Local biodiversity patterns in micrometazoans: Are tardigrades everywhere?

The aim of this paper was to assess the relationships between habitat type and diversity indices, richness and abundance of terrestrial tardigrade communities. Our analyses are based on results from a survey performed in Sierra de Guadarrama, in the centre of the Iberian Peninsula. We performed a multi-habitat intensive sampling in terrestrial environments (mosses and lichens on rocks, tree trunks and leaf litter), to test if local biodiversity patterns for tardigrades exist, to test the 'everything is everywhere' hypothesis, and if distributions can be attributed exclusively to environmental properties (as proposed for protists), or to a combination of historical and environmental events (as proposed for larger metazoans). Our analyses of richness, diversity indices and abundance of tardigrade communities revealed habitat-dependent patterns on these community descriptors. Leaf litter habitats showed high species richness and low abundances, rock habitats (mosses and lichens) showed intermediate species richness scores and high abundances, and trunk habitats (mosses and lichens) showed low numbers of both richness and abundances. Besides, common tardigrade species, and not scarce ones, determined the difference on tardigrade community composition between different habitats. We also describe two opposite patterns in tardigrade assemblages characterised by Heterotardigrada vs. Eutardigrada species; while Heterotardigrada-based communities presented low species richness and large tardigrade numbers, Eutardigrada-based assemblages showed high species richness and low abundances. in conclusion, tardigrade local biodiversity across habitats exhibited patterns closer to larger animals than to unicellular microorganisms even when environmental and biological conditions (small size, dispersal forms) are closer to these last organisms. Implications of these findings are discussed.