Coexistence of spiders in floodplain forests as an indicator of ecological stability and landscape sustainability in the inland Danube Delta

The coexistence of spiders in the Central European floodplains of the Danube River was assessed at 18 study sites (more than 43,000 individuals identified). Environmental conditions, i.e. groundwater level, flood regime, vegetation and tree shading, were defined as the main factors. The presence or absence of flooding and the depth of the water table had a significant effect on spider community structure. Based on the evaluation of 281 spider species found at sites in the inland Danube Delta, we conclude that the typical species of floodplain forests are the same, and thus the coexistence of these dominant and frequent species is a fundamental element of epigeic spider communities in floodplain forests in Central Europe (Piratula hygrophila Thorell, Pardosa lugubris Walckenaer as a leaders; and typical spiders − Bathyphantes nigrinus Westring, Diplocephalus picinus Blackwall, Diplostyla concolor Wider, Oedothorax retusus Westring, Pachygnatha clercki Sundevall, Pachygnatha listeri Sundevall, Pardosa amentata Clerck, Pirata piraticus Clerck, Agroeca brunnea Blackwall, Liocranoeca striata Kulczyński, Ozyptila praticola C. L. Koch and Trochosa ruricola De Geer and two euryhygrophilous species Centromerus sylvaticus Blackwall, Palliduphantes pallidus O. Pickard-Cambridge). We divided ground-dwelling spiders into four categories by using a simple dominance and frequency index (DF_flood = ∑ D_hygro · F_hygro · 0.01 / ∑ D_eury · F_eury · 0.01), which also characterize floodplain forest microhabitats and reflect their stability. The assessments of the status of floodplain wetland forest biota based on the DF-index values represents not only spatial but also temporal aspects, as well as the possibility of using it to determine the ecological stability of the landscape.