Large areas of tropical forests have already been converted to croplands, creating fragmented landscapes with high proportions of forest edges and isolated habitats. These changes severely affect upland forests by altering plant and animal communities, as well as associated ecosystem functions. Although riparian forests themselves typically remain intact, they are still threatened by land use changes that leave them smaller and isolated, which ultimately may change their microclimate, flora, fauna and ecosystem functions. The consequences of these transformations for riparian forest biodiversity and ecosystem services are not well understood in the rapidly developing southeastern Amazon Basin of Brazil, however. To address these gaps, we sampled trees and measured seed removal by ants and dung beetles, which are dominant faunal groups and major seed dispersers in tropical forests, in 10 riparian forests in southern Brazil – four surrounded by undisturbed forest and six surrounded by croplands. In each riparian forest, we sampled tree diversity in transects perpendicular to the stream, evaluated ant-seed interactions in a 5 ✕ 5 grid of myrmecochorous seed depots, and used 700 g of tapir dung containing 50 dummy seeds to assess seed removal by dung beetles.
Results/Conclusions
Undisturbed riparian forests had a more closed canopy (35%), more tree species (34%), and a more uniform distribution of tree species than disturbed riparian forests. Dung beetles dispersed two-fold more seeds in undisturbed forests (20% of total offered) than in disturbed forests (10%), but seed removal by ants did not differ. Fragmentation and edge effects often lead to forest degradation by increased tree mortality and changes in microclimatic conditions, ultimately eliminating sensitive species and favoring the invasion of open-habitat plant species. We show that forest change also affects a key ecosystem service, i.e., secondary seed dispersion by dung beetles. Habitat isolation has been shown to decrease the movement of large fauna and consequently their occurrence in fragmented habitats, thus, limiting resource availability for dung beetles. It is likely that seed dispersion was not altered by ants due to the type of seeds we offered them: myrmecochorous (i.e., bearing elaiosome), which were not associated with dung. These results suggest that a decrease in seed removal by dung beetles in disturbed riparian forests was mediated by altered mammal use of degraded riparian forests. This may cause reduced regeneration of riparian forests through lower seedling recruitment, causing further forest degradation across time.