Dynamics of insect-plant interactions: the role of biotic and abiotic parameters altering insect community relationships

Abstract

Interactions involving insects and plants are shaped by abiotic and biotic variations that alter the dynamics and structure of these communities. Anthropogenic changes are impacting these interactions with loss in insect diversity, thus studies that aim to understand the dynamics of these interactions are crucial. We assess how Copaifera langsdorffii (Fabaceae), a plant adapted to different environmental stresses, responds to nutrient and water supplementation and how insect communities are affected. In addition, we also evaluate how the presence of extrafloral nectaries (EFNs) throughout the phenological cycle of the plant modifies insect communities. We seek to answer the following questions: (1.) Does nutrient and water supplementation affect sclerophyll, leaf area, and fruit production in C. langsdorffii?; and (2.) Are herbivory rates, galling insect diversity, and patterns of gall co-occurrence in communities affected by nutrient and water supplementation and artificial EFNs? (3.) Is the stability of insect-plant interactions affected by biotic and abiotic factors?; (4.) Does the constant availability of sugar exudate (EFN) reduce the frequency of herbivore-plant interactions? The experiment was conducted in a rupestrian mountaintop ecosystem called Canga (ironstone outcrop), where we applied supplementation treatments to 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 sclerophyll and greater leaf area in individuals from plants that were supplemented with nutrients and water. Herbivory rates were lower and ant abundance was higher in plants with artificial EFNs available. Although we did not observe variations in the richness and abundance of galling insects, patterns of galling co-occurrence varied with the availability of resources (nesting space) on the plant. The networks of all treatments showed stability (robustness) and specialization, however, the network of the fertilizer treatment was less specialized. Free-living insects showed low specialization in the treatment with added fertilizer and ants were less specialized in the treatment with tubes with water. The proportion of occurrences of the free-living insect guilds varied among the treatments, the treatment with fertilizer had a higher proportion of free-living insects and a negative relationship with opportunistic ants. On the other hand, the treatment with added EFNs had a lower relationship with free-living insects and a positive relationship with some ant guilds. This study illustrates how abiotic and biotic variations modify the occurrence and interactions of insect communities.