Abstract
The aim of my study was to characterize the taxonomic and functional composition of the floral visitor insect fauna associated with Balanites aegyptiaca, Vachellia tortilis and Ziziphus mauritiana, three abundant tree species of great ecological and social importance in the Ferlo (northern Senegal), in the context of the Great Green Wall (GGW). The GGW is a pan-African initiative to combat environmental degradation and desertification in the Sahel. On the one hand, the aim of my thesis was to synthesize current knowledge of pollination and other ecosystem functions performed by flower-visiting insects in the Sahel; and on the other, to study the abundance, species richness and composition of this fauna, as well as the interaction networks between plants and flower-visiting insects in the Ferlo, located in the GGW intervention zone in Senegal. The data gathered will make it possible to estimate the impact of different restoration measures undertaken by the initiative on the interspecific interactions underlying ecosystem functions (pollination and other interactions) linked to flower-visiting insects. To assess the state of the insect pollinator community, I carried out an inventory of flower-visiting insects for the three tree species mentioned above, in four selected localities (Koyli Alpha, Widou, Téssékéré, Ranérou). Within each locality, the three types of sites sampled were: (i) afforested and/or fenced plots of the Great Green Wall, (ii) non-afforested/non-fenced plots subject to heavy grazing pressure, and (iii) non-afforested/non-fenced plots but in low-lying, wetter areas with denser tree cover. In addition, the same sampling was carried out at different seasons within the same locality (Koyli Alpha): wet season, mid-dry season and end of dry season. I then analyzed the composition (taxonomic, functional) and diversity (richness, equitability) of flower-visiting insect communities in the different sites, localities and seasons, examining the effects on these communities of various factors (afforestation, seasonality, density of woody vegetation, abiotic factors). My study revealed a surprising diversity of insects visiting the flowers of these trees in the region, including over 500 morphospecies of insects belonging to nine orders. I also found a strong effect of season on insect abundance and diversity, with different taxonomic and functional groups displaying somewhat different patterns. Effects of site on insect abundance and diversity were much less marked. The restoration actions of the GGW initiative appear to be too recent to have produced strong effects.