Differential survival rates in a declining and an invasive farmland gastropod species

Individual marking by using transponders represents a promising tool to further investigate causes of population trends of farmland invertebrates and the dynamics of farmland biodiversity. Once common in agricultural fields the slug Arion rufus is now restricted to forests, whereas the closely related and similar Arion lusitanicus recently built up high populations. Here, it was tested whether (1) transponder tagging combined with artificial shelters for recapturing is a suitable approach for quantifying adult survival of farmland gastropods under real field conditions, (2) whether captive bred and wild A. lusitanicus differ in survival, and (3) whether in agricultural fields the declining slug A. rufus shows reduced survival rates compared to the invasive slug A. lusitanicus. One hundred and thirty transponder-tagged slugs were released and individual-based mark–recapture models were performed. Individual marking by using transponders proved to be a successful method for quantifying survival rates of the slugs. The favoured model showed no differences in survival or encounter rates between captive bred and wild A. lusitanicus. A. rufus showed significantly reduced survival rates compared to A. lusitanicus coinciding with their population trends in farmland areas. This might be due to differential susceptibility to habitat degradation, differential predation rates or promotion of the invasive competitor species. A further decrease of A. rufus in agricultural landscapes is expected.