Vole cycle amplitude at Vindeln, Sweden, partly recovered in 2005–2025 while winters got warmer, and winter temperature does not explain the earlier dampening (1971–2025 trap data)

Question: Northern Swedish vole cycles became weaker in the 1980s and 1990s. Earlier studies linked this to mild, wet winters with poor winter survival. Winters continued to get warmer after these studies, so the hypothesis predicts that cycles stay weak. We tested this prediction with 21 new years of data. Approach: We used the Swedish national small-rodent snap-trap monitoring (Vindeln 1971–2025, Grimsö 1979–2025, three mountain areas 2001–2025), ERA5 and SMHI winter weather, and the NAO index. We compared three fixed periods (P1 1971–1984, P2 1985–2004, P3 2005–2025) with a block bootstrap. We fitted density models of winter population change to 1972–2003 and tested them on 2004–2024. Results: At Vindeln, the s-index of the three cyclic vole species was 1.50, 0.69 and 1.13 in P1, P2 and P3 (P3 − P2 = +0.44, 95% CI 0.17 to 0.69). November–March temperature was −6.7, −5.5 and −4.8 °C, and snow-cover days decreased from 184 to 166. The slope of density-adjusted winter change on winter temperature in 1972–2024 was −0.06 per °C (95% CI −0.19 to 0.07). A slope of −0.37 per °C is necessary for warming to cause the P1 to P2 drop. A density-only model predicted 2004–2024 winter change without clear bias (observed − predicted = +0.14, SE 0.28). The grey-sided vole did not recover and its winter change decreased with the NAO. At Grimsö, density increased in P3 and warm winters had smaller winter losses. In 2001–2024, warm winters had a weak negative effect in the five areas together (−0.25 per SD, CI −0.49 to −0.01), mainly in the mountains. Conclusion: In these data, winter warming does not explain the fading and partial return of the bank-vole-dominated cycle at Vindeln. Limitation: one long series carries the main test, each period has only 4–6 cycles, and point weather data do not measure conditions under the snow.

vole cycles · population cycles · Myodes glareolus · Myodes rufocanus · Microtus agrestis · winter climate · climate change · boreal forest · Sweden · long-term monitoring · out-of-sample test · density dependence · North Atlantic Oscillation · snow cover · ERA5