Population trends of four species of amphibians in western France; results from a 15 year time series derived from road mortality counts

Abstract

Reports of declining amphibians are a major concern for conservation given their role as indicators of environmental change. This includes potentially impacting on the ecology other species and highlights the importance of monitoring long-term changes in amphibian populations. The core difficulty is gathering long term data sets that as well as being time consuming, may be costly and impractical for logistical reasons. Road mortalities have value in this respect since they are highly visible and have been frequently used as a metric to monitor changes in a variety of animal populations. In this paper they have been employed as proxies to estimate long-term population changes in four European species of amphibians. From January 2005 to December 2019 a total of 747 amphibian mortalities were recorded, the majority of which were urodeles. Most individuals were adults that entered roads during the annual migration period. Long term annual counts fluctuated widely in all species and peaked during the 4-year period from 2009 to 2012 with second period of high numbers in Triturus marmoratus and Hyla arborea during 2018 and 2019. Long-term population trends were evaluated using regression analysis of the logarithmic (loge) transforms of annual counts against year as independent variables, which were then tested against a hypothetical 0 regression coefficient, indicative of population stability. The results indicated long-term stability in T. marmoratus and H. arborea but potential declines in Lissotriton helveticus and Pelophylax lessonae, the latter showing metapopulation extinctions followed by recolonisation. Stepwise regression of potential climate drivers in amphibian numbers suggested a 2-year lag of rainfall during October and December were potential factors involved in population change.