Ecological erosion of an Afrotropical forest and potential consequences for tree recruitment and forest biomass

• Complete defaunation is not a prerequisite to alter the strength of plant-animal interactions.
• Hunting reduced the mean seed dispersal distances of mammal-dispersed tree species.
• Hunted forest had reduced rates of seedling herbivory and lower above-ground biomass.
• Modification of plant-animal interactions could compromise forest carbon storage potential.

Unprecedented rates of logging and hunting threaten to transform the remaining primary tropical forest into a degraded mosaic, emptied of wildlife. Defaunation is expected to interrupt plant–animal interactions with cascading effects for forest structure, composition, and ecosystem services. In a Central African forest first logged 35 years ago, we evaluated this process of ecological erosion in 30 study sites distributed across forest disturbed by logging and hunting, logging alone, and neither logging nor hunting. Both logging and hunting tended to reduce abundances of large mammals, together shifting the relative abundance of the animal community towards squirrels and small birds. Through a series of experiments, we evaluated the effects of logging and hunting on seed dispersal, seed predation and herbivory. We demonstrate that complete defaunation is not necessary to significantly alter the strength of plant-animal interactions. Hunting reduced the mean dispersal distances of nine mammal-dispersed tree species by 22%. Rates of seed predation were similar among forest types, but hunted forest had significantly lower rates of herbivory that we attribute to the lower abundance of meso-herbivores. Hunted forest also had significantly lower above-ground biomass (301 Mg ha−1) than the logged only (358 Mg ha−1) and undisturbed (455 Mg ha−1) forest types, but similar numbers of tree species and individuals. Lower biomass in hunted forest is likely attributable to significantly lower wood densities at small tree size classes (<40 cm). We hypothesize that over time the human-mediated modification of plant-animal interactions can alter the composition of the forest to have a higher proportion of fast-growing, low wood density tree species, diminishing the long-term potential for carbon storage.