Endogeic Earthworms Avoid Soil Mimicking Metal Pollution Levels in Urban Parks

Abstract

In response to long-lasting high levels of metallic trace elements (MTEs) in urban soils, we expect soil invertebrates inhabiting urban environments to have evolved detection and avoidance and/or tolerance mechanisms to MTE pollution. In this study, I used artificial soils with concentrations of lead, zinc, copper, chromium and nickel that reflect pollution levels in the soils of Parisian parks. Using choice experiments, I compared habitat preference (i.e., the occurrence of individuals in the polluted vs. unpolluted soil) and health status (i.e., body mass maintenance, mobility, mortality) between three species of endogeic earthworms—Aporrectodea caliginosa, Aporrectodea icterica and Allolobophora chlorotica—originating either from urban or rural grasslands. This study highlights a clear avoidance of MTE-polluted soils in all three species, as well as MTE-induced health impairments, especially in A. chlorotica. Interestingly, earthworm response to MTE exposure only slightly differed between earthworms of urban and rural origin, suggesting the absence of widespread acclimatization or adaptation mechanisms to MTE pollution in cities. As a consequence, MTE pollution is expected to significantly shape earthworm spatial distribution in both urban and rural environments and, as a consequence, affect ecosystem functioning.

Metal Pollution Drives Earthworm Biodiversity in Urban Lawns

“…The mean concentrations observed in our study closely resemble those obtained in 2015 by Foti and colleagues across 30 urban lawns in Paris (Foti et al, 2017). These elements are characteristic of urban soils (Ajmone-Marsan and Biasioli, 2010; Cachada et al, 2013;Chatelain, 2023;Foti et al, 2017) where they accumulate due to various anthropogenic sources, including the combustion of fossil fuels, waste incineration, or the production of alloys (e.g. brass), paints, batteries, or tires (Binner et al, 2023).…”

“…Interestingly, while Pižl and Josens concluded in their study that soil pollution levels in Brussels did not significantly shape the composition of earthworm communities, their results clearly show that the earthworm community at the site with the highest concentration of Cu in the soil (28.83 mg/kg) was dominated by A. chlorotica (Pižl and Josens, 1995). Although this outcome might imply that A. chlorotica has a higher tolerance to Cu pollution, earlier research indicated that this species is one of the most susceptible to MTE pollution (Chatelain, 2023;Nahmani et al, 2003;Spurgeon and Hopkin, 1996). For example, when subjected to soil containing a blend of Zn, Pb, Cu, Cr, and Ni, designed to simulate pollution levels found in urban environments, several A. chlorotica individuals died, while no casualty was measured in the two other earthworm species studied (Chatelain, 2023).…”

“…However, in half of the parks, there were substantial differences in MTE pollution levels among the three sampled locations within each park. For instance, at Square du Temple, two of the sites displayed MTE concentrations (Hg, Pb, Cu, Cd, and Zn) at opposite ends of the spectrum: one site had MTE levels similar to those found in rural lawns, while another showed levels comparable to those measured near smelters (Chatelain, 2023;Lukkari et al, 2004); the third site showed intermediate levels of MTEs. This finding underscores the high heterogeneity of urban soils, emphasizing the importance of taking multiple samples when studying them.…”

“…These studies consistently demonstrated that earthworms can detect MTEs in soils and escape from contaminated to clean soil (e.g. Chatelain, 2023;Langdon et al, 2005;Lowe et al, 2016;. However, MTE sensitivity (e.g.…”

“…However, MTE sensitivity (e.g. survival, reproduction) varies between species (Chatelain, 2023;Langdon et al, 2005Langdon et al, , 2001. More specifically, previous research indicates that epigeic species may exhibit lower sensitivity to MTEs (Langdon et al, 2005;Lowe et al, 2016;.…”