Iešjávri, litterally ”Self-the-lake” is the largest body of water on the Fennoscandian tundra (area about 100 km2). As typical representative for Baltic Shield lakes it is shallow (14 m at deepest, 4 m at average). Glaci-fluvial ridges jut into it and rise here and there as islands (and as treacherous reefs, which you better watch up for). Where water table is high, the islands are covered by lush willow thickets and hummock bogs; elevated parts are occupied by heaths.




For the lush habitats, the difference between near-shore islands, visited by predators (left) and outer islands, where the introduced voles have been food limited (right) is striking





Grazing by food limited voles leads to expansion of resilient herbaceous plants. Before the introduction of voles the peat ridge on the island in the right picture was dominated by relatively unpalatable evergreens (crowberry, lingonberry). Now they are all gone…..


…..and palatable herbaceous plants are spreading, taking opportunity of the free growth space. Out of the woody plants, relatively palatable but resilient plants (bilberry, willows) do better than the unpalatable evergreens. When rodents are food limited in the arctic, defense does not count. Only resilience helps (Hambäck et al. 2004,  Dahlgren et al. 2009, Oksanen et al. 2009)



The islands are even interesting in the context of another important herbivore: the reindeer




Almost the entire North Calotte is used for reindeer grazing; in Norway and Sweden, the herds migrate between winter and summer ranges, crossing even national borders to some extent; in Finland, grazing is stationary. On annual basis, reindeer densities vary from one to a few reindeer per km2 (below; from C. Bernes 1996. Arktisk miljö i Norden – orörd, exploaterad förorenad?  Nordiska ministerrådet; København).



In the potentially lichen-dominated inland areas (inside the bright blue line) current lichen cover varies from thin (below) to almost nothing, depending on intensity and, even more, on the timing of the grazing. Summer grazing is maximally detrimental, even if reindeer then do not prefer lichens as food. However, lichens are brittle when dry and suffer from trampling.






The islands of Iešjávri are, in effect, reindeer exclosures as the lake is only partially frozen in early winter, when reindeer migrate through the area. The contrast as compared to mainland is striking in heath habitats