An assessment of survey methods to estimate spring density of two ptarmigan species in arctic Norway

摘要

Both Willow ptarmigan (Lagopus lagopus) and Rock ptarmigan (Lagopus muta) are important game species in mainland Norway, but were recently added to the Norwegian Red List due to long-term population declines. Yet the knowledge about the population dynamics and the underlying drivers of both species is limited, especially for L. muta. Developing robust methods for long-term monitoring of population densities is required to increase ecological insight. The partially overlapping distribution of L. lagopus and L. muta in our study area in northeastern Norway provided a rare opportunity for inter-specific comparison of methods of population surveys and analyses to estimate spring density of breeding pairs. We used identical survey designs conducted simultaneously during 3 weeks in April-May within the same geographical area for both species. Despite having similar survey efforts we obtained considerably more data for L. lagopus than for L. muta, mainly due to differences in their densities, but also because their different behaviours and spatial distributions influenced their detectability. While distance sampling appeared to accurately estimate the density of L. lagopus, the same method was not possible to apply to L. muta, as the number of observations was too low. When using point count methods, the density for both species seemed to be overestimated. This appeared to be mainly due to the violation of the assumption of closed population. This violation was due to a proportion of birds still being aggregated in mobile flocks and because unusually warm weather and loss of snow cover for transportation of observers caused the survey period to be terminated before the birds were stably established on their breeding territories. The density as well as species-specific phenological and behavioural traits should decide the choice of monitoring methods, herein timing, survey design and effort. The initiation of territorial behaviour in ptarmigan is mainly induced by change in day length, but can also be affected by weather conditions. In light of increasingly earlier onset of spring in alpine and arctic tundra under climate change, the temporal mismatch between the peak of territorial activities and sufficient snow cover for transportation of field personnel for the survey may increase. Thus, it may become even more challenging to perform manual spring surveys on ptarmigan in the Norwegian terrestrial arctic by conventional methods. The use of new automated survey technology, such as acoustic sensors, may help to overcome such challenges.