Estimating the Effect on Soil Organisms of Exceeding No-Observed Effect Concentrations (NOECs) of Persistent Toxicants

Abstract

In estimating the effects of toxic substances on ecosystems we generally lack information on the sensitivity (expressed as a no-observed effect concentration, NOEC) of individual species in the field, and have to rely on information from laboratory test species, expressed as a frequency distribution of NOECs. In this case we can express toxic stress as the fraction of organisms that is exposed above its NOEC: the potentially affected fraction (PAF). This paper describes a model of the soil food web and the effect of toxic stress by persistent pollutants. The model predicts that in the absence of competition, individual species disappear from the foodweb at toxic concentrations 3–5 times their NOEC. With competition present, species affected by toxic stress are replaced by less sensitive ones. This has a twofold effect: species disappear from the foodweb at a lower concentration because loss of competitiveness occurs well before absolute extinction, but the replacement of disappearing species implies that the effect on total biomass and diversity becomes only noticeable at a PAF level near 100%. Model predictions are in good agreement with observations on nematode communities in experimental fields contaminated with copper and zinc. The model serves to illustrate why overall measures of ecosystem functioning (total biomass, production, diversity) are affected by toxic stress only at high levels of pollution, which is particularly true for systems with a high diversity. This apparent robustness masks a considerable genetic ‘erosion’, i.e. the disappearance of sensitive species or genotypes. The PAF is a good indicator of the latter effect.