Late snow melt moderates herbivore disturbance of the Arctic tundra

Resilience of tundra vegetation to disturbance by herbivores can be low and lead to ecosystem state shifts. Pink-footed geese Anser brachyrhynchus are the most numerous herbivore on Svalbard and disturb vegetation when foraging for below-ground plant biomass (grubbing). We assessed grubbing extent (occurrence of vegetation disturbance) and intensity (proportion of vegetation disturbed) in 2006/07/08 when goose numbers were approximately 56,000 and in 2013 when they increased to approximately 81,000. Despite a 36% increase in population size, in 2013 the grubbing extent at pre-breeding sites was similar to that in 2007/08 but grubbing intensity was lower. Extensive snow cover in 2013 probably dispersed geese over larger areas in search of snow-free patches for feeding, thereby reducing grubbing intensity. At the largest known breeding site, both grubbing extent and intensity increased with more geese. Birds preferentially fed close to nests in previously grubbed wet habitat, probably aiding nest defence and permitting feeding on plants that were easier to remove from the soil. A greater impact on tundra vegetation may occur at nesting areas if the breeding population continues to grow. However, timing of snowmelt appears key in moderating the impact of disturbance on tundra vegetation since it controls spatial distributions of feeding geese.