Meta‐Analysis of the Effects of Human Disturbance on Seed Dispersal by Animals

Abstract

Animal-mediated seed dispersal is important for sustaining biological 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; all comparisons except one were conducted in tropical or subtropical ecosystems. We assessed the effects of forest fragmentation, hunting, and selective logging on seed dispersal of fleshy-fruited tree species. We carried out a meta-analysis to test whether forest fragmentation, hunting, and selective logging affected 3 components of animal-mediated seed dispersal: frugivore visitation rate, number of seeds removed, and distance of seed dispersal. 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 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, as the predominant seed dispersers for large-seeded plant species, are the first animals to be extirpated from hunted or logged forests. The reduction in forest area after fragmentation appeared to have weaker effects on frugivore communities and animal-mediated seed dispersal than hunting and selective logging. The differential effects of hunting and selective logging on large- and small-seeded tree species underpinned case studies that showed disrupted plant-frugivore interactions could trigger a homogenization of seed traits in tree communities in hunted or logged tropical forests.

Correlated loss of ecosystem services in coupled mutualistic networks

“…These changes in interaction structure are likely to limit the dispersal and recruitment ability of fleshy-fruited plants in secondary forest habitats 15,22 . Thus, our study supports previous concerns on the high vulnerability of animal-mediated seed dispersal to habitat degradation 18,23 .…”

“…Forest fragmentation was of minor importance (Table 1), which may be explained by the moderate degree of fragmentation and the diverse mosaic of forest and open landscape in our study area. This adds to previous findings that predict moderate habitat fragmentation to be of lesser importance than typically observed levels of habitat degradation 17,18 .…”

Using avian functional traits to assess the impact of land-cover change on ecosystem processes linked to resilience in tropical forests

“…Specifically, for insectivores and forest frugivores, there was a decline in the overall mean size of species persisting in disturbed habitats (electronic supplementary material, figure S2 and table S13). We attribute these shifts to a suite of factors well known to increase the susceptibility of large species to local extinction in degraded or fragmented environments, including increased hunting [60,61].…”

Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages

“…If animal-mediated seed dispersal is important for maintaining viable populations of trees, overhunting may lead to tropical forest degradation, including loss of biodiversity and decreased biomass [5][6][7]. Comparisons between hunted and non-hunted sites generally reveal lower rates of seed dispersal and lower seedling abundance of animal-dispersed tree species in hunted sites [8][9][10][11]. The most comprehensive study to date tracked changes in a tree community as hunting increased over a 15-year period and found increased spatial aggregation and decreased sapling recruitment for animaldispersed tree species, leading to an overall decline in sapling biodiversity [12].…”