Oksanen et al. coined the Exploitation Ecosystem Hypothesis (EEH) to predict how the number of aboveground trophic levels and trophic cascades should change along aboveground productivity (ANPP) gradients. Moore et al. postulated that EEH applies to belowground detritus based soil food webs as well, with the interaction among the aboveground and detritus realms as an important driver of ecosystem change. Here we tested how artificially increasing NPP affected plant and soil responses to mammalian herbivory in two low productivity Alaskan arctic tundra communities: dry heath (DH), dominated by dwarf evergreen shrubs and lichens, and moist acidic tussock tundra (MAT), comprised of sedges, dwarf evergreen and deciduous shrubs, and Sphagnum mosses. Previous fertilization studies in these communities in the presence of mammalian herbivores had shown that ANPP increases in both. Aboveground community biomass did not change at MAT because of offsetting species-specific responses, and actually decreased at DH because of the replacement of woody shrubs by a grass. Aboveground at MAT a deciduous shrub became more abundant at the expense of other plant species.
Results/Conclusions
Here, after 10 years, plots with ambient soil nutrients showed slightly opposite patterns when herbivores were excluded: at MAT, ANPP and plant community biomass slightly decreased, while at DH, both slightly increased. ANPP and biomass were greatest at MAT in fertilized plots exposed to herbivores, while ANPP at DH was greatest when fertilized and protected from herbivores. Herbivore activity seemed to stimulate ANPP of the newly dominant plant species at both sites under fertilized conditions: a deciduous shrub at MAT and a grass at DH. The soil food webs in both MAT and DH were altered by nutrient addition and fencing, with shifts in the relative activities of bacterial pathways (bacteria and their consumers) and fungal pathways (fungi and their consumers) tracking changes in ANPP, plant species composition, and plant litter quality and quantity. Seasonal and spatial separation in the placement and activities of the bacterial and fungal pathways within the soil food webs decreased with fertilization. These results point to a change in ecosystem structure and function that go well beyond the scope of EEH, to include shifts among trophic pathways and feedbacks both above and belowground that ultimately may control responses of arctic tundra to climate change.
