Dynamics and population structure of native Echinogammarus stammeri (Karaman, 1931) (Crustacea: Amphipoda) and non-native Gammarus roeselii Gervais, 1835 (Crustacea: Amphipoda) occurring in sympatry in Northern Italy

Gammarids are one of the most successful invaders in freshwater ecosystems due to both their diet plasticity and high reproductive capability. The Balkanic amphipod, Gammarus roeselii Gervais, 1835, has colonised a small canal in the southern part of the sub-lacustrine Ticino River basin (Northern Italy), where it lives in sympatry and shares the same habitat with the native species Echinogammarus stammeri (Karaman, 1931). We surveyed the populations of the two species over 12 months (from July 2014 to June 2015) to investigate their structure and dynamics. The overall densities of the two populations were similar in the study area, but we observed marked differences in the population structure: G. roeselii was in fact more successful than the native gammarid in achieving the adult stage, and E. stammeri exhibited a rather limited reproductive period, which does not justify the stable occurrence of juvenile individuals. We hypothesise that the population of the native gammarid in this small habitat is supported by a continuous upstream immigration of individuals from the Ticino River, while G. roeselii exhibits a well-structured and self-reproducing population.