EFFECTS OF ABUNDANT SNOW AND ROSS’S GEESE ON ARCTIC ECOSYSTEM STRUCTURE: PLANTS, BIRDS, AND RODENTS
Date
2021-06-15
Authors
Journal Title
Journal ISSN
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ORCID
0000-0002-9314-5043
Type
Thesis
Degree Level
Doctoral
Abstract
Migratory animals link and often have profound impacts on geographically distant ecosystems through trophic interactions and transport of nutrients, energy, toxins, parasites and pathogens. Moreover, when seasonally linked ecosystems differ in carrying capacity of migrant species, detrimental effects to biodiversity through trophic cascades can occur in ecosystems with lower carrying capacity. Access to agricultural production has increased carrying capacity of lesser snow (Anser caerulescens caerulescens) and Ross’s geese (Anser rossii, collectively, ‘light geese’) in southern agricultural landscapes where these species winter and stage during migration to and from northern breeding regions. Rapid population growth in response to increased carrying capacity during fall, winter, and spring has caused trophic cascades in less productive subarctic and arctic ecosystems during summer. I investigated changes to plant community structure in direct response to foraging and nesting by abundant and highly concentrated light geese within the Queen Maud Gulf (Ahiak) Migratory Bird Sanctuary, Nunavut, Canada, with particular reference to Karrak Lake, one of the largest nesting goose colonies in the world. I further studied indirect impacts of vegetation change on aspects of sympatric vertebrate response, including resident brown lemmings and ptarmigan, and migratory shorebirds and passerines.
Foraging by light geese created spatially heterogeneous landscapes composed of lightly and intensely foraged grazing lawns in the brood-rearing and molting region north of Karrak Lake, within the pre-existing mosaic created by variation in topography, moisture, and soil properties created by geomorphic processes. Although foraging light geese nearly depleted aboveground plant biomass in some areas in the Sanctuary, belowground biomass was largely intact and thus, plant communities have strong potential for aboveground regeneration. Nesting and foraging geese severely reduced aboveground plant biomass within the nesting colony at Karrak Lake. Colonizing plant species established on bare sediment or peat exposed by previous vegetation removal by geese, resulting in shifts in species composition of plant communities. High occupancy by nesting light geese resulted in transition of lowland communities dominated by grasses and sedges (collectively, ‘graminoids’), Sphagnum spp., and willows (Salix spp.) to those comprised of exposed peat, birch (Betula glandulosa), non-Sphagnum mosses, marsh ragwort (Tephroseris palustris), and mare’s tail (Hippuris vulgaris). Community changes were less apparent in upland regions that are naturally less vegetated even in the absence of geese, but fruticose lichens, crowberry (Empetrum nigrum) and white heather (Cassiope tetragona) dominated undisturbed plant communities whereas crustose lichens and bearberry (Arctostaphylos spp.) comprised disturbed communities.
Reduction of plant biomass and community shifts from graminoid dominance to those with high proportions of exposed peat and birch had negative effects on abundance of brown lemmings and occupancy by graminoid-specialist shorebirds; however, some open-nesting and generalist cover-nesting avian species showed neutral or positive responses to establishment of birch in altered habitats. Intact graminoid communities are important to ecosystem structure and function and population health of many resident and migratory arctic vertebrates. However, light geese often nest in localized, dense aggregations, and thus negative impacts on sympatric species may be spatially limited and confined to large nesting colonies and severely altered brood-rearing and molting regions. Moreover, altered upland and lowland habitats remained largely vegetated in the Sanctuary, contrasting sharply with altered coastal marshes in subarctic regions.
Description
Keywords
arctic, landscape ecology, vegetation, herbivores, lesser snow geese, Ross's geese, population ecology, trophic cascade
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Biology
Program
Biology