Introduced Rangifer alter above and belowground ecosystem processes in maritime-tundra on sub-arctic islands
Temporal and spatial variation in grazing pressure from ruminant herbivores can accelerate or decelerate nitrogen cycling and plant growth by altering the composition and quantity of plant material entering the decomposition cycle. Changes may be particularly pronounced after these herbivores are introduced to islands without native herbivores and predators, and then display irruptive population dynamics. We utilized a natural experiment enabled by staggered introductions of caribou and reindeer (Rangifer tarandus, hereafter ‘Rangifer’) to islands in the Aleutian archipelago of Alaska to ask how herbivory might accelerate (e.g., increase graminoids and plant-soil-N) or decelerate (e.g., increase dwarf-shrubs and lower plant-soil-N) ecosystem processes in maritime tundra. We measured variation in plant community structure and nitrogen properties of plants and soils during summer across three islands representing a chronosequence of elapsed time post-Rangifer introduction (Atka: ~ 100 years; Adak: ~50; Kagalaska: ~0). Distinct patterns of Riney-Caughley irruptive population dynamics relative to K-carrying capacity were nested within each island (Atka: irruption, K-overshoot, decline, K-re-equilibration; Adak: irruption, K-overshoot; Kagalaska: initial release). We indexed spatial variation in Rangiferisland use from pellet group counts to account for non-uniform grazing effects within islands.
After accounting for environmental covariates, biomass and cover of graminoids increased while dwarf-shrubs, lichens, and mosses decreased along the chronosequence. In contrast, increasing Rangifer spatial-use within all islands was correlated linearly with reductions in graminoids and forbs, and non-linearly with reductions in dwarf-shrubs. These results support the hypothesis that Rangifer population persistence on islands is facilitated by the use of graminoids as winter forage after preferred lichens are decimated. On mainland arctic-tundra, Rangifer-induced shifts in vegetation towards graminoids with lower foliar C:N compared to ratios in shrubs and lichens often correlate with more soil-N. In our study, however, net N-mineralization, NH4+ pools, and soil 15N declined markedly along the chronosequence and did not correlate strongly with spatial-use. Overall plant and soil patterns generally fit Riney-Caughley model predictions. Although 100 years of herbivory on Atka associated with more graminoids overall, lower soil nitrogen and fewer graminoids and forbs in heavily grazed areas indicated herbivore-depleted resources as predicted at the conclusion of an irruptive cycle. Intermediate changes in palatable forage abundance and soil nitrogen on Adak may indicate a depletion of resources relative to the recent irruption. Rangifer have had negligible impacts on Kagalaska.