How complementary are large frugivores for tree seedling recruitment ?A case study in the Congo Basin

Large frugivores provide critical seed dispersal services for many plant species and their extir- pation from forested ecosystems can cause compositional shifts in regenerating plant cohorts. Yet, we still poorly understand whether large seed-dispersers have complementary or redun- dant roles for forest regeneration. Here, to assess the functional complementarity of large- bodied frugivores in forest regeneration, we quantified the effects of varying abundance of hornbills, primates and the forest elephant on the density, species richness and the mean weighted seed length of animal-dispersed tree species among seedlings in five sites in a for- est–savanna mosaic in D. R. Congo, while accounting for percentage forest cover and the local presence of fruiting trees. We found that the abundance of primates was positively associated with species richness of seedlings, while percentage forest cover was negatively associated (R2 = 0.19). The abundance of hornbills, the presence of elephants and percentage forest cover were positively associated with mean seed length of the regenerating cohort (R2 = 0.13). Spatially explicit analysis indicated that some additional processes have an important influence on these response indices. Primates would seem to have a preponderant role for maintaining relatively high species richness, while hornbills and elephant would seem to be predominantly responsible for the recruitment of large-seeded trees. Our results could indicate that these taxa offrugivores play complementary functional roles for forest regeneration. This suggests that the extirpation ofone or more ofthese dispersers would likely not be functionally compensated for by the remaining taxa, hence possibly cascading into compositional shifts.