Effect of soil contamination with lead (Pb) on the multitrophic web of kale-associated insects

Abstract

Anthropogenic activities such as mining, industry, urban waste, and the excessive use of agricultural inputs can result in soil contamination with heavy metals. These elements can be absorbed from the soil by plants and cause toxic effects to organisms that feed on these plants and to trophic chains, due to their cumulative potential in tissues, which can impact the ecosystem. Therefore, this thesis had as main objective to investigate the effect of soil contamination with heavy metal on the insect community, structured in a trophic network, which lives in the aerial part of the plant. For this, we used as an experimental model lead (Pb), at different concentrations, and the insect community associated with kale (Brassica oleracea L., var. acephala). Kale plants were grown in soil experimentally contaminated with four concentrations of Pb nitrate: 0 (control), 144 (T1), 360 (T2), and 600 (T3) mg/Kg of soil. The concentrations of contaminated treatments correspond, respectively, to the prevention value, limit value for agricultural soil, and limit value of Pb for residential soil. The experiment was conducted in an open greenhouse for the natural colonization of insects. Insects were collected twice in the field through trap bags attached to the plant leaf and by direct removal. In the first article, we evaluated the effect of Pb concentrations on accumulated species richness, composition, and abundance of different functional groups of insect in the community. We found that the concentration of Pb in the plant increased with the increase in soil contamination, even presenting values above those allowed for this plant. The control treatment showed a greater richness of accumulated species. In addition, the treatments had an effect on community composition, with parasitoid species as indicators of the absence of the contaminant. The abundance of chewing and sucking herbivores, their respective parasitoids, predators, and predator parasitoids was negatively affected. However, hyperparasitoids were not affected in abundance, but rather in richness. In the second article, we evaluate the effect of contamination on the structural and functional food web properties. We evaluated the effect of Pb concentrations on the overall food web architecture and on the following metrics: number of species and links, links density, connectance, predator-prey ratio, and trophic generality and vulnerability. In general, we observed a food web simplification as Pb concentrations increased, reflecting in visually smaller webs. There was a reduction in the food web metrics from treatment T2. Structurally, there was a reduction in the number of species and links density. Connectance increased with contamination. Functionally, there was a reduction in the predator/prey ratio, vulnerability, and trophic generality. This work showed that soil contamination with Pb can negatively affect the insect community associated with a plant and the structure and function of the food web, even at permitted concentrations.