Explaining declines of newt abundance in northern Italy

1. Amphibians are an exemplary case of the current biodiversity crisis, being among the vertebrates suffering the fastest decline. Population dynamics of amphibians can result from processes acting at different scales. Both the local characteristics of breeding wetlands and the features of the surrounding landscape can strongly affect the temporal dynamics of amphibian populations. European newts are particularly threatened by land-use change and invasive alien species. While it is known that newts are declining across Europe, few studies have performed broad-scale assessments of their decline, either because abundance dynamics are more complex to analyse than presence/absence data or because they require a high sampling effort and long-term monitoring. 2. In this study, we show that long-term distribution data can be combined with demographic models to quantify the decline in abundance of newt species at the regional scale, and to assess the importance of multiple factors in determining abundance dynamics. We performed multiple surveys between 1996 and 2020 and used N-mixture models in a Bayesian framework. We then calculated abundance changes between the first and the last sampling season, which were performed with an average timespan of 13 years across all wetlands. 3. Both Italian crested newts and smooth newts showed large declines, with an average estimated abundance loss between the first and last sampling season of 57% and 63%, respectively. Local characteristics of the wetlands were the main determinants of abundance dynamics: the abundance of both species showed a positive relationship with the area and the permanence of the wetland and a negative relationship with the presence of fish. Additionally, the abundance of Italian crested newts was negatively related to the presence of invasive crayfish. No relationship was detected between abundance and terrestrial habitat availability or connectivity measures. 4. Despite uncertainties in the absolute values of estimated abundance, the striking regional-scale decline of newts is evident. Among the major determinants of population dynamics, fish and crayfish presence increased their prevalence in the study area, while other factors remained more stable. Management actions aimed at eradicating or controlling invasive fish and crayfish might halt abundance loss and even revert this declining trend. 5. The application of N-mixture models to long-term data from representative sites permits the analysis of temporal trends of species at the regional scale even when data come from complex monitoring schemes. We found large declines in abundance of two newt species, suggesting that European newts may be more threatened than previously thought.