Are laboratory derived toxicity results informative for field situations? Case study on earthworm populations contaminated with heavy metals

The relevance of laboratory tests on toxicants for field situations is often disputed given that laboratory tests are conducted under, next to the toxicant stress, optimal conditions which are not expected in field situations. In this paper we confront the results of laboratory tests on growth, reproduction and survival of earthworms, in a polluted and a reference field soil with a field inventory of earthworms. The field inventory includes population density, biomass and demographic composition in life stages measured monthly over a period of one year. The field inventory showed that density and biomass was higher at the polluted field site, a result in conflict with the extrapolation of the laboratory tests that showed a decrease in population growth rate by 23% at this site compared to the reference. The field inventory and laboratory derived results agreed in the demographic composition of the population with more individuals in the younger age class at the polluted site compared to the reference. Abiotic and biotic conditions that differ between sites and could possibly explain the lower earthworm biomass and density at the reference site are discussed. We suggest that predation by the two to five times higher densities of meadow birds in spring may have caused the lower density and biomass of earthworms at the reference site.