Population dynamics of an Arctic migrant: From individual fitness to long term trends

The natural world has been transformed since the Great Acceleration of the mid-20th century. Today, humanity is the dominant influence on nature, by way of land and sea modification, exploitation and climate disruption. The studies presented in this thesis aim to provide an overview of human impacts on the population dynamics of Arctic-breeding birds, with a particular focus on the Barnacle Goose Branta leucopsis, a migrant bird that breeds in the high-Arctic and winters in temperate Europe. The literature review for this thesis revealed that Arctic-breeding birds are strongly impacted by overexploitation (hunting for subsistence or sport), industrial activity (agriculture, hydrocarbon and mineral extraction, chemical manufacture and fisheries) and climate disruption (weather regime shifts, vegetation shifts, phenological mismatches and diminishing sea ice). It is well understood that Barnacle Goose numbers are strongly affected by exploitation and agriculture: in the first half of the 20th century, the population declined due to hunting pressure, but recovered once hunting became regulated and intensively managed agricultural grassland expanded across Northern Europe. The following three studies in this thesis explored ways in which weather regime shifts, phenological mismatches and chemical contamination may relate to individual fitness and long-term trends in Barnacle Geese. The first study uses a 50-year demographic dataset to analyse relationships between survival and productivity rates and temperature and precipitation shifts. This study demonstrates that weather regime shifts towards warmer and wetter conditions during spring migration positively influenced long-term survival rates and contributed to a population increase of Barnacle Geese. The second study uses satellite telemetry to determine the relative importance of photoperiod, air temperature and forage plant growth to migrating Barnacle Geese. Spring migration was closely associated with forage plant growth and autumn migration was closely associated with falling temperatures, both of which are vulnerable to phenological shifts associated with climate disruption, and, hence, vulnerable to phenological mismatches. The third study uses morphometric measurements and biosamples to analyse relationships between the key avian hormone corticosterone and key morphological traits associated with individual fitness: body weight, body size and facial plumage. This study concludes that exposure to levels of chemical contaminants in the environment that interfere with the normal functioning of corticosterone in Barnacle Geese could have consequences for body weight regulation, which is a key factor in survival and productivity. Thus, human activity has shaped the population dynamics of Barnacle Geese through the direct effect of attitudes to exploitation on survival (initially negative effects of overhunting, then positive effects of legal regulation) and the indirect positive effect of agricultural intensification and a warming climate on survival through bottom-up effects on forage plants. Barnacle Geese are one of a number of Arctic and northern terrestrial herbivorous birds and mammals with thriving population trends today (e.g. geese, swans, deer). This suggests that in the northern hemisphere, low-trophic herbivores (especially those with legal protection) are at an advantage. However, many of these species are actually in an ecological trap; while agricultural land modification and weather regime shifts currently provide plentiful forage resources, changes to agricultural practises or exacerbated climate disruption could remove this resource, with severe consequences. Furthermore, exceptionally high abundance in thriving species is often accompanied by negative externalities on other species in the ecosystem. Maintaining the balance of nature, biodiversity and ecosystem function should be a priority.