Earthworms in Environmental Research - Earthworm Ecology - page 317

and field studies of earthworm toxicology for 10 pesticides. Assuming an even pesticide distribution
in the upper 5 cm of the soil, the laboratory reproduction studies were reported to be at least 5 to
10 times more sensitive than the field responses of all earthworms. However, as stated, there is an
impressive difference in sensitivity among various earthworm species. Jones and Hart (1998)
reviewed laboratory-field comparisons for the toxicity of pesticides and underlined the impact of
sensitivity differences among species. They considered that differences among soils are less impor-
tant. Bembridge et al. (1997) showed, in a long-term experimental field treatment and by ordinations
and principle response curve analysis, the different responses of various earthworm species to
benomyl. Allolobophora spp. and Nicodrilus spp. proved to be the most sensitive to chemicals.
Jones and Hart (1998) stressed the element of recovery by earthworms from exposure to
chemicals. For pesticides with a half-life in soil of less than 50 days, earthworm population recovery
occurred within a year, and it was possible to recover from reductions in numbers of other soil
invertebrates of over 90%. With more persistent pesticides, the rate of recovery was reduced. This
is effectively also the principal difference in assessing the impacts of persistent heavy and degradable
organic contaminants.
In correlating laboratory test results on heavy metal toxicity to earthworms with field sampling,
Spurgeon et al. (1994, 1999), Posthuma et al. (1998), and Nahmani et al. (2002) reported on
earthworm populations and diversity in the vicinity of large smelter works or deposits of smelter
ashes. Spatial distribution in a dose-effect relationship was expressed by Spurgeon et al. (1994)
based on the OECD 14-day LC 50 values of the heavy metals present in terms of square kilometers
from the source in the vicinity of the smelter works, which should be devoid of earthworms. For
Zn, this was 75 km 2 ; for Cu, it was 42 km 2 ; and for Pb, it was 4.7 km 2 . Posthuma et al. (1998)
plotted the numbers of earthworms found in a sigmoid distance-response curve, which enabled the
calculation of an EC 50 that could be compared with the results of standard OECD tests. Posthuma
et al. (1998) and Spurgeon et al. (1994) reported an overestimation of the potential toxic effect of
the heavy metals using the OECD test. Posthuma et al. (1998) explained this by noting a difference
in the uptake rate of Cd accumulation by earthworms that they actually observed ( Figure 17.7 ) and
hence concluded a difference in bioavailability. This was confirmed by the observation of Spurgeon
et al. (1994) that, despite their extrapolation that no earthworms should be present in the close
vicinity of the smelter, they found some still present there. In addition, Posthuma et al. (1998)
pointed at a possible antagonistic effect of binary mixtures of heavy metals compared with effects
in the single-metal OECD test (see also the section on current interest in earthworm ecotoxicology).
Clearly, these studies confirm for field conditions a difference in sensitivity among various
earthworm species. The endogeic Allolobophora and Nicrodilus spp. appear to be the most sensitive,
followed by epigeic species such as Lumbricus rubellus and L. castaneus , whereas organic matter-
oriented species like Dendrobaena veneta are sometimes found in soils with high heavy metal
contents. Callahan and Linder (1992) sampled soil from a contaminated site and tested this in the
laboratory or under outdoor conditions for toxicity to earthworms. This last approach was also used
by Bengtsson et al. (1992) when they studied the possible adaptation of earthworms to long-term
exposure to chemicals in the soil surrounding a copper brass mill dating from the 17th century.
Bengtsson and Rundgren (1992) discussed the possibilities for recovery of the observed scattered
and low-density Dendrobaena octaedra population in the vicinity of the old mill.
L AND I MPROVEMENT AND E ARTHWORMS AS B IOENGINEERS
A rather new role of earthworms in environmental research can be related to the concept of their
suggested impact as soil bioengineers (Jones et al. 1994). Earthworms contribute to a better soil
environment by their burrowing behavior. This is especially useful not only in ameliorated soils
(former mining soils); eroded soils; and remediated, formerly contaminated soils, but also in
deposits of mine waste or contaminated sediments. In all these instances, earthworms were able to