Insights into the plant-insect network topology across land-use types at RAB-Rubona forest and its surrounding agroecological landscapes.

Abstract

Pollination ecology, particularly the interaction between plants and their pollinating insects in different land use types, has not been well explored in Rwanda. It has been challenging to inform the public about how some plant species interact with their flower-visiting insects, how they depend on each other, and how pollinating insects contribute to global GDP, through pollination services. This research aims to explore the topological networks among plants and their pollinating insects under different intervention land-use types such as forest, forest edge, coffee, apple, mango, and avocado farmlands. It documented the diversity and richness of plant species and associated pollinating insect diversity and richness. The research elucidated how these species interact with each other using network graphs and how the interaction might be stable in one land-use type compared to others. Further, the study recorded observations and network parameters namely Specialization index (H2), Nestedness index (NODF), Selectiveness index (d), Normalized degree (K), and Mueller's index to estimate the plant species that may influence pollination services in selected crop plantations. The research took place in Huye district, Rusatira sector, Southern Province of Rwanda. Data have been collected for a period of 13 months in 2022. Some plant species showed generalist or specialist behavior depending on flower-visiting insects. Wild bees such as Xylocopa sp, Amegilla, hypotrigona and Halictus sp, managed bees (Apis mellifera), flies mostly from the Syrphidae family, butterflies, wasps etc were recorded while visiting and pollinating the flowering plant species that presented the inflorescence during the data collection. These species include Gymnanthemum amygdalinum and Psychotria leptophylla recorded from the edge transect. They are more specialists while Tithonia diversifolia and Albizia gummifera were more generalists under the edge transect, Callicarpa americana and Acanthus pubescens were more specialist while Dovyalis caffra and Echinocystis lobata were more generalists under the forest transect, Black-jack (Bidens pilosa from Asteraceae family), Lantana camara, Tommy Atkins (mango) were more specialist species. Further, Washington (Citrus), Florida (orange) and Amonica (orange) were the most generalist species under Mango transect. In coffee transect, 4476AC and 4873AC (d=0.34) varieties were more specialist while W20-La and JACKX6AAC varieties were the most generalists’ ones. Golden (apple) was more specialist species in Apple transect. American hog-peanut and Arsenic bush plant were more specialist in abocado transect whereas the Donald (avocado) and Fuerte (avocado) presented the most generalist behavior than other plant secies in the Avocado transect. Additionally, Mango farmland showed a perfect and iii strong network, whereas the poorest network was recorded from the Apple transect. The study also revealed some plant species that can be planted or mixed with crop plantations to facilitate pollination interaction in crops, which might increase crop productivity. These plant species include Persea americana, Triumfetta rhomboidei, Gymnanthemum amygdalinum, Acacia sieberiana, Dovyalis caffra, Ocimum tenuiflorum, Ageratum conyzoides, Descurainia pinnata and Bidens pilosa. This research suggests maintaining the pollination ecological services through pollinating insect conservation and plant conservation, raising awareness on the proper use of pesticides and fertilizers, and promoting biopesticide markets and biological indicators instead of using chemical fertilizers and pesticides that harm both pollinating insects and their host plants.