Influence of weather on a population of soprano pipistrelle bats in West Wales, UK: a 20 year study estimates population viability

A nursery roost of the soprano pipstrelle bat Pipistrellus pygmaeus has been moni- tored continuously between 1 April and 27 September (Weeks 1�26) for 20 yr to promote conser- vation of the species, which declined over the last century. The long-term study, essential to esti- mate a reliable population trend, is linked to environmental changes as possible causes of decline. The main emergence was from May to July (Weeks 6�18) when 550 ± 190 (SD) P. pygmaeus were counted. Analysis showed that the year-to-year change in population size of female P. pygmaeus (ΔN)and the time of the peak exit count of the females from the roost in May to June could be pre- dicted from the integrated air temperature (degree days, D) between 1 January and 31 March. The regression of ΔN on D showed a statistically significant linear regression line defined by ΔN = 1.31�0.0015 D (R 2 = 38.4%; p = 0.005). ΔN = 1.04 to 1.33 reflected a stable to increasing population (Years 1�7 and 15�20). ΔN = 0.78 to 1.00 reflected a stable to decreasing population (Years 8�14). The regression of the time of the first peak exit count, calculated as the number of weeks from 1 April, on D showed a statistically significant linear regression line defined by Weeks = 12.75�0.020 D (R 2 = 32.8%; p = 0.008). Rising temperatures between January and March would be detrimental to P. pygmaeus populations.