A trait based-approach to the diet of grass snakes (Colubridae:Natrix): opportunists with preferences

Understanding the mechanisms by which predators select their prey can help us understand the evolution of ecological interactions and community-level patterns and can also have conservation implications. Grass snakes (Natrix spp.) are predators specialising on amphibians and fish. In this study, we used citizen science data and observations from social media to understand the mechanisms of prey selection in the snakes by coupling dietary and occurrence data to morphological and ecological traits of prey. Based on our results, the Iberian grass snake (N. astreptophora) and the dice snake (N. tessellata) specialises on amphibians and fish, respectively, while the barred grass snake (N. helvetica), the viperine snake (N. maura) and the grass snake (N. natrix) feed on both groups. We used Generalised Linear Models to test for the effects of prey ecological traits on the number of predation records for each species, and we found that among amphibians, most species prefer larger and more aquatic species, while prey abundance is also an important factor in prey selection. Among fish, most species prefer more abundant species, while the two species that consume fish to a lesser extent – N. helvetica and N. natrix – prey mostly on benthic species that are less agile swimmers and thus might be easier to catch. Defensive traits of both fish and amphibians were largely unimportant, however, amphibian toxicity had a negative effect on the number of predation records in the case of N. helvetica, while N. natrix selected fish species with defensive dorsal spine modifications and lower scale complexity more often.