Citizen-science data shows long-term decline of snakes in southwestern Europe

It is difficult to obtain evidence of population trends for some animals due to their elusive behaviour and/or low density, hindering the acquisition of quantitative data to measure population size that would allow an evaluation of their vulnerability. This is the case for top predators such as snakes. This study explores the usefulness of citizen-science datasets, gathered between 1980 and 2018, in two large regions of southwestern Europe (Spain and the area of Languedoc-Roussillon in southern France) to estimate long-term snake population trends. We used the TRIM (Trends and Indices for Monitoring data) program to analyse trends in 14 snake species during the chronosequence, and modelled the response of each species (the slope of the trend) with a generalized linear mixed model (GLMM) in order to identify the functional traits that explain population trends. Overall, the results showed a general decline of snake fauna in both regions. The GLMM detected that this decline is attenuated in Eurosiberian species compared to Mediterranean species, as well as in species that dwell in forested habitats. The open Mediterranean landscapes in both study regions have been anthropically transformed, and the pressure on their biota has accelerated over recent decades due to habitat loss and degradation. This degradation is affecting Mediterranean organisms such as snakes. The framework we developed allows citizen-science data to be used as a tool to detect population trends and to apply IUCN criterion A (Population size reduction) to species with a high level of imperfect detection or that are suspected to be inaccurately ranked in the assessment of their vulnerability.