Richard Collins: How voles make an unlikely contribution to science

They have provided insights into one of the enduring mysteries of Arctic rodents; the cyclical boom-and-bust swings in the numbers of these little mammals.

Bank voles may have stowed away in cargoes destined for the Shannon hydroelectric scheme in the 1920s. We are not the only islanders to have experienced a rodent invasion; sibling voles were discovered on Spitzbergen in 1960. They had hitched rides in animal fodder being transported to the Nordic archipelago on Russian ships supplying the mining industry.

Auks and kittiwakes, breeding on Spitzbergen’s cliffs, fertilise the soils around them with their droppings. The voles, being vegetarian, thrived on the resulting rich grasses.

Richard Collins: 'Do the peak-and-trough fluctuations result from temporary food shortages, predator pressure, weather changes, or a combination of all three?'

Alien invasions can have catastrophic effects on native flora and fauna but the voles on Spitzbergen have made an unlikely contribution to science. They have provided insights into one of the enduring mysteries of Arctic rodents; the cyclical boom-and-bust swings in the numbers of these little mammals.

The most famous Arctic swinger is the lemming. Almost absent in some years, with as few as three animals to the hectare, their numbers start to increase in succeeding seasons, with up to 300 lemmings per hectare at peak. These population explosions take place every three to five years or so.

The little rodents, ‘behaving like lemmings’, are seen occasionally moving in hordes across the country. Are they searching for cliffs overlooking a river or lake, from which to hurl themselves down to a watery grave? No. This suicide myth, needless to say, is nonsense.

Vole populations in the Arctic tend to fluctuate like those of lemmings, the number of animals present in an area varying from a handful to about 120 per hectare.

Various explanations of the cyclical behaviour have been offered. Do the peak-and-trough fluctuations result from temporary food shortages, predator pressure, weather changes, or a combination of all three?

Could food shortage be the culprit? When there is plenty to eat, creatures thrive, reproduce vigorously and their numbers soar. But after the feast, comes a famine. Starvation decimates a population already swollen, leading to catastrophic decline. Irish trees provide an example of the mechanism. Oak mast is eaten by species such as deer. The trees hit back; they protect their seeds by synchronising production. All the trees bloom together but produce hardly any seed in following years. The seed-eaters thrive in a mast year but their numbers decline in the lean years which follow. When the next mast year arrives, there are far fewer eaters to attack the crop.

The Johnny-come-lately sibling voles are the only small mammas on Spitzbergen where there are no predators to attack them, apart from the odd Arctic fox or scavenging gull. Their place in the ecosystem, therefore, is fairly straightforward to study, enabling Dominique Fauteux, an expert on Arctic food webs, to isolate the factors triggering population cycles.

Most significantly, Fauteux found, the introduced voles on Spitzbergen don’t exhibit the peaks and troughs typical of Arctic voles elsewhere. Herbivore-plant interactions are not cyclical and don’t account for rodent population fluctuations. He and his team conclude that predation pressure and weather factors, rather than food instability, give rise to the famous rodent cycles.

• Dominique Fauteux et al. Climate variability and density-dependent population dynamics. Lessons from a simple high arctic ecosystem. Proceedings of the National Academy of Sciences. 2021.