A long-term study of population dynamics of tardigrades in the moss Rhytidiadelphus squarrosus (Hedw.) Warnst

Variation of abundance, dominance and diversity of tardigrades were studied over a period of 54 months in a carpet of the moss Rhytidiadelphus squarrosus that covered a lawn in the Black Forest (Germany). Altogether 19,909 individuals belonging to 24 species were extracted from the moss. Macrobiotus hufelandi (56%), Macrobiotus richtersi (18%) and Diphascon pingue (12%) were the most abundant and dominant species. Dominances of the other species varied between 0.01 to 2.15%. Generally, species diversity (Shannon-Index and evenness) was highest during the winter. A temporal variation in numbers with a decline in winter and an increase in spring until fall was seen in M. hufelandi, D. pingue and less clear in M. richtersi. Three species (Diphascon rugosum, Hypsibius dujardini, Hypsibius cfr. convergens) showed a cyclic variation with clear peaks in wintertime. The moist season favoured species that were considered as hygrophilous in the literature, whereas the relatively dry sunny seasons promoted the relative increase of euryhygric species. The composition of the tardigrade community was strikingly robust over the years. Fourteen species were always present during the entire period of investigation. However, changes in the species composition over the years, and cyclical temporal presence of certain species stress the significance of long-term studies to estimate species richness in a given habitat. Application of the Spearman rank correlation test to the data set of the whole study period showed different correlations between the mean number of tardigrades as well as the mean number of individuals of selected species (dominance >1) and rainfall, humidity and temperature. Correlations confirmed some of the ecological dependences of tardigrade species known from literature. Conflicting classifications suggest dependence from other variables or perhaps presence of ecotypes.