Size matters: Meta-analysis of the effects of human disturbance on seed dispersal by animals

Background/Question/Methods

Animal-mediated seed dispersal is important for sustaining plant diversity in forest ecosystems, particularly in the tropics. Forest fragmentation, hunting, and selective logging modify forests in myriad ways and their effects on animal-mediated seed dispersal have been examined in many case studies. However, the overall effects of different types of human disturbance on animal-mediated seed dispersal are still unknown. We identified 35 articles that provided 83 comparisons of animal-mediated seed dispersal between disturbed and undisturbed forests in tropical and subtropical ecosystems. We carried out a meta-analysis to test whether forest fragmentation, hunting, and selective logging affected three components of animal-mediated seed dispersal of fleshy-fruited tree species: frugivore visitation rate, number of seeds removed, and distance of seed dispersal.

Results/Conclusions

Forest fragmentation, hunting, and selective logging did not affect visitation rate and were marginally associated with a reduction in seed-dispersal distance. Hunting and selective logging, but not forest fragmentation, were associated with a large reduction in the number of seeds removed. Fewer seeds of large-seeded than of small-seeded tree species were removed in hunted or selectively logged forests. A plausible explanation for the consistently negative effects of hunting and selective logging on large-seeded plant species is that large frugivores are the first animals to be extirpated from hunted or logged forests. The reduction in forest area after fragmentation had weaker effects on frugivore communities and animal-mediated seed dispersal than hunting and selective logging. We conclude that the disruption of plant-frugivore interactions is likely to cause changes in tree species composition in disturbed tropical forests because large-seeded plant species were more susceptible to direct human effects than small-seeded plant species. This finding underscores the central role of vital plant–animal interactions for maintaining species-rich tropical tree communities.