Seed dispersal and predation of the African walnut tree, Coula edulis Baill. (Coulaceae): contrasting fates in intact and hunted rainforests.

backgroundSeed dispersal and predation are critical processes shaping the dynamics of plant communities in tropical forests. However, many seed dispersers and predators, particularly mammals, are increasingly threatened by deforestation and hunting. While the impacts of defaunation on seed dispersal and predation have been extensively studied, key knowledge gaps persist for many economically important African forest tree species. The objective of this study was to assess the effects of defaunation and habitat disturbance, as well as fruiting fluctuations on seed dispersal and predation patterns in Coula edulis. It is a widespread Guineo-Congolian tree species whose seeds are heavily traded by local communities. We tested three predictions in natural forests: (i) defaunation and habitat disturbance reduce fruit removal, (ii) fruit removal declines during high fruiting events, and (iii) dispersers and predators may use spatial memory and prior experience to locate fruiting trees (non-random search). Over two consecutive years (2022-2023), we established 288 fruit removal monitoring plots, 81 of which equipped with camera-traps, at two contrasting forest sites in Cameroon: a relatively intact forest in Campo Ma'an National Park and a disturbed forest near Mbalmayo city where large mammals have disappeared. Experimental plots consisting of 20 fresh fruits placed on the ground and monitored for 14 days were established beneath fruiting and non-fruiting C. edulis trees, as well as beneath individuals of other tree species.

resultsOver 1,120 camera trap days, we recorded 1,605 interactions between C. edulis fruits and animals. Rodents (Atherurus africanus, Cricetomys emini, and several squirrel species) were the primary fruit removers (60% of interactions), acting both as predators and dispersers. Their activity was seven times higher at Campo than at Mbalmayo. Large-bodied fauna, such as forest elephants (Loxodonta cyclotis) and mandrills (Mandrillus sphinx), occurred only at Campo and behaved as seed predators, whereas humans harvested fruits at Mbalmayo. Fruit removal rates were significantly higher in the intact site, highlighting the combined effects of defaunation and habitat disturbance, and lower during the high fruiting year (2022), showing a consumer satiation effect. Moreover, fruit removal rates were highest under C. edulis fruiting trees, lower under non-fruiting C. edulis trees, and lowest under other tree species, suggesting that animals selectively target predictable fruit locations.

conclusionsOur findings suggest that defaunation and habitat disturbance alter seed dispersal systems by reducing the involvement of large mammals, while resilient rodent assemblage can sustain some dispersal functions, potentially buffering regeneration of C. edulis in defaunated forests.