Estimating population trends of a riparian bird community in low-Arctic tundra.

Abstract

In the Northern Norway tundra, willow thicket along river valleys create a more productive environment than the surrounding open tundra and host a rich community of bird species by providing food, shelter from predators and nesting site. The state of Arctic ecosystem is expected to undergo long-term changes due to opposite effects of climate change and herbivory pressure. It is also the background of a large natural temporal variability in small rodent population cycles and date of onset of spring. This study aims at evaluating the status of the riparian bird community and at investigating the effects of temporal variation in rodent abundance and onset of spring on the growth rate of bird species. Bird abundance was recorded in the Varanger Peninsula, during seven years over the period 2005-2016, using a spatially and temporally repeated point count sampling method. A hierarchical model (the generalised N- mixture model of Dail and Madsen) is used to estimate the abundance and growth rate while taking into account the detection probability of the recorded species. The effects of spatial and temporal variables on abundance, detection probability and growth rate are estimated through the inclusion of environmental covariates in the model. Two species, the rough-legged buzzard and the willow grouse had a neg- ative population trend, to the point of reaching local extinction. No other species exhibited discernible population trends, despite the general loss of total abundance, species richness and occupancy of the bird community. No clear long-term change in the onset of spring or the rodent abundance was found, but a few species were found to benefit from an earlier onset of spring or from higher rodent densities. This indicates that the studied bird community is generally getting poorer. However, if long-term environmental changes happen in accordance to climatic predictions, spring events will occur earlier, rodent cycles might dampen and the shrub thicket habitat could gain in areal extent. Thus, species will probably show differential responses to these environmental changes. Species able to adapt their phenology to future temporal changes, species that already benefit from earlier onset of spring and species with a low level of habitat or diet specialisation will be expected to perform better than other less adaptable species. This study provides more insight in the status of the bird community associated with the low-Arctic riparian willow thickets by revealing the virtual local extinction of two species in the studied region and the general impoverishment of the community. Longer-term monitoring will be needed to observe clear trends in the population of other species and to determine the effects of environmental changes on the bird species.