In the context of global change, plants are facing new constraints such as rising temperatures or the presence of invasive non-native species, combined with other local constraints. In such conditions, colonisation of more suitable habitats is a key mechanism in plant population dynamics. Plants can disperse in a variety of ways, by auto-, anemo-, hydro- or zoochory, although natural polychory occurs. Zoochory can be the result of external transport of seeds (epizoochory) or internal transport following consumption of seeds (endozoochory). Endozoochory is a mutualistic interaction between the animal and the plant whose seeds are dispersed. However, once consumed, the seeds undergo a number of filters before establishing in a new site, most of which depend on the biology and ecology of the animal vector. In our study, we looked at the whole germination process from seed consumption to germination in order to identify the strength of the different filters: initial consumption, mastication, digestive transit and finally, the overlooked filter of germination success within the faeces matrix. To do this, we focused on the omnivorous Eurasian brown bear (Ursus arctos arctos), a unique seed disperser, highly mobile, monogastric with a long retention time.

We used 86 bear faeces from the critically endangered Pyrenean population to assess (i) its diet, (ii) seeds intact in faeces, (iii) seeds viable in faeces and (iv) germinating seeds from faeces. To characterise the plant component of the brown bear's diet (i.e. consumption), we carried out a metabarcoding approach using the TrnL molecular marker. To identify which seeds had been consumed and were intact (i.e. mastication), we carried out a coproscopic analysis of the faeces to collect and identify all the seeds. To determine which seeds are viable (i.e. digestive transit), we carried out a tetrazolium viability test on the seeds. Finally, we carried out a germination test under greenhouse over 6 months using two modalities, raw and filtered faeces, to determine the effect of the faeces matrix as a filter on seed germination.

The metabarcoding revealed ~600 operational taxonomic units, belonging to 120 species. Coproscopic analysis and tetrazolium test are currently underway. Germination tests were highly selective with only nine species that germinated including one tree, two fleshy fruits, five herbaceous and also for the first time a fern.

Our results point out the critical phases in the dispersal process from seeds potentially consumed to seeds that finally germinate in the faeces. We also pinpoint the role of the brown bear in the dispersal of the Pyrenean unique flora, and invite more broadly to consider the impact of animal dispersal in montane regions that are particularly impacted by climate changes.