Does the alien Lemna minuta show an invasive behavior outside its original range? Evidence of antagonism with the native L. minor in central Italy

The American duckweed Lemna minuta Kunth is considered an invasive species in several European countries (e.g., Belgium, Germany, Hungary, Ireland, Italy), but its invasiveness outside its original range remains poorly studied. We therefore analyse: (i) the speed with which L. minuta has spread throughout Europe; and (ii) its capability to compete with L. minor L., the most common European native duckweed. Based on literature data, the increasing number of observations of L. minuta since it was reported for the first time in France (1966) indicated the wide extent reached by this species in most of Europe, highlighting its high capacity to colonize new areas. By sampling Lemna populations from 41 stations in standing waters in Central Italy, it emerged that the frequency of the two species was similar, but the alien species was more abundant than the native one, showing generally higher coverages, and predominating in mixed Lemna populations. A negative correlation between L. minuta and L. minor was observed in our study. This relationship was not explained by different ecological requirements, since the two species responded similarly to the main environmental gradient, but was linked to an antagonistic dynamism between both species. A multitemporal analysis, based on the comparison of Lemna coverage data recorded at the same stations at two different times (last vs. first record), revealed that L. minuta had appeared where it had been absent previously and that its abundance had increased to the detriment of L. minor, which in some cases disappeared within a few years. The arrival of L. minuta in a site can be considered a real threat to conservation of the native plant diversity due to its competition with the congeneric L. minor.