Disentangling trophic relationships in a High Arctic tundra ecosystem through food web modeling

Abstract

Determining the manner in which food webs will respond to environmental changes is difficult because the relative importance of top-down vs. bottom-up forces in controlling ecosystems is still debated. This is especially true in the Arctic tundra where, despite relatively simple food webs, it is still unclear which forces dominate in this ecosystem. Our primary goal was to assess the extent to which a tundra food web was dominated by plant-herbivore or predator-prey interactions. Based on a 17-year (1993-2009) study of terrestrial wildlife on Bylot Island, Nunavut, Canada, we developed trophic mass balance models to address this question. Snow Geese were the dominant herbivores in this ecosystem, followed by two sympatric lemming species (brown and collared lemmings). Arctic foxes, weasels, and several species of birds of prey were the dominant predators. Results of our trophic models encompassing 19 functional groups showed that <10% of the annual primary production was consumed by herbivores in most years despite the presence of a large Snow Goose colony, but that 20-100% of the annual herbivore production was consumed by predators. The impact of herbivores on vegetation has also weakened over time, probably due to an increase in primary production. The impact of predators was highest on lemmings, intermediate on passerines, and lowest on geese and shorebirds, but it varied with lemming abundance. Predation of collared lemmings exceeded production in most years and may explain why this species remained at low density. In contrast, the predation rate on brown lemmings varied with prey density and may have contributed to the high-amplitude, periodic fluctuations in the abundance of this species. Our analysis provided little evidence that herbivores are limited by primary production on Bylot Island. In contrast, we measured strong predator-prey interactions, which supports the hypothesis that this food web is primarily controlled by top-down forces. The presence of allochthonous resources subsidizing top predators and the absence of large herbivores may partly explain the predominant role of predation in this low-productivity ecosystem.

Long-term monitoring at multiple trophic levels suggests heterogeneity in responses to climate change in the Canadian Arctic tundra

“…This high Arctic site is located in the eastern Canadian Arctic Archipelago and several key components of the tundra food web have intensively been monitored during the past two decades [29][30][31]. Like many other Arctic sites, this region has experienced warming for the past 40 years [30].…”

Thermo-erosion gullies boost the transition from wet to mesic tundra vegetation

“…as dominant moss species (Zoltai et al, 1983). As a result, the valley houses many herbivores such as snow geese (in summer) and brown and collared lemmings (Gauthier et al, 1995;Gruyer et al, 2008), thus representing a critical environment for tundra food web (Gauthier et al, 2011;Legagneux et al, 2012).…”

“…Above-ground biomass was harvested inside the exclosures near peak production in early August 2012 using random grids of 25 cm × 25 cm for wet and recently disturbed polygons and of 50 cm × 20 cm for older disturbed polygons and mesic environments. Two different grid sizes were used because of the difference in structure of the vegetation (herbaceous vs. shrubs) associated with the habitat heterogeneity (Legagneux et al, 2012). All graminoids present in the random grids were cut to a standard height, i.e.…”

“…Indeed, while we focused on wet polygons dominated by Carex aquatilis, Cadieux et al (2008) and Gauthier et al (2012) worked on wet polygons dominated by Dupontia fisheri and Eriophorum scheuchzeri. Because our study was part of a large-scale multisite project on wetland carrying capacity for snow geese (Legagneux et al, 2012;Doiron, 2014), we only focused on forage plant (i.e. graminoids) biomass and did not sample forbs or shrubs.…”

“…In contrast, little is known about how thermo-erosion gullying affects plant community structure and plant species abundance. Yet, this information is urgently needed as vegetation plays an important role in structuring Arctic ecosystems and regulating permafrost response to climate change (Jorgenson et al, 2010;Gauthier et al, 2011;Legagneux et al, 2012). Wetlands serve as preferred grounds for Arctic herbivores such as snow geese (Gauthier et al, 1996;Massé et al, 2001;Doiron et al, 2014).…”