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Population density cycles of small rodents create a boom and bust system driving the dynamics of the tundra food web, both in their functions as prey and as consumers. Warmer and wetter winters are expected to hamper rodents access to food, resulting in dampened, irregular or lost rodent population cycles. How will these changes cascade through the food web?

 

Small rodents in tundra foodwebs

Small rodents are a key component of the low arctic tundra ecosystem at Varanger. Svalbard lacks native small rodents, but has a local population of an introduced vole species (the Sibling voles) that shares a dangerous (zoonotic) parasite with the Arctic fox. Population cycles of arctic small rodents have typically a period of three to five years, but there is considerable variation in cycle period and amplitude. During the last decades, small rodent cycles have been fading out in several localities in the Arctic.

Population time series of three most common rodent species in Varanger Peninsula the Norwegian lemmings, Grey-sided voles and Tundra voles. Note that although the species are synchronized their cycles have different amplitudes, shapes and regularity (i.e. the lemming is missing in the last peak in 2015).

The population density cycles create a boom and bust system driving the dynamics of the tundra food web, both in their functions as prey and as consumers. The three rodent species (figure to the right) have very different diets, different sensitivity to climate and value to predators. It is therefore important to consider them separately in many respects.     

  • All tundra carnivores generally consume small rodents. Rodents are, however, particularly important as a subsistence prey for several specialist predators, such as arctic fox, stoat, least weasel, long-tailed skua, rough-legged buzzard and snowy owl. Lemmings have a larger impact than voles on some predators, especially arctic fox and snowy owl. These predators have high reproductive output during the peaks of the cycles, while they may skip breeding entirely during the lows. Several generalist predators, such as red fox and corvids, also respond to the rodent cycle. Their increased abundance causes affects also other prey species such as ptarmigan (so called alternative prey).
  • During peak years, rodents consume vast quantities of plant foods. This affects both the biomass and species composition of plant communities. Small rodent module addresses these interactions in heaths and snow bed habitats. The effect of tundra voles on tall shrub encroachment is addressed in the Tall Shrub module.

 

Expected climate impact

Climate change is expected to lead to warmer and wetter winters. This will lead to increased proportion of precipitation coming as rain instead of snow, leading to melting-freezing events and ice crust formation. Icy snow limits the rodents, especially lemmings, access to plant foods. Dampened, irregular or lost rodent population cycles are a likely result. Sensitivity of lemmings may render the rodent guild dominated by voles.

These changes are predicted to have cascading impacts in the ecosystem. Boreal generalist predators will replace the lemming dependent arctic predators. Reduced rodent herbivory will contribute to vegetation state changes.

 

Expected effects of climate change on tundra food web through small rodents. The direct impact of winter climate on the rodents populations are considered most important. We also expect an indirect effect of climate through its effect on plant communities’ function as food and cover for the rodents. Generalist predators’ abundance is also determined by availability of ungulate prey and ungulate management. Besides the tree rodent species small rodent module focuses on climate responses of the plant communities tundra heaths and snow beds and the specialist predators least weasel, stoat, long-tailed skua, rough-legged buzzard and snowy owl.

 

Management relevance

  • Information about the population dynamics of rodents (and in particular lemmings) are important when deciding on the when and where to allocate management actions to support populations of red listed arctic predators such as the Arctic fox .
  • Rodent population dynamics have indirect effects on small game, like ptarmigan. Information about rodent abundance can therefore aid forecasting game population dynamics and devising efficient game harvesting strategies.   

 

Monitoring methods

Rodents: Snap trapping (summer and fall) and fecal counts (summer) with existing time-series since 2005. The snap trapping will be gradually replaced by year-round camera monitoring in an extensive system of permanent camera boxes.

Plant communities: Annual maximum biomass of all functional plant groups in tundra heaths based on point intercept frequency (time-series since 2005). Moss and vascular plant cover in snow beds. 

Predators: Least weasel and stoat abundance by small mammal camera traps (see rodents). Rough-legged buzzards and snowy owls breeding population density by visits to traditional nest sites. Long-tailed skua breeding population density by line transects in breeding habitats and demography by means of capture-resighting of ringed birds.  

 

Lemming. The Norwegian lemmings is a particularly important prey species for the arctic fox and snowy owl, whose reproduction in the Varanger peninsula tends to depend on abundant lemming prey. Photo Rolf. A Ims

Tundra vole. Riparian willow thickets and meadows are the main habitat of tundra voles. During their peak densities they cut down the small life stages of the tall shrubs and open up the grassland.  Photo: Geir Vie

Grey-sided vole. The grey-sided vole is the dominant small rodent species of dwarf shrub heaths at the Varanger peninsula. Photo: Geir Rudolfsen

Dwarf shrubs in heath habitats have often clear signs of feeding by rodents after rodent population peaks. Photo: Eeva Soininen

Feeding signs in snowbed vegetation are abundant after lemming population density peaks., Photo: Eeva Soininen

Novel camera methods enable us to monitor elusive predators, stoat and least weasel (in the photo) year-round, also below the snow. Photo: COAT camera trap

We check annually the camera boxes which monitor small rodents and their predators. Photo: Leif Einar Støvern

Lemming abundance in snowbeds and their impacts on vegetation have been monitored since 2009. Counts of feces and dwarf shrubs are done in permanent plot. Photo: Eeva Soininen

Module members

Module leader
Researcher,UiT - Arctic university of Norway
eeva.soininen@uit.no
Professor, UiT - Arctic university of Norway
Researcher, UiT - Arctic university of Norway
Researcher, UiT - Arctic university of Norway
Professor, UiT - Arctic university of Norway
Professor, UiT - Arctic university of Norway

 

 

Selected papers

Soininen, E. M., Zinger, L., Gielly, L., Yoccoz, N. G., Henden, J. A., & Ims, R. A.
Not only mosses: lemming winter diets as described by DNA metabarcoding
2017. Polar Biology, 1-7.
Soininen, E. M., Jensvoll, I., Killengreen, S. T., & Ims, R. A.
Under the snow: a new camera trap opens the white box of subnivean ecology
Remote Sensing in Ecology and Conservation, 1(1), 29-38.
Ims, R. A., Henden, J. A., Thingnes, A. V., & Killengreen, S. T.
Indirect food web interactions mediated by predator–rodent dynamics: relative roles of lemmings and voles
2013. Biology letters, 9(6), 20130802.
Ims, R. A., Yoccoz, N. G., & Killengreen, S. T.
Determinants of lemming outbreaks
2011. Proceedings of the National Academy of Sciences, 108(5), 1970-1974.
Ims, R. A., Henden, J. A., & Killengreen, S. T.
Collapsing population cycles
2008. Trends in Ecology & Evolution, 23(2), 79-86.