Evol Ecol Res 15: 1-24 (2013)     Full PDF if your library subscribes.

Impact of marine-subsidized predators on lemming–plant oscillations

Tarja Oksanen^1,2, Lauri Oksanen^1,2, Gunnar Söderbacka², Katrine S. Hoset^1,2, Lise Ruffino^1,2 and Maria Tuomi^1,2

¹Department of Biology, Section of Ecology, University of Turku, Turku, Finland and  ²Section of Sciences, Finnmark University College, Alta, Norway

Correspondence: T. Oksanen, Department of Biology, Section of Ecology, University of Turku, 900 14 Turku, Finland. E-mail: tarja.oksanen@utu.fi

ABSTRACT

Questions: Assuming that arctic lemming oscillations are generated by interactions between lemmings and depletable plants, how should these oscillations change in response to varying densities of marine-subsidized predators and differences in the production of herbaceous forage? Are the patterns thus generated consistent with existing data?

Features of the model: Rosenzweig-type predator–prey model, with lemmings interacting with depletable plants and herbaceous plants. The impact of marine-subsidized predators is included as an extra mortality factor, influencing lemmings in summer.

Ranges of parameters: We set the carrying capacity of depletable plants at 2000 kg · ha⁻¹. We used carrying capacities for herbaceous plants of 1000 kg · ha⁻¹ (grassland/meadow) and 100 kg · ha⁻¹ (heath). In the absence of predation, we assume that the population growth rate of lemmings in summer is zero when their foraging rate is 44% of the saturation rate. In subsequent simulations, we successively increased the foraging rate required for a zero population growth rate to simulate the impact of a successively larger pool of marine-subsidized predators.

Predictions: On grassland/meadow tundra, increasing intensity of summer predation reduces the amplitude and period of lemming and plant oscillations and increases the minimum and average density of lemmings and depletable plants. The predicted predator–lemming relationship is thus mutualistic rather than exploitative. In heath-type tundra, the predicted positive impact of summer predation on lemmings is even stronger because, without intense summer predation, the predicted oscillations are violent enough to lead to local extinctions. In tundra areas with such dynamics, natural selection should favour lemmings that react to high population densities by emigrating.

Keywords: arctic, herbivory, lemming, oscillations, population cycles, predation, tundra, vegetation.

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