Dung beetles, but not rodents, contribute to Brown Bear faeces removal, disaggregation and secondary seed dispersal.

Résumé 0

Seed dispersal by endozoochory is essential to plant dynamics, but once released in the faeces, the seeds face a hostile environment that is not always favourable to germination. Indeed, faeces may contain inhibitors, high seed density and be densely structured. However, faeces visitors such as vertebrates and invertebrates may play an essential role in secondary seed dispersal (SSD) and limit the chemical and physical constraints of the faeces. In this study, we designed a field experiment in the French Pyrénées mountains to disentangle the relative role of vertebrate and invertebrate on Brown Bear (Ursus arctos) faeces removal, disaggregation and secondary dispersal of small and medium-sized seeds. For this purpose, we used 30 fresh faeces and separated each of them into three sub-samples submitted to different treatments allowing total access to any visitor, access restricted to invertebrates and no access respectively. We inserted eight raspberry (Rubus idaeus) and five blueberry (Vaccinium myrtillus) seeds in each sub-sample to assess SSD. In parallel, we used camera and pitfall traps to catch vertebrate and invertebrate visitors, respectively. After ten days we weighted the fecal matter removed, counted the remaining seeds and assessed the faeces disaggregation based on visual examination and objective criteria. We observed a significant effect of invertebrates on faeces removal, disaggregation and secondary dispersal of both seed species. Vertebrates did not visit the faeces. Dung beetles caught in pitfall traps appear as the main disaggregation agent in this area. We also pinpointed that diet composition and structure of Brown Bear faeces affect their disaggregation and subsequent SDD. Our study in a temperate mountainous area identifies dung beetles as key agents in the disaggregation and secondary dispersal of small and medium-sized seeds. Diet composition and the fecal matter trapping the seeds affect seed fate by modulating dung beetle activity. By releasing variable fecal contents, omnivorous primary dispersal vectors have an even more complex effect on seed fate than expected.