| dc.description.abstract | Ongoing climate change can be expected to have major consequences for many species and ecosystems. Effects have been reported on reproduction, survival and movement ecology, with potential impacts for population dynamics and species interactions. Mountains are among the ecosystems that are expected to be particularly exposed to climate change, and observed changes include increased temperatures, shorter winters, reduced snow cover, and changed precipitation patterns. For many montane species there is limited information of the relationships between climatic variation and life history traits and demography. Such lack of knowledge makes it challenging to predict population dynamics under a changing climate, and potentially mitigate the effects of climate change.
In this thesis, I investigated how short-term variation in climatic factors affects life history traits and demography of willow ptarmigan (Lagopus l. lagopus). Willow ptarmigan is a common and iconic bird species in northern hemisphere mountains, with long-term population declines at least in parts of its distribution range. It is resident to Arctic and low-alpine tundra, including the tundra-forest ecotone, but can perform shorter seasonal migrations within these environments. Empirical research on the detailed responses of ptarmigan to climatic variation is so far limited. I used data from two study areas in Norway, one historical (Dovrefjell, 1978-1994) and one ongoing (Lierne, 2015-2021), to test hypotheses related to reproductive strategies, movement strategies and survival in seasonal environments, with particular focus on the role of climatic variation.
I found support for a common clutch size across individuals, meaning that the number of eggs laid was independent of body mass, age, and climatic variables. Although females with high body mass did not produce more eggs, they did produce more hatchlings from large clutches than females with low body mass. Further, I found that increased spring temperatures advanced onset of egg-laying, and that early egg-laying was followed by a higher number of offspring. Both timing of egg-laying and clutch size were highly repeatable within individuals.
Willow ptarmigan selected for well-covered nest sites and for nest sites among trees. As a relative measure of how strongly the individuals selected, I calculated individual nest site selection scores based on the relative availability of nest cover and habitat features. The nest site selection scores were not affected by body mass, age, or climatic variation. However, nest site selection scores were related to nesting success, where successful nests were characterized by stronger nest site selection, in addition to a positive effect of an early arrival of spring.
The study population in Lierne was partially migratory, and the probability that juveniles migrated increased with lower body mass. Migratory strategy for adults was not affected by body mass. There was high repeatability in migratory strategy between seasons. Migrants and residents did not differ in terms of clutch size or nesting success.
When the snow arrived early in the previous autumn, there was a delayed mortality risk during spring for yearlings, but not adults. This may be caused by a reduced access to nutrient-rich food after the arrival of winter, resulting in a carry-over effect on mortality risk in the following spring. For both yearlings and adults, spring mortality risk also increased with increased snow depth during spring. This may reflect limited access to food resources in a time when body mass reserves are low after the winter. I found no relationship between snow depth and autumn mortality risk.
The findings presented in this thesis indicate that the study species is sensitive to short-term variation in climatic factors, in terms of both seasonal survival and annual reproduction. In light of the projected climatic changes in the future, the results per se may apparently point in a positive direction for willow ptarmigan, where earlier and warmer springs increase reproductive success, and a later arrival of snow in the autumn and less snow in the spring increase survival prospects. However, responses in reproduction and survival to climatic factors may involve a complex set of ecological mechanisms and trade-offs between life history traits. Integrated approaches may further improve our understanding of the vulnerability of willow ptarmigan and the mountain ecosystems to climatic changes. | en_US |
