Inventories of small rodents at Grimsö

Grimsö is unique as a centre for wildlife ecology studies in Sweden. Results from the annual and long-term monitoring of wildlife species are transferred to knowledge and used in research and natural resource management. SITES have spoken to three key persons and asked them about the environmental monitoring, inventories and research about small rodents. Let’s meet, Gunnar Jansson, Frauke Ecke and Petter Kjellander.

Two times every year Grimsö set up and perform the small rodent survey, in May and September. This monitoring series is one of nineteen at Grimsö, where the others focus e.g. moose, fox and starling. This year, 2017, is the 45th year in a row with this small rodent survey, which includes voles, mice and shrews. The larger rodents, beaver, hare and rabbit are not included in this inventory.

Gunnar Jansson is a researcher and coordinates SITES operations at Grimsö. He explains that the purpose of the inventories is to follow the population dynamics of the small rodents, which is a key factor in Scandinavian ecosystems as voles indirectly influence ups and downs in several other species populations.
In the long run, years with many voles (population peaks) are also reflected in the fox population, during the same and the following year. In general there is also a nice correlation with the dynamics of the mountain hare. When the quite easily caught voles are abundant, fox predation on other species like the capercaillie, black grouse and hares are reduced. Thus, a vole peak year favors many other small game species.
Vole photographed by Rolf Sagerstedt
Vole photographed by Rolf Sagerstedt

How is the inventory operated?

Gunnar Janson says that the survey is designed to represent the whole research area, 13000 ha. Traps (50 per plot) are placed in 20 systematically distributed plots of 1 ha each. In total, the 1000 traps are out during three days per season and are checked, and if necessary recharged, every day.

In Sweden there are several different species of small rodents. At Grimsö commonly four-five species are found and the bank vole dominates in numbers before field voles and mice species. The Vole Index, in total and for each species are presented as Catches/100 trap nights.

The national inventory of small rodents

The survey at Grimsö´s constitute along with several other locations in Sweden the sampling areas in the national environmental monitoring program of small rodents. The program is part of SLUs assignment within environmental monitoring and assessment (Foma), where Frauke Ecke at SLU in Umeå is the coordinator.

The inventory at Grimsö represents the Bergslagen region, and another location is for example Vindeln, nearby Svartberget, where the monitoring series of voles started in 1971 with 58 permanent catch plots. In addition, monitoring in the mountain region has run since 1995 in Ammarnäs with 44 catch plots and since 2001 in Vålådalen/Ljungdalen with 42 catch plots and Stora Sjöfallet with 41 catch plots.

Despite the geographical distance there are many similarities between the vole population dynamics in Grimsö and Vindeln, both regarding the cycles and the density says Frauke Ecke. Both these areas are characterized by decreasing densities primarily of field voles from 1990 until 2010, whereas the mountain areas still have large population fluctuations.

”Vole-years” occurs from time to time, what does that mean and what causes the phenomenon?

Generally we divide the population dynamic in low, in-between and peak years, so called “vole-years”, and the later ones are nowadays rare in southern and middle Sweden. However, the vole population is still cyclic in the Berslagen region but at lower densities, hence less distances between low and peak years compared to the 1970-80; ies says Frauke Ecke.

The occurrence of peak “vole-years” depends most likely on several factors that need to coincide. Overall however, it seems that winters with a lot of snow and extended snow cover provide good opportunities for voles. This might give the voles favorable conditions to hide under the snow from predators, easier access to food and they may thus reproduce almost all year around, or at least start the reproduction earlier compared to seasons with less favorable winter conditions. After a winter with a lot of snow you can expect to find more voles in the landscape.

The most recent peak years at Grimsö were 2005, 2010 and 2014, but the fluctuations show a general decrease in recent decades, says Gunnar Jansson. In the period 1988-98 (of which the first five winters can be classified as "green") Grimsö had e.g. only one medium year, otherwise very low records. The reason for why years with strong peaks have become increasingly rare in Götaland and Svealand, is generally assumed to be linked to the milder winters.

With voles comes vole fever - or?

Vole fever is acknowledged in media at times, mainly during the spring, when the garden should be cleaned, the firewood handled and so on. Vole fever belongs to the category of zoonoses, i.e. diseases carried and spread by animals to humans, a growing research topic in recent years. Both in Sweden and internationally, and an area where Frauke Ecke is active. Vole fever is caused by Puumala virus (PUUV) which has voles as its single host species. In a research project Frauke Ecke lead, voles from bio-banks caught within the national environmental monitoring in the Vindeln area was examined.
The study showed that the proportion PUUV infected animals is directly linked to the number of voles, so with more voles you have more PUUV infected animals. Moreover, it seems as a species rich small mammal fauna and presence of vole predators can counteract PUUV infection among voles. But, for still unknown reasons, the disease in humans seems to be most common in the four northernmost counties of Sweden, even though voles are common species throughout Sweden explains Frauke Ecke.

More research on voles - needs and use of data

Petter Kjellanders´ research is also linked to zoonoses and epizooties (animal diseases), the latter involves infectious diseases that may constitute a serious threat to human or animal health. From the catches made at Grimsö tissue samples of the brain, heart, lung and spleen of rodents are taken. These samples are analyzed for any tick-borne pathogens, such as Borrelia, Anaplasma and TBE-virus. Furthermore, the rodents are checked for ticks they may carry and such ticks are then analyzed for the same pathogens. Right now, the main interest is to investigate the importance of rodents for the ticks to spread Borrelia diseases. That is, is the abundance of ticks and Lyme disease prevalence in an area affected by if it is a low or peak year among rodents?

Predictions of the future population trends of voles are also important for e.g. the forestry, where especially the field vole may be a major problem in young tree plantations. These forecasts can be obtained thanks to the long-term data series collected within the national environmental monitoring.

Data from our national monitoring of small rodents have also been used internationally for various comparative studies. It has for example been shown that the prolonged decreasing of primarily Grey red-backed voles, a vole sub-species, and field voles coincides with the long-term decline in voles in other parts of Europe.

The 44th and the now approaching 45th year of the small rodent inventory

The spring 2016 showed low numbers of small rodents in Bergslagen, with a total Vole Index of  0.6 catches / 100 trap nights. In the autumn, however, it was a really good peak with 4.98 catches / 100 trap nights.

The results until the 44th season is compiled as population dynamics data freely available via Grimsö wildlife research station or SITES website. In May 2017 a new inventory round starts!
Gunnar Jansson, Researcher and coordinator of SITES Grimsö
Frauke Ecke, Coordinator of the national environmental monitoring and assessment program of small rodents, located at SLU in Umeå.
Petter Kjellander, Professor in wildlife ecology at SLU, Grimsö Research Station.

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