Spot (or hear) the vole - in the snow
From the snowshoe hare and the lynx to the spider and the fly, predator prey relationships have been studied in depth. The cycling of numbers, too, in response to their partner's population size is also well-recorded. Voles are the primary consumers for many food webs in the northern hemisphere, with weasel and stoat often their main predators.
The rodent cycles in Finnish ecosystems over 26 years have been studied by Katri Korpela of the University of Jyva¨skyla¨ and colleagues from the Finnish Forest Research Institute, , the Finnish Museum of Natural History and the Universities of Oulu and Turku. They publish their paper in Proc. Roy. Soc. B as- Predator-vole interactions in northern Europe: the role of small mustelids revised.
Specialist, generalist and avian predation were simulated, using as a base the density indices of the Vantaa Game and Fisheries Research Institute. Snow track counting extended to even the smallest mammal tracks between January and March for each year. The least weasel, Mustela nivalis, must have been the hardest tracks to spot, with all of the problems caused by frequent snowfall and bitter weather over an area extending to 300,000km2.
However, it is the small mustelids that seemed to play the greatest role in the complex boreal vole cycles of population. This was especially evident in the north of Finland, but was surprisingly obvious even in the brief summers. To the south-west, coastal conditions were able to cause the small mustelids to eat their voles mainly during summer. This finding is quite important to vole cycles throughout northern Eurasia. Summer predation near mild coastal areas could prove to be crucial to some ecosystems. Mild winters haven't weakened the impact on the poor voles either in the SW part of Finland.
In the continental climate of the east, vole cycles disappeared years ago, with predation, mainly in winter, insufficient to maintain any regular variation. Owls and some migratory raptors, alongside red fox, Vulpes vulpes in the north, do seem to help to maintain these vole cycles. With few alternative prey, the generalist predator, the red fox, relies on the vole when they are cubs in the spring, creating a great effect on summer vole population reductions. However, the weasels and stoats (small mustelids) reign supreme in controlling the dynamics of the vole population. If we measured the predators' populations more than once in the year, perhaps greater insights could be gained.
Many variations in vole numbers couldn't be explained by predator influences. With reduced specialist predation in the winter, the collapse of small mammal population cycles is unlikely to be caused by any cold weather factor. Mortality is bound to have alternative causes, such as food supply, snow cover, temperatures, parasites or disease. But summer predation by animals such as weasels certainly maintains a healthy balance.
For mustelid lovers everywhere, there is a black side. Europeans took many animals, including rats, rabbits and stoats, to New Zealand centuries ago, but the horrendous effects are still being realised!