Abundance, diversity and seasonal succession of dytiscid and noterid beetles (Coleoptera: Adephaga) in two marshes of the Eastern Po Plain (Italy)

The dytiscid (plus Noteridae) communities of two neighbouring marshes (A and B) in the Eastern plain of the Po river (Italy) was investigated. The two marshes showed similar surface area, plant species and abundance, quantity of detritus, exposure and depth variations. Physical and chemical variables of the two habitats were similar. Phenology curves based on larval and adult water beetles showed two peaks, in spring and autumn. However, the number of specimens and the variety of species were significatively higher in marsh A than in marsh B. The occurrence and the abundance of the main vertebrate and macroinvertebrate taxa in the two habitats were measured using a relative method. There was little difference in densities of prey animals (tadpoles, newt larvae, crustaceans. ephemeropteran nymphs and dipteran larvae) in the two marshes. Instead, a higher abundance of odonate larvae and hemipteran nymphs and adults were found in marsh B compared to marsh A. Difference in the number of species of aquatic beetles and their abundance could be partially due to the stability of marsh B compared to marsh A. Probably, competition and predation by hemipterans and odonate larvae on dytiscid larval stages could be implicated in determining water beetle communities structure in the two neighbouring marshes; thus, this phenomenon is discussed here.