The effects of long-term experimentally modified hydration conditions on populations of moss-living tardigrades were investigated in a naturally dry South-Swedish alvar environment at the Island Öland. Carbonite rocks with mosses were collected from rock fences and arranged in three experimental groups: increased dehydration, increased hydration, and control. The total experimental period was 18 months, with treatments applied during two 6 month periods. The density of tardigrade populations was recorded. The total population of tardigrades, all species included, tended to be lower under watering treatment, but the difference was only marginally significant. Populations of Richtersius coronifer and Echiniscus spiniger did not respond to the treatments, while populations of Milnesium tardigradum declined under conditions of increased hydration. The density of eggs in R. coronifer was also lower in the watering treatment. Thus, no positive response to increased hydration was recorded. These results suggest that the tardigrade populations either were not limited by the amount of hydrated periods, or that some other factor(s) counteracted the expected positive response to increased hydration. All populations showed a high variability in density among different moss samples, and the rock from which a sample was taken explained a significant part of this variability. This confirms a commonly believed, but seldom quantified, high heterogeneity in density of semi-terrestrial tardigrades, also among seemingly very similar substrates.
tardigrades, desiccation, population change, Richtersius coronifer, Milnesium tardigradum, Echiniscus spiniger