Simplification effect of lead soil contamination on the structure and function of a food web of plant-associated insects

Anthropogenic activities such as mining, industry, urban waste production, and the overuse of agricultural inputs may result in the contamination of the environment by heavy metals, which are toxic to organisms mainly due to their potential to accumulate in tissues. This contamination causes the simplification of aquatic and edaphic food webs, which show impaired structure and function. We investigated whether soil contamination with heavy metal at the permitted concentrations affects the structural or functional properties of a terrestrial food web of insects living in the aboveground parts of a plant. As study models we used lead (Pb) and kale-associated insects. Kale plants (Brassica oleracea var. acephala) were grown in soil experimentally contaminated with four lead nitrate concentrations: 0 (control), 144 (T1), 360 (T2), and 600 (T3) mg/kg of soil. The experiment was conducted in an open greenhouse for natural insect colonization. The insects were collected twice in the field using trap bags attached to the plant leaf and by direct removal. In general, we observed a simplification of the food web as the Pb concentrations increased, resulting in visually smaller food webs. The food web metrics decreased from the T2 treatment (the Pb limit for agricultural soils). Structurally, the Number of Species (whose values were highly positively correlated to the Number of Links and Link Density values) decreased with increasing contamination, while Connectance increased. Functionally, the Predator/Prey Ratio and Trophic Vulnerability and Generality became lower. This study shows that soil contamination by Pb can simplify insect food webs in the aboveground parts of plants, negatively affecting the structure and function of the webs, even at permitted concentrations.