Fine-scale patterns in micrometazoans: tardigrade diversity, community composition and trophic dynamics in leaf litter

Studies on micrometazoan local distribution patterns are needed to understand microscopic organisms' evolution in the light of the related influences of historical and ecological events. Tardigrades encompass a set of rare and unique characteristics to study evolutionary hypotheses. Particularly interesting are leaf litter tardigrades, due to their connection with soil dynamics: although tardigrades have been frequently considered occasional or accidental inhabitants coming from nearby habitats. In this study, leaf litter tardigrade diversity from a central Spain mountain system was determined using four diversity descriptors: species richness, abundance, and two diversity indexes (Shannon and Simpson), and a diversity trophic index (TD). In addition, community composition was analysed using Canonical and Discriminant Analyses. Fifteen types of leaf litter were explored in the present survey, many not previously studied, all of them containing tardigrades. We have found 39 species, 20 are being recorded for the first time in leaf litter, and increasing to 20% the global tardigrade diversity reported for leaf litter worldwide, with the total number of species now being 99. The same genera that were reported being present in high species diversity at global scale were found to be highly diverse at local scale in this study. Three eutardigrade families previously reported as beech litter inhabitants were not found in our survey, probably because beech litter was not included in the present survey, suggesting that further work studying a more diverse range of litter would increase reported tardigrade diversity worldwide. Tardigrade species abundances depended on high level plant categories (plant divisions and type of tree), while tardigrade diversity patterns were related with type of leaves. Broad leaves had higher diversity values (Shannon, Simpson, species richness, trophic groups) than any other leaf, probably due to their association to dense forests presenting humid conditions and high resources availability, associated with this leaf. Xerophilous leaves presented high diversity of species (heterotardigrades) well-adapted to xeric open space in our survey. Type of leaves also changed abundance dynamics of different tardigrade trophic groups. Different biotic and abiotic conditions in different types of leaves promoted varying abundances of carnivore, herbivore and microbivore tardigrades on those different micro-ecosystems. Implications and consequences of these findings in evolutionary hypotheses for microfauna are considered.