Spatial Patterns and Dynamic Responses of Arctic Food Webs Corroborate the Exploitation Ecosystems Hypothesis (EEH)

Abstract

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.

Keystone effects of an alien top-predator stem extinctions of native mammals

“…Viewed collectively, these studies constitute strong evidence that this recently introduced top predator regulates a trophic cascade in arid Australia and has positive benefits for the conservation of small native mammals. Our results also add to a growing body of evidence suggesting that mammalian carnivores can generate strong trophic cascades in terrestrial ecosystems (Hebblewhite et al 2005;Beyer et al 2007;Aunapuu et al 2008;Fey et al in press). Like previous field studies on the effects of dingoes, our study relied on a pre-existing land-use framework, the dingo fence, for the 'experimental' treatment.…”

Birds help plants: a meta-analysis of top-down trophic cascades caused by avian predators

“…However, the plant responses are often not reported at all. If they are reported, they cannot be separated between the diVerent predator groups (mammalian, avian or reptilian), because all predators are usually excluded (see, e.g., Norrdahl et al 2002;Aunapuu et al 2008). Incorporating measures of plant responses to these studies would widen our understanding of the eVects of birds in trophic cascades among diVerent ecosystems.…”

Toward a trophic theory of species diversity

“…from exclosures led to the conversion of shrubland to grassland in the Chihuahuan Desert, United States (84). In addition, in Arctic Eurasia, small rodents, principally voles, are capable of transforming tundra vegetation when experimentally transferred to predator-free islands (85). Similarly, salt marshes (Spartina alterniflora) in the southeastern United States are reduced to barrens by removing predators of the major herbivore, the snail, Littoraria irrorata (86).…”

“…At the other extreme, where herbivores are present and predators absent, excessive herbivory can transform vegetation from a competitionstructured to a herbivore-structured state, also lacking in diversity, composed of resistant and/or tolerant plant species, as seen on experimentally manipulated islands in both tropical and arctic settings (81,85). Again, this suggests with respect to plant diversity that there is an optimum at intermediate levels of herbivory [reminiscent of Connell's (6) intermediate disturbance hypothesis] at which diversity is maximum (117).…”