Understanding climate change impacts in an Arctic ecosystem: an integrated approach through the prism of Svalbard reindeer

Arctic ecosystems are natural laboratories for climate change studies, because they are simple and undergo rapid warming. We focus our research around the paradox that the reindeer population has increased 2-3 fold in a time period with rapid climate change, by investigating if this change is due to changes in the diet plants, a relaxation of parasite load or ongoing behavioral and physiological adaptations. In the botanical work package, we build on our 2021 discovery that experimental heating in autumn delayed senescence of a grass species but no other functional plant groups (a shrub and a forb). This year we tested if onset of senescence was also temperature dependent, by advancing the experimental heating by one month. These results will be the basis of Caroline Ivelands master thesis. We also conducted a moisture manipulation experiment of selected forage plant species, which will be analyzed by master student Emilie Kvåle Sletterød Andersen. In the parasite work package, we ask the question: what has happened to the parasite regulation of Svalbard reindeer that was important to depress fecundity of reindeer 20 years ago. Despite nearly three times increase of the reindeer population, the parasite egg counts in reindeer feces were lower than two decades ago, both last year and this year. This substantiate that something has interrupted the host-parasite cycle. One possibility is that the infection rate is lower because forage plants grow taller in a warmer environment and/or that the parasitic larvae are forces deeper in the substrate to avoid high temperatures and desiccation. This is investigated experimentally and through observational studies by PhD student Tirza Moerman and master student Kia Karina Tahmin. In August and October 2022, 30 female reindeer was culled and adult parasite in the abomasum will be counted in the lab this winter to provide the gold standard for assessing parasite load, to corroborate the declining egg counts found in fecal samples. In the physiology work package, we focus on investigating the mechanisms of energy conservation in Svalbard reindeer. Previously, we have detected that there are large individual differences in thermoregulation; some individuals strongly reduce their subcutaneous body temperature during rest, while others don’t. This year we for the first time obtained multiple years of body temperature data for the same individuals. This clearly show that the ability to reduce body temperature in winter is a plastic trait. Individuals showed much higher heterothermy in 2018 compared to 2021, and Post Doc Monica Trondrud is searching for mechanisms. In the diet work package (with connections to all others) we have invested heavily in field work. A total of 55 female reindeer have GPS-collars and body temperature loggers. Three master students repeatedly visited as many as possible of them during 90 days from summer through autumn. The main aim has been to record their diet to search for items allowing for heterothermy in winter and to investigate which foraging strategy result in maximum fatness in autumn. Autumn is singled out as a focal season because of the strong impact on reproductive fitness (Loe et al 2021). From the grazing observations we found that many individuals focus on flower heads in summer (in particular Ranunculus), known to be rich in omega 6 fatty acids, known to facilitate heterothermy. We also observed grazing on roots and rhizomes, that may be important ephemeral resource in late autumn when conditions are right (mild and little snow). Oline Eikelands master will be based on this material. Detailed recordings of activity type were conducted for the GPS-animals. This will provide validation data for integrated accelerometers in the collars. Emma Djurberg will in her master investigate if accelerometer data can be used to separate grazing from other activity types, and even if different types of grazing activities can be separated (walking grazing and standing grazing). This will answer if individuals have different grazing strategies depending on which dietary items they target. Even Unsgård will in his master investigate if individual differences in diet selection and feeding strategies impact on fat accumulation, utilizing the GPS-marked individuals shot in October 2022. We have continued the work on characterizing the diet of Svalbard reindeer using DNA metabarcoding. This work is reinforced through the NFR network project «PIECEMEAL: Cracking the diet of the Svalbard reindeer by integrating old and modern tools». We are in a phase where we are problem shooting why three different methods of diet characterization yield different results. We are also finalizing a manuscript describing the microbiota of the reindeer gastrointestinal tract and their functional role.

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Terrestrial Arctic ecosystems are natural laboratories to study the effects of climate change, because of their simplicity and fast rate of warming. In Svalbard, reindeer are a keystone species directly affecting lower (plants and parasites) and indirectly higher trophic levels (carrion for Arctic fox), and therefore central to understanding ecosystem resilience. For more than 20 years we have studied tundra vegetation dynamics, reindeer impacts on habitat structure and ecosystem processes. At 4500 captures of 1000 individual reindeer we have the largest dataset for any Arctic herbivore. Two decades ago, we published what became a textbook study reporting that reindeer were strongly food limited and regulated by gut parasites, rendering the subsequent doubling of the population size an unsolved paradox. In our current RCN project, we discovered two phenotypes with contrasting thermoregulatory physiology; one drops subcutaneous body temperature to save energy, while the other keeps a stable high temperature throughout winter. The two phenotypes also differed in their diet, gut microbiome, body mass dynamics and reproduction, begging the question if one is better adapted to the ‘old’ and the other to the ‘new’ climate regime. We will develop innovative models of how the different phenotypes contribute to reindeer population growth, as the climate continues to warm. However, such models would not be realistic without considering trophic interactions, leading us to return to our unsolved population increase paradox, by investigating the role of higher plant productivity and the possible loss of parasite regulation. By looking at the issue of climate change in the Arctic through the prism of an ecosystem engineer, Svalbard reindeer, our work plan is both novel, because of its holistic scope, working across boundaries of sub-disciplines in biology, and ambitious, in moving empirical measurement out of traditional laboratory settings, into a rapidly warming world.