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.