Accurate measures of species abundance are essential to identify conservation strategies. N-mixture models are increasingly used to estimate abundance on the basis of species counts. In this study we tested whether abundance estimates obtained using N-mixture models provide consistent results with more traditional approaches requiring capture (capture-mark recapture and removal sampling). We focused on endemic, threatened species of amphibians and reptiles in Italy, for which accurate abundance data are needed for conservation assessments: the Lanza’s Alpine salamander Salamandra lanzai, the Ambrosi’s cave salamander Hydromantes ambrosii and the Aeolian wall lizard Podarcis raffonei. In visual counts, detection probability was variable among species, ranging between 0.14 (Alpine salamanders) and 0.60 (cave salamanders). For all the species, abundance estimates obtained using N-mixture models showed limited differences with the ones obtained through capture-mark-recapture or removal sampling. The match was particularly accurate for cave salamanders in sites with limited abundance and for lizards, nevertheless non-incorporating heterogeneity of detection probability increased bias. N-mixture models provide reliable abundance estimates that are comparable with the ones of more traditional approaches, and offer additional advantages such as a smaller sampling effort and no need of manipulating individuals, which in turn reduces the risk of harming animals and spreading diseases.

Many organisms live in networks of local populations connected by dispersing individuals, called spatially structured populations (SSPs), where the long-term persistence of the entire network is determined by the balance between two processes acting at the scale of local populations: extinction and colonization. When multiple threats act on an SSP, a comparison of the different factors determining local extinctions and colonizations is essential to plan sound conservation actions. Here we assessed the drivers of long-term population dynamics of multiple amphibian species at the regional scale. We used dynamic occupancy models within a Bayesian framework to identify the factors determining persistence and colonization of local populations. Since connectivity among patches is fundamental for SSPs dynamics, we considered two measures of connectivity acting on each focal patch: incidence of the focal species and incidence of invasive crayfish. We used meta-analysis to summarize the effect of different drivers at the community level. Persistence and colonization of local populations were jointly determined by factors acting at different scales. Persistence probability was positively related to the area and the permanence of wetlands, while it showed a negative relationship with the occurrence of fish. Colonization probability was highest in semipermanent wetlands and in sites with a high incidence of the focal species in nearby sites, while it showed a negative relationship with the incidence of invasive crayfish in the landscape. By analyzing long-term data on amphibian population dynamics, we found a strong effect of some classic features commonly used in SSP studies, such as patch area and focal species incidence. The presence of an invasive alien species at the landscape-scale emerged as one of the strongest drivers of colonization dynamics, suggesting that studies on SSPs should consider different connectivity measures more frequently, such as the incidence of predators, especially when dealing with biological invasions.

Invasive predators can strongly affect native populations. If alien predator pressure is strong enough, it can induce anti-predator responses, including phenotypic plasticity of exposed individuals and local adaptations of impacted populations. Furthermore, maternal investment is an additional pathway that could provide resources and improve performance in the presence of alien predators. We investigated the potential responses to an alien predator crayfish (Procambarus clarkii) in a threatened frog (Rana latastei) by combining field observations with laboratory measurements of embryo development rate, to assess the importance of parental investment, origin and exposure to the crayfish cues. We detected a strong variation in parental investment amongst frog populations, but this variation was not related to the invasion status of the site of origin, suggesting that mothers did not modulate parental investment in relation to the presence of alien predators. However, cues of the invasive crayfish elicited plastic responses in clutches and tadpoles development: embryos developed faster when exposed to the predator. Furthermore, embryos from invaded sites reached Gosner’s development stage 25 faster than those from non-invaded sites. This ontogenetic shift can be interpreted as a local adaptation to the alien predator and suggests that frogs are able to recognise the predatory risk. If these plastic responses and local adaptation are effective escape strategies against the invasive predator, they may improve the persistence of native frog populations.

Aim Understanding which factors determine the variation in population size across space and time is crucial to plan sound conservation interventions. Amphibians are often characterized by large demographic changes, therefore a better understanding of factors driving these changes could help mitigate their global crisis. We investigated drivers of abundance dynamics of two similar frog species to understand why they met different fates in the same study area. Location Northern Italy. Methods In seven different years between 2004 and 2020, we performed repeated counts of egg masses of two similar frog species, the agile frog (Rana dalmatina) and the Italian agile frog (Rana latastei), in 31 wetlands, and used Bayesian models to estimate the relationships between frog abundance and candidate drivers acting at the scale of (i) site: wetland surface, shading percentage and the presence of the invasive red swamp crayfish (Procambarus clarkii); (ii) landscape: forest cover around wetlands; (iii) climate: yearly precipitation and (iv) spatially structured population (SSP): clutch incidence of the focal species and crayfish incidence in the surrounding landscape. Results The two species showed sharp differences in population size and trends: R. dalmatina was abundant and showed a stable trend throughout the entire study period; R. latastei showed low abundance in the first years and then almost disappeared. The abundance of R. dalmatina was positively related to forest cover, shading, wetland area and precipitation, while negatively related to the occurrence of invasive crayfish at both local and SSP level. R. latastei abundance increased with wetland area and precipitation, while models were not able to detect relationships with other factors. Main conclusions The high sensitivity to drought and demographic stochasticity could have contributed to the quasi‐extinction of R. latastei in the study area, highlighting that similar species can meet different fates even under the same environmental conditions.

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. 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. 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. 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. 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.