Study of voles on Arctic island advances knowledge of small-mammal population dynamics

An East European vole on Spitzbergen island. These animals were introduced to Svalbard islands in the last few decades. Their presence gives scientists a living system that allows them to evaluate the effects of predators on Arctic mammal populations. Credit: Nigel Yoccoz
An extensive study of voles in the Norwegian archipelago Svalbard has provided insights into an enduring puzzle of Arctic ecology. It reveals what is driving the established population cycles of small Arctic mammals like lemmings and voles.

These rodents, which eat plants, are some of the Arctic's most common mammals and are crucial to the Arctic food web. A team of Canadian and Norwegian scientists published their findings online in the journal PNAS (Proceedings of the National Academy of Sciences). They found that predators are a major factor in the three- to five-year cycles of small mammal abundance. The study clearly shows that herbivore-plant interactions from the bottom do not generate such cycles.

Dr. Dominique Fauteux (Canadian Museum of Nature research scientist and the lead author of the study) says that Svalbard presents a unique situation in which the voles are free from specialized predators. This means that we now have a simple, real-life model to examine the effects of predators on populations.

Fauteux spent 10 years studying the ecology and food webs small mammals in the Canadian Arctic. Here, Arctic foxes are the common predators. Ermines, raptors, and Arctic foxes are the most common predators. It's difficult to determine the relative importance and influence of predators on herbivores and other non-biological factors like weather and climate.

Svalbard's unusual circumstances offer unique insights. From the 1920s to 1960s, the East European vole (Microtus Levis) was probably introduced to Spitzbergen Island (part of Svalbard archipelago). The island was an isolated ecosystem that had no predators and grasses where the voles thrived.

Norwegian scientists, which included study co-authors Dr. Rolf A. Ims and Dr. Audun Sttien, and Dr. Nigel G. Yoccoz, at the Arctic University of Norway, as well as Dr. Eva Fuglei, at the Norwegian Polar Institute monitored vole populations over two decades, in the 1990s to 2000s. They used live-trapping to tag and track them. They burrow in rocky outcrops and feed on the grass that covers the fertilized soil on coastal slopes. These slopes are fertilized with the droppings of Thick billed Murres or Black-legged Kittiwakes which breed on nearby cliffs.

The scientists lived on grassy slopes and rocky hills, where they trapped East European voles in order to monitor their population. Credit: Dominique Fauteux

Over the past two decades, the vole population has fluctuated greatly from peaks of 120 per hectare to almost complete absence. The scientists discovered that the population dynamics of the voles from Svalbard was not consistent with other Arctic and boreal areas.

Fauteux used these data to bring to bear demographic analyses in order to support the team's hypothesis regarding the importance of predators in controlling the population dynamics. To determine if similar population cycles could be observed elsewhere, the team tested theoretical models using Fauteux's analysis. If food availability was the primary factor, then Svalbard's voles would expect to follow the three-to-five-year cycle. However, this was not true.

Our study has a strong point: because it is so simple, there is good evidence that food access cannot be the only factor that creates the cycles that are typical for small mammals. He notes that this shows the importance of predator-prey regulation to regulate small mammal population sizes.

The study also examined the impact of climate on population fluctuations. The Norwegian researchers have previously shown that snow accumulated during winters can create crusty layers which prevent the voles from reaching the plants beneath.

Extreme population drops were caused by snowy winters that saw a lot of rain. Fauteux and collaborators have shown that population declines can also occur when there is a high density of voles in the falllikely, which leads to overgrazing and reduced survival during long Arctic winters.

These empirical results have informed a theoretical model that shows that climate variability and winter overgrazing are both factors contributing to the unpredictable "boom-bust” dynamics of the High Arctic vole population.

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More information: Climate variability, density-dependent population dynamics and lessons from a simple High Arctic ecosystem. Proceedings of the National Academy of Sciences (2021). Information from Proceedings of the National Academy of Sciences: Climate variability and density dependent population dynamics: Lessons learned from a simple High Arctic ecosystem (2021). DOI: 10.1073/pnas.2106635118

Canadian Museum of Nature