Experimental manipulation of biotic and abiotic parameters changes the outcome of insect-plant interactions

Abstract

Plants usually respond to environmental shifts with morpho-physiological adjustments, which trickles down to biotic interactions in the insect-plant system. We evaluate how Copaifera langsdorffii, a widespread tree species adapted to multiple ecosystems, responds to shifts in nutrient and water availability through experimental supplementation and how it affects its insect communities. We also evaluate how the presence of extrafloral nectaries (EFN) exudates modifies galling insect diversity and herbivory rates. Such experimental approach is scarce, as we simultaneously evaluate biotic and abiotic factors and cover the entire reproductive cycle of a species in natural environments, bringing important contributions to better understand how bottom-up factors drive species interactions and possibly community assembly. The experiment was set in an ironstone outcrop vegetation, where we deployed supplementation treatments in 80 plant individuals divided into the following field treatments: (T1 = Fertilizer, T2 = EFN simulation (Eppendorf tubes with a 20% sugar solution), T3 = fertilizer + EFN simulation, T4 = water spray, T5 = EFN control (microtube with water), and T6 = Control. We observed lower sclerophylly and greater leaf area in plant individuals that were supplemented with nutrients and water. Herbivory rates were lower and ant abundance was higher in plants with artificial EFNs available. While we did not observe variations in richness and abundance of galling insects, the patterns of co-occurrence of galls varied with the availability of resources (nesting space) in the plant. This study illustrates how variations in nutrient availability to plants modify interactions with insect communities. Ant-plant interactions can have a negative impact on general herbivory rates, however ants seem to have a more harmonious relationship with the galling insects.