Deciphering the patterns and drivers of tardigrade diversity along altitudinal gradients

Altitudinal gradients offer a unique opportunity to understand the drivers of species richness, as mountain regions cover vast areas and contribute disproportionately to global terrestrial biodiversity. However, most studies have focused on larger organisms, often neglecting microscopic animals such as meiofauna also in mountain biodiversity research. In this study, we investigated patterns of tardigrade diversity and distribution in the Western Alps (Northern Italy) by compiling an extensive inventory of taxa inhabiting bryophytes. We analyzed 546 bryophyte samples collected across a broad altitudinal gradient and used DNA metabarcoding to characterize tardigrade communities. For each taxon, we gathered functional trait data to assess how species characteristics influence distribution. We then evaluated the effects of macroenvironmental variables (altitude, vegetation type, slope exposition) and microhabitat-level traits (bryophyte biological and structural features) using spatially explicit statistical modeling. We found that species richness decreased with altitude, whereas standardized phylogenetic and functional diversity increased, indicating higher redundancy at lower elevations. Community composition was not driven by specific bryophyte species but rather by general bryophyte functional traits. Our results reveal that tardigrade communities in bryophyte microhabitats are highly heterogeneous, with strong species turnover and prevalent phylogenetic and functional underdispersion. Despite the influence of stochastic processes in shaping their distributions, we show that macroenvironmental variables such as altitude and geographic location drive species turnover, while microhabitat traits govern trait-based community structure. These findings suggest that macroenvironmental gradients shape species distributions, whereas trait-based environmental filtering operates primarily at the microhabitat scale.