4. Discussion
Heavy metal pollution had drastic effects on macrofauna communities. Density sharply decreased at the highest
concentrations. A threshold of 2000 ppm zinc was identified beyond which A. caliginosa were no longer found. Similar
order of sensitivity was measured for zinc effects on A. caliginosa in laboratory tests [14]. Bengtsson [3] pointed
out that this density reduction may also be due to a decrease in pH that is often associated with heavy metal pollution. It
was not the case at our site where no significant variation in pH was observed. Pollution seemed also to affect invertebrate communities through the soil organic matter quantity and quality. These changes were certainly due to the
decrease of the decomposition rate in conditions when earthworms and/or microorganism activities decreased.
These special conditions might explain the large increase in Hoplinae larvae population. This species lived in larvae
form during the winter and we suppose that the organic matter layer protected them from the cold. These rhizophagous
larvae may either be more metal tolerant than endogeic earthworms or they ingest less zinc because of their special
feeding habits. However, the toxicity of heavy metals to these two species should be compared in the laboratory to
test the field observations. Species richness, which is often considered as suitable indicator of disturbance [11,13], did
not reflect the spatial metal distribution across the site. The increase of species richness observed at intermediate levels
of pollution was mainly due to the great difference in intrinsic diversity among large taxa. In the low polluted
zone, we suppose that the concentration of bio-available zinc was not sufficiently high to exert toxic effects on the
non-social arthropods, especially Coleoptera and Arachnida.