The Role of Vegetation Structure in Controlling Distributions of Vertebrate Herbivores in Arctic Alaska

Abstract

ABSTRACTClimate-driven shrub expansion is altering the distribution of animal communities in the Arctic. A better understanding of habitat requirements is needed to accurately predict the response of herbivore communities to shrub expansion. We examined patterns of browsing by moose (Alces alces), snowshoe hare (Lepus americanus), and ptarmigan (Lagopus lagopus, L. muta) across the tundra of northern Alaska to determine whether forage requirements explain the distribution of herbivores in this ecosystem. In addition, we examined the potential for competition among these three shrub-dependent species. We recorded shrub characteristics and browsing levels at 59 sites along a 568 km riparian transect spanning from the Brooks Range to the Arctic Coast. Mean shrub height was positively correlated with browsing intensity for all three species (r = 0.40–0.71). The minimum shrub height threshold for hare occurrence (≥87 cm, 95% CI: 67–94) was similar to that for moose (≥81 cm, 95% CI: 65–96), whereas ptarmigan were nearly ubiquitous (≥3 cm, lower 95% CI = 0). Diet overlap among herbivores was nearly complete, with all three species heavily browsing feltleaf willow (Salix alaxensis). Our findings indicate that unlike moose and ptarmigan, forage availability does not appear to control the distribution of snowshoe hares in the Arctic. Resource competition may further affect distribution patterns within this guild as shrub cover continues to expand.