Seasonal variation in insect assemblages at flowers of Balanites aegyptiaca , an ecologically and socially important tree species in the Ferlo region of Senegal's Great Green Wall corridor

The Sahel is among the world's regions most affected by ecosystem degradation, caused by intensive land use and climate change. The importance of conserving biotic interactions is increasingly recognised in actions that attempt to counter and reverse this degradation, such as the Great Green Wall initiative in the northern Sahel. Interactions between flowers and flower-visiting insects play central roles in ecosystem functioning, ensuring pollination. Floral resources are also crucial for numerous other biotic interactions, as flower-visiting insects use them as fuel in their search for prey, hosts, breeding sites, and other resources. Despite their importance in ecosystem functioning, very little is known about insect-flower interactions in the northern Sahel. Using two capture methods, netting and pan traps, we studied interactions between insects and flowers of Balanites aegyptiaca, sampling in three sites in a single locality: a topographical depression with higher tree density, a "restored" site with exclusion of livestock and an "unrestored" site in intensively grazed rangeland. Each site was sampled at three different seasons to examine variation in this extremely harsh and seasonal environment. Balanites aegyptiaca is the region's most abundant tree. Its open-access flowers, produced in multiple flowering events each year, are a critical resource for insects. This species also produces economically important resources for the region's inhabitants. We found a surprisingly high diversity of insects visiting its flowers, comprising 371 morphospecies from 10 insect orders, with a predominance of Hymenoptera and Diptera. Insect abundance and diversity differed between seasons but only little between the different sites. Bees appear to be important pollinators. The bee fauna is dominated by solitary bees of the family Halictidae, which were abundant at all seasons. Diptera, particularly diverse families of small flies, were also frequent visitors and were particularly abundant and diverse in the wet season. Ants were abundant, but captures at flowers were frequent only in the dry season, when few resources other than flowers and flower-visiting insects were likely available to them. Insect abundance and observed diversity differed only little between sites, but estimated total diversity (Chao1 richness) was highest in the depression site. Insects visiting flowers of B. aegyptiaca included herbivores, decomposers, and parasitoids and predators of diverse arthropods, underlining the role of these floral resources in numerous facets of ecosystem functioning. The floral resources of B. aegyptiaca and other trees, which can flower throughout the year, are likely critical to assure the persistence of numerous insect species through the long dry season. It is crucial to integrate understanding of biotic interactions in management aimed at conservation and restoration of Sahelian ecosystems.