Gut shuttle service: endozoochory of dispersal-limited soil fauna by gastropods

Numerous important ecosystem functions and services depend on soil biodiversity. However, little is known about the mechanisms which maintain the vast belowground biodiversity and about the filters shaping soil community composition. Yet, biotic interactions like facilitation and dispersal by animals are assumed to play a crucial role, particularly as most soil animal taxa are strongly limited in their active dispersal abilities. Here, we report on a newfound interaction of potentially high ubiquity and importance in soil communities: the endozoochorous dispersal of soil fauna by gastropods. We focus on the dispersal-limited group of oribatid mites, one of the most diverse and abundant soil animal groups. In a field survey in a German riparian forest, 73% of 40 collected slugs (Arion vulgaris) egested a total of 135 oribatid mites, belonging to 35 species. Notably, 70% of the egested mites were alive and survived the gut passage through slugs. Similar results were found for Roman snails (Helix pomatia), indicating the generality of our findings across different gastropod taxa. Complementary laboratory experiments confirmed our field observations, revealing that oribatid mites are, indeed, ingested and egested alive by slugs, and that they are able to independently escape the faeces and colonise new habitats. Our results strongly indicate that gastropods may help soil organisms to disperse within habitats, to overcome dispersal barriers, and to reach short-lived resource patches. Gastropods might even disperse whole multi-trophic micro-ecosystems, a discovery that could have profound implications for our understanding of dispersal mechanisms and the distribution of soil biodiversity.