Megafaunal engineering in the tropical rainforest: African forest elephants ( Loxodonta cyclotis ) reduce avian species richness but elevate functional diversity

Abstract African forest elephants ( Loxodonta cyclotis ) are critically endangered ecosystem engineers of African tropical forests. Despite their rapidly declining population trend, little is known of how forest elephants interact with other vertebrate assemblages. To explore this, we studied avian communities within a continuous tropical rainforest on the slopes of Mt. Cameroon. The area's topography allowed us to survey birds in elephant‐grazed primary forest and in adjacent portions that elephants avoid, isolating the effect of megafaunal grazing within an otherwise intact system. We additionally surveyed nearby secondary forests fragmented by human activity, contrasting the disappearance of a single ecosystem engineer against the reshaping of forest by anthropogenic disturbance. We assessed the taxonomic diversity (species richness, Shannon index), functional diversity (Rao's quadratic entropy), functional redundancy (species overlap in functional space), and composition of functional traits (abundance‐weighted trait averages). Previous research suggested that loss of forest elephants would negatively impact avian assemblages through reduced habitat heterogeneity. In contrast, we show that elephant grazing itself significantly reduced species richness, to levels comparable with heavily disturbed secondary forests. Functional diversity nevertheless increased in both the grazed and secondary forest, but at the cost of functional redundancy: in both of these habitats, the avian assemblages were taxonomically impoverished, yet functionally more dispersed, with some foraging niches negatively affected. The elephant‐grazed forest was dominated by understory and terrestrial invertivores, whereas the ungrazed forest supported frugivores and larger birds. The secondary forest, in turn, harboured a functionally distinct assemblage of open‐habitat species with high dispersal ability and divergent foraging strategies. Together these patterns show that although elephant grazing reduces taxonomic diversity to a degree comparable with anthropogenic disturbance, it reshapes functional composition in fundamentally different ways. The presence of forest elephants facilitates stable populations of forest specialist birds, such as terrestrial invertivores. Conversely, the adverse effects of elephant grazing on frugivores and large birds could be minimized by establishing larger reserves of lightly grazed forest, possibly alongside ungrazed refuges. Overall, our results show that loss of keystone megaherbivores has widespread consequences for other rainforest vertebrates, potentially resulting in a cascading restructuring of key ecosystem functions.