier studies. The analysis of bag records from Fennoscandia found a number of differences compared to earlier studies. We found only limited evidence for the presence of 4-year cycles, and found that the pattern and distribution of cycles has changed in the last 20 years since the fox mange epizootic. Data coverage from central Europe and Asia was very limited and no meaningful conclusions could be drawn.
Populations of mountain hares in Scotland show unstable dynamics with 7-12 year fluctuations in abundance. I describe the findings of three studies which tested the hypothesis that intestinal parasites i) reduce female survival, body condition and fecundity, and ii) are randomly distributed within the host population. Parasite reduction experiments suggested that the intestinal parasite Trichostrongylus retortaeformis had little affect on female survival, but reduced female body condition and fecundity. A large scale crosssectional study revealed that T. retortaeformis was loosely aggregated within the hare population and had a negative affect on body condition. These results are consistent with the hypothesis that parasites may destabilise some mountain hare populations in Scotland.