Distribution of soil tardigrades as revealed by molecular identification across a large-scale area of Australia

Tardigrades, also known as ‘water bear’ or ‘moss piglet’, inhabit diverse environments ranging from marine to freshwater and terrestrial habitats. They occupy various trophic levels in the micro-food web attributed to their different feeding preferences and different predators, which also heralds the complexity of their ecological functions. Therefore, understanding the ecological preference of tardigrades and their interactions with other organisms is crucial for uncovering the changes in ecosystem functions performed by these organisms under future scenarios of climate change. Here, we investigated the diversity and community composition of tardigrades, and their driving factors from 194 soil samples across south and eastern Australia, based on amplicon sequencing of 18S rRNA gene. We further validated the presence or absence of tardigrades in selected soil samples using morphological detection. Eleven tardigrade genera were observed in 53 samples, predominantly from coastal soils, with Eremobiotus as the most dominant genus. Notably, mean annual temperature (MAT) was the most important factor influencing the presence of tardigrades, revealing a decreased relative abundance of tardigrades as MAT increased. Other abiotic factors, including soil pH, total nitrogen, and mean annual precipitation, as well as biotic factors, including bacteria, fungi, protists, algae and nematodes, were also critical to the distribution of tardigrades, as revealed by structural equation modelling. Morphological identification broadly aligned with our molecular findings; it also illustrated the sporadic distribution pattern of tardigrades. Taken together, our findings provide the first empirical evidence for the relationships between soil tardigrades and the environmental factors using environmental DNA and demonstrated the importance of both biotic and abiotic factors in shaping the large-scale distribution patterns of soil tardigrades. Additionally, our findings imply a certain degree of feasibility for soil tardigrade researches using environmental DNA, and highlight the potential risk of a decline in tardigrade communities in the face of increasing global temperatures.