Mass invasion of the Ponto-Caspian amphipods in Masurian Lakeland associated with human leisure activities

Non-indigenous species (NIS) contribute to the decrease of native species’ diversity on a local and global scale. One of Europe's most significant donors of freshwater invasions is the Ponto-Caspian region. Following the construction of artificial canals connecting isolated water bodies and resulting heavy boat traffic, the Ponto-Caspian Amphipoda started to spread in Europe. Four species: Dikerogammarus haemobaphes, Dikerogammarus villosus, Pontogammarus robustoides and Chaetogammarus ischnus invaded Masurian Lakeland (North-Eastern Poland). Based on the literature and our data, we studied their distribution in 14 lakes in the region in the years 2001 - 2016. We analysed their distribution against several water quality parameters and levels of anthropogenic pressure. Our results are also the first records of two new invaders - D. villosus and C. ischnus in the studied area. We show that the relative abundance and frequency of these two species rapidly increase, and simultaneously the populations of the older invaders, D. haemobaphes and P. robustoides, decrease. The native species - Gammarus lacustris - seems to be negatively affected by NIS richness as well as by the proximity of cities. The NIS found in the lakes appear to be facilitated by boating and the lower complexity of the shoreline. Our study shows how anthropogenic pressure and tourism in the specific, may directly aid bioinvasion, mining the survival of native biodiversity without proper regulation.