To see the edible dormouse populations trend in Catalonia an annual rate of change was calculated using the TRIM method (TRends and Indices for Monitoring data) with the statistical package rtrim.
The TRIM method calculates indices for each time period based on a Generalised Linear Model (GLM), assuming that counts follow a Poisson distribution. In case that a station has not been sampled for some periods, TRIM still calculates its expected values from the changes observed in the rest of the stations. These indices do not reflect the real abundance of the species, but indicate the percentage change between two periods, with an associated standard error. Moreover, they are corrected to minimise the effect of two aspects: overdispersion, when the variance of the results is higher than we would expect by chance, and autocorrelation, the fact that the number of individuals captured tends to depend on those captured during the previous season.
From the total number of stations of the Dormouse Project, we have selected those with data from 2012, and we have not considered the stations located outside the Catalan territory, such as Andorra, having a total of 48 valid stations for Catalonia. Therefore, the trend of the edible dormouse in Catalonia has been calculated for the ten-year period from 2012 to 2021.
For this calculation, the maximum number of adult dormice observed per checking day of each season was used, and for each checking day the individuals were added up and divided by the number of nest boxes checked. Finally, prevailed the checking day with the highest number of adult dormice observed of each season. This calculation allows us to include both advanced and basic protocol stations.
The figure shows the change in abundance of the edible dormouse ( Glis glis ) between different sampling seasons of the Dormouse Project (2012-2023). The stations where there is data from 2012 onwardshave been selected and stations outside the Catalan territory have not been considered. The rate of change represented is scaled according to the TRIM method and shows a moderate increase of thespecies in Catalonia.