Insect visitation and pollination networks across traditional rangeland management categories in a Northern Tanzanian rangeland

Abstract

The structure of pollination networks is critical to ecosystem stability and functioning. We investigated pollinator-plant interactions to understand the foraging preference and develop pollination networks in a semi-arid rangeland of different grazing management categories in Tanzania. Along three line transects, each measuring 100 m, in each of the four grazing man- agement categories (private and communal enclosures, wet and dry season grazing areas), we laid out three quadrats measuring 5 m x 5 m (25 m2,) located 30 m apart. We recorded insects visiting flowering plants for two consecutive days in each quadrat every week at each site from April to May, in 2019 and 2020. Aspilia mossambicensis received the most significant proportion of insect visitors (28%), followed by Justicia debile (21%). The mean protein concentration in sampled pollen varied significantly between plant species (χ2 = 25.9, P = 0.001), with Solanum incanum containing the highest concentration (299.3 ± 0.68) g/100 g. We did not notice any correlation between honey bee visitation and protein concentration in pollen (r = 0.471, P = 0.239) nor with fatty acids concentration (r = 0.253, P = 0.546). When comparing pollinator-plant network properties including connectance, nestedness, robustness, number of links, modularity, network diversity and linkage density across rangeland management, we found that the private enclosure contained significantly larger networks than the communal enclosure, the dry and the wet season grazing sites. We conclude that particularly private enclosures are vital to promote pollination networks in our studied rangeland system as they include important pollinator forage plants.