The effects of tropical deforestation are conspicuous and well documented but further attention needs to be focussed on forest degradation, relating to the more cryptic impacts of human disturbance on underlying ecosystem functions. While disturbances, including fire and selective logging, are well known to have detrimental effects upon species richness and other measures of biodiversity, loss of function remains more difficult to assess. We focus on seed dispersal, one of the key ecosystem functions in tropical forests with many birds and animals depending upon fruits as a food resource, and a high proportion of plants in turn relying upon them to disperse their seeds. There is growing evidence that overhunting large-bodied frugivores can alter recruitment processes and long-term plant communities but structural forest disturbances, such as logging or wildfires, also dramatically alter plant community composition, with possible cascade effects upon the functional trait landscape presented to frugivores. We combined tree inventories with fruit and seed trait data, collated from 24,400 herbarium records and literature sources, to investigate the effects of (1) forest disturbance and (2) forest recovery from clear-felling on the functioning of seed dispersal processes that will determine future plant-animal interactions.
Results/Conclusions
We inventoried 230 forest plots (57.5 ha) in eastern Brazilian Amazonia, along a disturbance gradient including logged, burned, logged-and-burned and undisturbed primary forest, as well as secondary forests (aged 6-22+ years). We measured a total of 27,267 stems ≥ 10 cm DBH, with 99.1% of all stems identified to species level (933 species in 80 families), which we matched to functional traits for each species. We found that the shift in plant composition towards an early succession community in increasingly disturbed forests was not accompanied by a swing from abiotic (e.g. anemochory) to biotic (e.g. endozoochory and synzoochory) modes, probably because of the predominance of plants with bat-dispersed seeds, but there was a reduction in seed diameter within endozoochorous species. In addition, we report the reversal of this trend with increasing age of secondary forests, although seed size in mature secondary forests only returned to levels comparable to the most heavily disturbed primary forests. This study thus shows how focussing on fruit dispersal modes and seed traits provide a key insight to help us understand and quantify the impacts of landscape-level forest disturbance and the process of forest restoration.