According to the exploitation ecosystems hypothesis (EEH), productive terrestrial ecosystems are characterized by community-level trophic cascades, whereas unproductive ecosystems harbor food-limited grazers, which regulate community-level plant biomass. We tested this hypothesis along arctic-alpine productivity gradients at the Joatka field base, Finnmark, Norway. In unproductive habitats, mammalian predators were absent and plant biomass was constant, whereas herbivore biomass varied, reflecting the productivity of the habitat. In productive habitats, predatory mammals were persistently present and plant biomass varied in space, but herbivore biomass did not. Plant biomass of productive tundra scrublands declined by 40% when vegetation blocks were transferred to predation-free islands. Corresponding transfer to herbivore-free islands triggered an increase in plant biomass. Fertilization of an unproductive tundra heath resulted in a fourfold increase in rodent density and a corresponding increase in winter grazing activity, whereas the total aboveground plant biomass remained unchanged. These results corroborate the predictions of the EEH, implying that the endotherm community and the vegetation of the North European tundra behaves dynamically as if each trophic level consisted of a single population, in spite of local co-occurrence of >20 plant species representing different major taxonomic groups, growth forms, and defensive strategies.

Oksanen, T., Oksanen, L., Schneider, M. and Aunapuu, M. 2001. Regulation, cycles and stability in northern carnivore-herbivore systems: back to first principles. -Oikos 94: 101-117.In studies on dynamics of northern predator-prey systems, two assumptions are often made. First, the bifurcation from stable to cyclic dynamics is seen as a consequence of changing generalist-specialist ratio, ultimately due to reduced prey diversity at high latitudes and the negative impact of snow on the efficiency of generalists as predators of small, folivorous mammals. Supposedly, the primary mechanism is the qualitative difference between the functional response of specialist and generalist predators. Second, the interaction between large predators and ungulates is supposed to be prone to lead to two alternative equilibria, one where predation regulates ungulates at a relatively low equilibrium and another, where ungulate densities are close to carrying capacity. In the first-mentioned issue, our analysis corroborates the general idea of snow favoring specialists and leading to cycles. However, differences in functional response appear to be of secondary importance only, and rather special conditions are required for generalists to have a stabilizing type III functional response. A destabilizing type II functional response or a slight modification of it should be common in generalists, too, as also indicated by the classical experiments. Stability of generalist dominated systems seems primarily to derive from their relative inefficiency, allowing prey's density-dependent mechanisms to play a bigger role in the neighborhood of the equilibrium. Moreover, the main destabilizing impact of deep, long-lasting snow cover appears to lie in the protection it offers to the efficient but vulnerable specialists, which are eliminated or marginalized by intraguild predation in areas with snow-free winters, unless the habitat offers some other form of efficient protection. As for the conjecture of multiple equilibria in northern wolf-ungulate systems, it seems to be derived from an erroneous operational definition of numerical response and has little if any empirical support. Available data suggest that predation limitation of folivorous mammals prevails along the entire gradient from relatively productive low arctic habitats to the humid parts of the temperate zone, provided that the numbers of predators are not controlled by man. Scientists' view of nature and how it functions is inevitably influenced by both empiricism and theory. As long as the pieces seem to fit, we tend to retain tacit assumptions, derived from a mixture of past empirical and theoretical work. Whenever a closer look at the explanation reveals logical problems in the underlying framework, it is time for reconsideration. This can open new avenues worthwhile to explore. The problem became actualized for us when we observed that weakened cyclicity in two northern vole populations coincided with the invasion of an alien generalist predator -the American mink (Mustela 6ison) (...

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