Quantitative metabarcoding reveals the effects of ecological factors and invasive species on functional diversity of freshwater insects

Elia Lo Parrino, Isabel Cantera, Alessia Guerrieri, Benedetta Barzaghi, Mattia Falaschi, Raoul Manenti, Martina Muraro, Serena Pozzi, Gentile Francesco Ficetola

Aquatic insects represent the majority of freshwater biodiversity, yet they are less studied than other taxa. Responses of aquatic communities to stressors are rarely assessed and are usually measured in terms of taxonomic richness derived from presence-absence data, despite certain mechanisms that may cause a reduction in the biomass of certain species, without causing their disappearance. The integration of taxonomic and functional measures with biomass is often disregarded due to methodological difficulties, particularly concerning aquatic insects. The red-swamp crayfish Procambarus clarkii is an invasive crustacean capable of disrupting trophic webs in aquatic environments. As traditional survey methods for insects can require considerable effort, metabarcoding can be a useful tool for ecological investigations, but it lacks information on biomass.
We monitored 44 waterbodies in Northern Italy in 2021 – 2022, measuring ecological characteristics, including P. clarkii presence, and collecting insect specimens. We weighted insect specimens before DNA extractions. The relationships between metabarcoding data and biomass were used as correction factors to retrieve biomass of the seven insect orders in our communities. Then we calculated indices of taxonomic and functional diversity and assessed the effects of ecological parameters on functional traits using fourth-corner analysis.
We found weak negative effects of P. clarkii on taxonomic diversity, while functional diversity resulted strongly affected by it. Traits associated with mosquitos were negatively associated by Rana tadpole presence. We showed that functional diversity is more sensitive to P. clarkii presence than taxonomic diversity, thus being a more accurate measure of insects’ response to invasive species. Additionally, the presence of traits associated with invasive mosquitoes were negatively affected by the presence of Rana tadpoles. Given the negative impacts of Procambarus clarkii on amphibians in the study area, the role of this invasive species in the potential facilitation of mosquito invasion should be further investigated.