Reduction in biomass of freshwater arctic vegetation by foraging and nesting hyperabundant herbivores shows recovery

Arctic-nesting geese are specialist herbivores of grasses and sedges (collectively, graminoids). Under moderate grazing pressure, these migratory herbivores can create and maintain arctic grazing lawns with high nutritional content and low aboveground biomass. Nutrient and energy subsidies from southern agricultural landscapes during winter have improved survival among populations of Ross's (Anser rossii) and lesser snow geese (Anser caerulescens caerulescens), leading to marked population growth. Resulting goose hyperabundance has raised conservation concern for resilience of arctic ecosystems to withstand cumulative and intense pressures of herbivory and nest construction. We used both design-based (experimental herbivore exclosures) and model-based methods to investigate changes to plant community structure in direct response to foraging and nesting by these species within the Queen Maud Gulf (Ahiak) Migratory Bird Sanctuary, Nunavut, Canada. Annual nest construction and foraging by up to ~1.3 million geese at a large colony at Karrak Lake markedly reduced aboveground biomass of forage (graminoids) and non-forage (foliose and fruticose lichens) vegetation, with spatial variation in reduction associated with intensity of use by geese. Within vast brood-rearing regions, foraging reduced above- and belowground plant biomass of lowland plant communities by 61% and 29%, respectively, between 1994 (when herbivore exclosures were established) and 2017. In addition to landscape diversity associated with abiotic properties created by geomorphic processes, long-term herbivory by geese further increased spatial heterogeneity in vegetation at the landscape scale. Although foraging geese nearly completely depleted aboveground plant biomass in some parts of their brood-rearing areas, belowground biomass was largely conserved, and thus, plant communities had strong potential for aboveground regeneration. We propose that effects of high-density nesting and foraging by Ross's and lesser snow geese in the central Canadian Arctic, while extensive, are ephemeral and prone to reversion following cessation of grazing and nesting pressure, such as that afforded by shifts in herbivore distribution or local population decline.