In broad outlines, the results of our long-term (1977-2017) study on dynamics of small rodents on the Finnmarksvidda tundra conformed to the prediction the Exploitation Ecosystems Hypothesis (EEH). Productive shrublands supported boreal-type vole cycles, where vole declines were accompanied by intense predator activity. Conversely, the unproductive high tundra was characterized by irregular outbreaks of Norwegian lemmings (Lemmus lemmus), ending in abrupt winter crashes and in profound destruction of perennially available plants. In contrast to the Siberian and North American tundra, densities of avian predators where chronically low. Lemmings thus increased unhinged during peak summers. As proposed by Kalela, young adults responded to the discrepancy between numbers and resource supply by migrating. Tens of young lemmings were trapped on a small island in the middle of Iešjávri, a 10 by 8 km tundra lake, and many lemmings were seen starting to swim across the lake. The risky crossings of a major water barrier indicated that these lemmings were on a long journey. We propose that the unique dynamics and traits of Norwegian lemmings are a legacy of the Last Glacial Maximum (LGM), which Norwegian lemmings survived in a small, extremely isolated refugium archipelago, unlikely to have been exploited by avian predators specialized to Lemmus spp. On the basis of the results summarized above and earlier simulation studies, we argue that pure lemming-vegetation interactions create extremely violent, chaotic density oscillations, where periods with low lemming numbers are long, and high lemming densities are only achieved late in peak summers. When Norwegian lemmings lost their specialized avian predators during LGM, the Fennoscandian tundra shifted to the alternative state of pure lemming-plant oscillations, characterized by autumnal peaks and long periods of low lemming density, which is thus resistant to the invasion of avian predators adapted to exploit Lemmus spp.
