Estimating the effect on soil organisms of exceeding no-observed effect concentrations (NOECs) of persistent toxicants

In estimating the effects of toxic substances on ecosystems we generally la ck information on the sensitivity (expressed as a no-observed effect concen tration, NOEC) of individual species in the field, and have to rely on info rmation from laboratory test species, expressed as a frequency distribution of NOECs. In this case we can express toxic stress as the fraction of orga nisms that is exposed above its NOEC: the potentially affected fraction (PA F). This paper describes a model of the soil food web and the effect of tox ic stress by persistent pollutants. The model predicts that in the absence of competition, individual species disappear from the foodweb at toxic conc entrations 3-5 times their NOEC. With competition present, species affected by toxic stress are replaced by less sensitive ones. This has a twofold ef fect: species disappear from the foodweb at a lower concentration because l oss of competitiveness occurs well before absolute extinction, but the repl acement of disappearing species implies that the effect on total biomass an d diversity becomes only noticeable at a PAF level near 100%. Model predict ions are in good agreement with observations on nematode communities in exp erimental fields contaminated with copper and zinc. The model serves to ill ustrate why overall measures of ecosystem functioning (total biomass, produ ction, diversity) are affected by toxic stress only at high levels of pollu tion, which is particularly true for systems with a high diversity. This ap parent robustness masks a considerable genetic 'erosion', i.e. the disappea rance of sensitive species or genotypes. The PAF is a good indicator of the latter effect.