Evol Ecol Res 11: 1189-1203 (2009)     Full PDF if your library subscribes.

Plant defences to no avail? Responses of plants of varying edibility to food web manipulations in a low arctic scrubland

Jonas Dahlgren1,2, Lauri Oksanen3,4, Tarja Oksanen1, Johan Olofsson1, Peter A. Hambäck5 and Åsa Lindgren5

1Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden,  2Department of Forest Resource Management, Swedish University of Agricultural Sciences, Uppsala, Sweden,  3Department of Biology, Section of Ecology, University of Turku, Turku, Finland,  4Department of Natural Sciences, Finnmark University College, Alta, Norway and  5Department of Botany, Stockholm University, Stockholm, Sweden

Correspondence: L. Oksanen, Department of Biology, Section of Ecology, University of Turku, FI-200 14 Turku, Finland.
e-mail: lauoks@utu.fi


Background: According to the Green World Hypothesis of Hairston, Smith, and Slobodkin, all plants are edible for some herbivores. Hence, the copious abundance of plant biomass, typical for terrestrial ecosystems, depends on the collective regulatory action of predators on the herbivore guild. According to the counterarguments of Polis and Strong, the defensive traits of terrestrial plants attenuate terrestrial trophic cascades to species-specific trickles, so elimination of predators might lead to increased abundance of inedible plants but will not influence community-level plant biomass.

Question: Does the elimination of predators from a low arctic scrubland, with high-quality forage plants and poorly edible evergreen ericoids, lead to a reduction of community-level plant biomass or to an increased abundance of well-defended evergreen ericoids?

Methods: In 1991, we introduced grey-sided voles (Myodes rufocanus) to islands, initially harbouring dense scrubland vegetation, and established permanent plots there. In 2000, we transplanted vegetation blocks from a large three-trophic-level island with voles and predators, to two-trophic-level islands with introduced voles but without resident predators, and also to vole-free one-trophic-level islands, and back to the three-trophic-level island. Vole densities were monitored by semi-annual live trapping. Vegetation was monitored by the point-frequency method.

Results: In the absence of predators, vole densities increased 3.7-fold and the community-level plant biomass was decimated. The least palatable plant group, evergreen ericoids, suffered especially heavily, whereas palatable herbaceous plants increased in abundance. However, all three functional plant groups responded positively to the elimination of grey-sided voles.

Conclusions: Our results corroborate the Green World Hypothesis, indicating that in the absence of predators, plant defences do not prevent runaway consumption of the vegetation. The fate of plants in predator-free systems with browsing vertebrates depends primarily on the accessibility of each plant during the limiting season. Evergreen ericoids then form the most sensitive functional group.

Keywords: arctic, food web dynamics, herbivory, Myodes rufocanus, plant defences, tolerance, trade-offs.

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