Does foraging impact tropical forest composition?

Growing evidence suggests that low-intensity anthropogenic activities affect ecological communities. The resultant changes in the forest structure and composition can also be influenced by elevational gradients. During foraging for food collection, humans can cover a wider spatially and elevational range compared to other high-intensity activities. These foraging activities can alter the species richness and evenness patterns either through preferential planting or intentionally or inadvertently influence the propagule pool. This study investigated the impact of foraging on forest composition and structure along an elevation gradient in Cross River National Park in the tropical rainforest zone of Nigeria. Fifteen permanent 40 x 40 m sample plots, covering an area of 2.4 hectares, were established in the forest located in the park along an elevational gradient of 120m to 460m. All trees of 10cm diameter at breast height (dbh) and above in all the plots were measured for dbh and identified to species level. Structured questionnaires on tree species utilization amongst forest-dependent rural communities were used to create edible and inedible tree species categories. Species abundance distributions were calculated at community level. Pairwise beta diversity between all plots along the elevational gradient was calculated using Sorensen’s dissimilarity index (β_sor) and the turnover component of β_sor using Simpson’s dissimilarity index (β_sim). A total of 35 edible species with a density of 128 stems/hectare and basal area of 11.99 m²/ha and 109 inedible species with a density of 364 stems/hectare and basal area of 22.42 m²/hectare were encountered. A reversed j-shaped function characteristic of a ‘natural’ uneven-aged tropical forest stand was observed in the diameter size distribution for all species categories. The pairwise beta diversity trend of inedible species showed that there was a positive trend between beta diversity and elevation, and this was driven by turnover; the replacement of species in the species pool along the elevational gradient. In comparison, edible species differed significantly from inedible species and showed a negligible trend indicating that the species turnover was not enough to cause a change in beta diversity with increase in elevation. These results suggest that low-impact activities such as foraging may be subtly but notably altering the composition of tropical forest, perhaps by preferential planting, or influencing the propagule pool along elevational gradients. Further tests across a wider range of sites are required to assess if this is a widespread phenomenon and to identify the driving mechanisms.