A mesocosm experiment testing the competition between the invasive species Heteranthera reniformis Ruiz & Pav. and the endangered fern Marsilea quadrifolia L

Biological invasion is one of the most severe threats to biodiversity, with invasive species altering ecosystem functioning and services. Once they arrive in the recipient community, invasive species are likely to spread and outcompete natives through higher competitive ability, with phylogenetically distant species expressing lower degree of competition than close related species. We present the results of a mesocosm experiment where a wet habitat was reproduced aiming at understanding the competitive ability of the threatened fern Marsilea quad-rifolia L. and the invasive monocot Heteranthera reniformis Ruiz & Pav., two species that can coexist in paddy fields of S-Europe. The species were cultivated both in monoculture and in mixed cultures at different pro-portions. We assessed interspecific competition using relative growth rate, specific leaf area, leaf dry matter content, chlorophyll content, leaf nitrogen and phosphorus content as main indicator of plant efficiency in acquiring resources and physiological stress. Contrarily to what we expected, M. quadrifolia expressed high competitive ability in terms of plant growth versus the invasive species, with H. reniformis maintaining some of the traits typical of invasive species, i.e., higher specific leaf area, nitrogen and phosphorus content. The per-formance of the threatened species depended on plant mixtures, whereas H. reniformis performed best in monoculture. Although higher competitive ability of the invasive species cannot be ruled out in crop cultivation context, the mesocosm experiment demonstrated that the threatened species M. quadrifolia may have chances of survival in co-occurrence with the invasive species H. reniformis. In view of a robust conservation plan for the species, field-based experiments in the real context need to be performed to completely understand the rela-tionship between species, also taking into account the crop species.