Grey red-backed vole (Myodes rufocanus) ©Sari Holopainen

A recent hiking trip to Lapland got me thinking of voles. The little critters were absolutely everywhere, happily (or with a vengeance) gnawing at our rucksacks during the night in hope of finding food. Several notes left in hiking huts along the way gave more proof of their high numbers: two people had had their rucksacks eaten through and one had lost a bag of nuts when a vole ate a hole through the tent.

Back home in southern Finland, not hair nor hide of a vole. Why not? What actually constitutes a vole cycle, how does it work, and why does it periodically crash? And how does it affect other animals?

Voles can reach sexual maturity as early as three or four weeks, depending on the species in question. This combined with large brood sizes and giving birth to several broods during the breeding season means that when conditions are right there will be an Awful Lot of Voles (called the peak of the cycle). This leads to increases in the numbers of predators (e.g. stoats Mustela erminea and least weasels Mustela nivalis), which marks the start of vole population declines and crashes as their mortality increases. The whole cycle appears to take around three to five years in northern Europe, but it is not synchronous everywhere. Hence a deluge of voles in Lapland, while populations in southern Finland crashed last winter and currently remain small.

Vole populations fluctuate on a yearly basis. Graph by Hannu Pietiäinen.

Voles are of course, paramount to many predators. During our hike we saw a hawk owl (Surnia ulula) and several rough-legged buzzards (Buteo lagopus). Signs of martens and stoats were numerous. Unfortunately the elusive snowy owl (Bubo scandiacus) did not make an appearance. These are all species whose life cycles are influenced by voles. They produce larger numbers of offspring during vole-rich years, while breeding may plummet to zero when vole cycles are down. Many strict vole eating owl species are nomadic, wandering large distances to ensure being in the right place at the right time when it comes to food. However, several owl species are residential and will not travel in search of food, e.g. the tawny owl (Strix aluco). For these species in particular the vole cycle does not only determine breeding success or brood size, but actually influences the number of breeding pairs in the total population, the timing of nesting onset, even a young owl’s entire life expectancy and quality of life. For example, the phase of the vole cycle (aka the nutritional state of an owl) during the first year of an owl’s life correlates with the number of parasites it carries as an adult. The vole cycle also determines at what age residential owls are recruited into the population as breeders (e.g. whether at one or two years of age), as owls will not breed if the female is too weak.

Phase of vole cycle affects nesting and owling survival. Modified from picture by Hannu Pietiäinen.

A worrying phenomenon has possibly begun to unfold during the last few decades concerning vole cycles. Twice the cycle has been disrupted, and an anticipated peak has not occurred. If disturbances in the cycle become more frequent, this will play havoc for countless northern species. The reasons behind this disturbance are unclear, but a warming climate with thin snow cover has been suggested.

Rough-legged buzzards use voles as their primary food source. ©Sari Holopainen