Life-history patterns in reference and metal-exposed earthworm populations

The growth and development of F1 Lumbricus rubellus bred from reference and one metal-exposed (smelter) populations were assessed in soils from both sites. In the clean soil, faster growth (as reflected by mean weight) and maturation were found at selected time intervals for the smelter worms. This result is in agreement with predictions from life-cycle models, which indicate that polluted-site populations will be adapted for faster growth, earlier maturation and increased reproductive effort. Life-history adaptation is not however the only explanation for the differences in maturation rates found. During the exposures, mortality of smelter worms was higher than for the reference strain. This resulted in a reduction in the density of the smelter strain. In earthworms, both growth rate and maturation time are known to be density dependent. Thus it is probable that density-mediated responses of growth and development, rather than adaptation, are primarily responsible for the observed life-history. Exposure of the two strains to the smelter site soil was carried out to quantify differences in performance indicative of physiological resistance in the smelter worms. In fact, no consistent differences in growth or maturation were found. Thus it is unlikely that growth and development responses of Lumbricus rubellus are adapted to the metals present in soils at the smelter site.