Communities of insect pollinators along a logging gradient in Budongo Forest reserve.

Abstract

Despite the evidence suggesting declines in pollinators in many parts of the world with consequent direct and indirect negative effects on biodiversity conservation, natural regeneration of forests and food security, little attempts have been made to understand the role of past forest management practices in maintaining insect pollinator populations in tropical natural forest ecosystems. This study assessed the community of potential insect pollinators among six compartments of Budongo Forest Reserve, Uganda with different logging history to understand the responses of insect pollinators to logging intensity. Specifically, the study assessed the abundance, species richness, diversity and community composition of potential insect pollinators among forest compartments and between two tree species in the compartments along a logging gradient. It examined possible directional patterns in the abundance, species richness and diversity among forest compartments along the logging and age gradients. The compartments consisted of an unlogged climax forest, unlogged mixed primary forest, a pit sawn compartment and those mechanically logged at 34.8, 36.1 and 80 m3ha-1 in periods ranging from 58-80 years prior to the study. Coloured pan traps were used for sampling potential insect pollinators (flower visitors) of Tabernaemontana holstii and Alchornae laxiflora at 5-20 m above the ground for three flowering seasons. The abundance, species richness and diversity were compared among the compartments and between the two tree species using one way ANOVA. To compare community composition, Bray–Curtis’s dissimilarity index was computed and visualized using NMDS ordination. Spearman’s rank correlation was used to test for possible directional pattern in the predictor’s variables along the logging and age gradients. The compartments which were subjected to very high logging intensities had higher pollinator abundance, species richness and diversity than the unlogged climax forest compartment and those compartments that had low logging intensities. This demonstrates that selective mechanical logging in tropical high forests can benefit insect pollinators when the disturbed areas are allowed to recover. The primary forest compartments shared the least pollinator species than when they are compared with the logged compartments, and the pollinator community in the mixed primary forest was distinct from the communities in the other compartments. These results indicate that the two primary forests are further apart in pollinator communities than the climax forest is with the logged compartments. The overall insect pollinator abundance, species richness and diversity were higher on T. holstii than A. laxiflora, but the reverse was true for Hymenopterans. There were positive directional patterns in pollinator abundance and species richness along the logging gradient when the climax forest was considered. This indicates that the recovery of insect pollinators, to the state of the climax forest, in compartments subjected to different logging intensities follows the logging gradient. Overall, this study indicates that logging can be used as a forest management tool to enhance pollinator abundance, species richness, diversity and communities in tropical high forest. However, such interventions require careful implementation with the aim of diversifying vegetation and floral resources rather than maximising timber profits.