Drivers of Population Trends for Waterfowl

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Each year, management agencies conduct aerial surveys of waterfowl populations across Alaska and the rest of North America to document annual changes in the numbers of breeding and wintering waterfowl.  These surveys yield long-term data sets of population change, but often the drivers of increases and decreases in population size are unknown.  The USGS Alaska Science Center waterfowl research program aims to understand drivers of population trends to inform the annual changes in waterfowl populations observed during aerial surveys.  Our research examines both landscape and species level processes driving population trends.

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Landscape Processes

Climate

The Arctic is warming at more than twice the global rate and is undergoing an environmental transition typified by loss of summer sea ice and permafrost thaw. Warming temperatures and retreating sea ice are influencing the characteristics of Alaskan coastlines, lakes, and rivers.  Wetlands are changing in extent and distribution as riverine and coastal habitats become subject to increased erosion, saltwater inundation, and permafrost degradation.  Warming temperatures are advancing the start of the growing season, which may require migratory birds to arrive on breeding grounds earlier than in the past so that they can match their peak nutrient demands (during nesting and brood rearing) with peak forage production.  USGS Alaska Science Center waterfowl research is quantifying how climate factors (temperature, precipitation) are influencing the quality and quantity of waterfowl habitat and forage and if these changes are resulting in increases or decreases in populations.

Flooding

With increased temperatures, a decrease in sea ice extent, and greater weather variability in the Arctic, coastal tundra areas used by waterfowl for nesting and molting are changing.  Changes include inundation and flooding by salt water, thawing permafrost, and increased sedimentation of coastal habitats.  In some areas of Alaska, these changes appear beneficial for waterfowl populations, whereas in other areas these changes may impact waterfowl in more negative ways.  For example, on the Arctic Coastal Plain of northern Alaska, loss of sea ice has increased ocean wave action, leading to erosion and salt water inundation of coastal habitats. Saltwater tolerant plants are now thriving in these areas and this appears to be a positive outcome for geese in the Arctic. This finding is contrary to the deleterious effects that declining sea ice is having on habitats of ice-dependent animals, such as polar bear and walrus.

Fire

Fires are the major natural disturbance in the boreal forest, and their frequency and intensity will likely increase as the climate warms. Terrestrial nutrients released by fires may be transported to boreal lakes, stimulating increased primary productivity, which may radiate through multiple trophic levels, including to waterfowl that nest and raise young on lakes in the boreal forest.  USGS research is examining how fire influences nutrient and chlorophyll levels, macroinvertebrate diversity and abundance, and ultimately if these bottom-up processes affect waterfowl populations.  Such research answers questions about the resiliency of ecosystems to forest fires, thus allowing predictions for positive or negative effects as fire seasons become more frequent.

Sea Ice and Oceanic Conditions

Sea ice plays a major role in the breeding and wintering ecology of many northern latitude waterfowl species.  Additionally, broad-scale, multi-species changes in waterfowl population trends have been linked to changes in sea ice extent and oceanic conditions.  This link is due to the fact that many species of waterfowl spend more time during the year at-sea (such as sea ducks) than they do on land and that many other species winter and breed in coastal areas that are undergoing landscape change with declines in sea ice.  For example, declines in populations of North American sea ducks for unknown reasons has been a management concern for the past several years. However, USGS research into the influence of oceanic conditions on waterfowl population trends found that the perceived population declines of sea ducks appear to have halted >20 years ago, and populations have been relatively stable or increasing since that time.

Forage and Habitat Conditions

The quantity, quality and availability of habitats for waterfowl can be a major driver of population dynamics at breeding, wintering and stop over areas during migration. Migratory birds are reliant upon good quality habitats at specific times of the year to fuel long-distance migrations and energetically demanding events such as egg production, brood rearing, and surviving the over-wintering period. The USGS waterfowl research program has quantified habitats that are important to waterfowl species, such as marine areas used by sea ducks, eelgrass beds in bays and lagoons of the marine environment, coastal sedge grazing lawns, and interior lakes of the boreal forest. Habitats are constantly changing and waterfowl have been found to respond both positively and negatively to these changes.

Species Level Processes

Annual Survival and Reproduction

Species level biological processes are short- and long-term changes that determine the size, age composition, and overall health of populations.  USGS waterfowl research determines how populations are affected by birth and death rates, migration, wintering, and the flightless feather molting period that all waterfowl go through in late summer.  By quantifying species level processes during multiple times of the year, we can determine where potential population bottlenecks occur that may be limiting population growth or driving a population decline.