Large frugivores contribute a notable role in forest dynamics and hence occupy a central position in seed dispersal networks. However, large-bodied vertebrates are disproportionately prone to local extinction caused by increasing human disturbances than are smaller-bodied species. Observed changes in the structure and composition of juvenile plant cohorts following the extirpation of large-bodied seed dispersers constitute evidence for the long-term alteration of the whole tree stand, resulting in a loss of biodiversity and a reduction of ecosystem services. The western lowland gorilla (Gorilla gorilla gorilla Savage, 1847) is the largest arboreal frugivore, facing rapid population depletion in recent years and becoming locally extinct in many parts of its historical range. To understand the likely consequence of their disappearance for the future of Afrotropical forests, this thesis aims at determining the seed dispersal effectiveness of a western lowland gorilla population (“La Belgique” research site, southeast Cameroon). Seeds of 58 species were identified in 1030 faecal samples collected over 36 months. Despite this diversity of species dispersed, seeds of the genus Uapaca (N=5), dispersed in 72.2% of months sampled,represented 36.5% of the total seed load. With a retention time of seeds in the gorilla digestive tract averaging 50.6 ± 28.1 h, and an average daily travel distance of 2 km, the western lowland gorilla may transport seeds over considerably long distances. This contribution to (meta)population dynamics would be effective since seed viability is not reduced by the time seeds are exposed to digestive enzymes (N=4 spp.) and may even be improved (N=1 sp.). Overall, seeds received a good treatment in the mouth and gut as 50% and 41.7% of species tested (N=12 spp.) had a germination success unchanged (including Uapaca spp.) or improved, respectively, after gut passage. At the seed deposition site, however, seed viability may be further altered by dung beetles (coprophageous Scarabaeidae) through burial at unsuitable depths, as demonstrated by one species of Uapaca (73% of buried seeds were relocated >10 cm deep where the emergence probability dropped to zero). This negative impact is nonetheless lower at sleeping sites where fewer seeds are buried and at shallower depths (mean depth: 8.7 ± 3.9 cm). Sleeping sites are important seed deposition sites as half of the daily faeces production occurs there, and they are characterized by an extant tree stand with a low and sparse canopy (mean openness: 19.1 ± 13.1%) and a relative overrepresentation of light-demanding individuals as well as individuals of small diameter (10–19.9 cm), suggesting recent disturbances such as the fall of emergent trees. The higher amount of light reaching the forest floor in these microhabitats encourages the recruitment of dispersed species, as demonstrated by species of the genus Uapaca. For these, recruitment probability is further iv improved when sleeping sites are established on hygromorphic soils; a habitat type predominantly used at the end of the dry season (50% of sleeping sites discovered during this period were found in this type of habitat). As this period also corresponds to the peak in Uapaca seed dispersal quantity (mean of 31.7 ± 25.5 seeds dispersed per 100 g of faecal material; 38.6% of the total Uapaca seed load of the whole study period), the gorilla population at La Belgique performs ‘directed dispersal’ and hence contributes highly effectively to the dynamics of this genus. In return, the fruits of the genus Uapaca provide a food resource, known as fallback, that contributes to the fitness of the gorilla population in ways other fruit resources do not, therefore shedding light onto a particular case of plant–animal mutualism. Using the western lowland gorilla as a model, this thesis supports the unsurpassed role of large-bodied frugivores in shaping tropical forests and ensuring their long-term integrity. Appropriate management actions are urgently needed to cease the ongoing extirpation of such key seed dispersers from tropical forests.