The impact of agricultural contaminants on populations of the European common toad

Amphibians are the most threatened class of vertebrates worldwide, according to the IUCN Red List. Habitat loss and degradation are considered to be major threats causing population declines. Even common and widespread species, such as the European Common toad (Bufo bufo), are facing gradual population declines. Even a minor decline of a common species results in a significant decrease in biomass, which is expected to have major effects on ecosystem functions and services. Therefore, it is crucial to understand the causes of toad population decline in order to formulate effective conservation strategies. Agricultural contaminants were analysed in water samples from ten ponds with declining toad populations and compared to ten ponds with stable populations, selected throughout different ecoregions in Flanders (Belgium) and sampled monthly over a period of 3 months. Pesticides (n=112), mycotoxins (n=20), and antiparasitic drugs (n=15) were analyzed by targeted ultra-high performance liquid chromatography-tandem mass spectrometry methods (UHPLC-MS/MS). Furthermore, 46 antimicrobial drug residues were detected using UHPLC combined with high resolution MS (UHPLC-Orbitrap-HRMS). For the analysis of 11 heavy metals, both inductively coupled plasma - optical emission spectrometry (ICP-OES) and ICP-MS were used. In addition, nutrient (n=5) levels were measured using ion-exchange chromatography (IC), and total nitrogen content was determined using a Kjeldahl digestion combined with a distillation-titration method. An exploratory data analysis of a subset of samples indicates that declining populations are linked to slightly higher total concentrations of nutrients and pesticides, but heavy metals and total nitrogen content do not appear to be related to population status. Other analyses are currently in process. A Principal Component Analysis (PCA) will be carried out to explore the relationships between the measured concentrations of contaminants and population status, and potential interactions between contaminants. By linking toad population decline to the presence of certain contaminants, we can provide effective conservation strategies to mitigate threats to an ecologically valuable species. Future research will focus on contaminants that were identified as possible drivers of population decline, in order to fully understand how these contaminants affect toad health.