C. Barata et al., Comparing population response to contaminants between laboratory and field: an approach using Daphnia magna ephippial egg banks, FUNCT ECOL, 14(4), 2000, pp. 513-523
1. The life-history responses of one field and two laboratory populations o
f Daphnia magna were studied under exposure to cadmium and ethyl parathion
to assess by how much the response to toxic chemicals of laboratory populat
ions with low genetic diversity differs from the response of genetically di
verse field populations.
2. The field population was represented by at least 50 unique clones hatche
d from resting eggs (ephippia) collected at the beginning of the growing se
ason from a temporary water-body located in the north-east Mediterranean Sp
anish coast. The laboratory populations were clonal lines established from
two geographically and genetically distinct genotypes, which differed in th
eir tolerance to cadmium and ethyl parathion. Toxicant effects on the mean
and the variance of life-history responses of the laboratory and the field
populations were determined. For the field population, toxicant effects on
the components of variance of primary fitness traits were also studied.
3. In addition to lethal effects, exposure to cadmium had strong sub-lethal
effects on clutch size and age at first reproduction whereas ethyl parathi
on only affected juvenile survival. The results reported for life-history r
esponses showed that the field population had a similar or greater mean tol
erance to cadmium and ethyl parathion than the laboratory populations, but
the breadth of its tolerance distribution (measured as the coefficient of v
ariation, CV) was higher. Furthermore in contrast with the field population
, laboratory populations did not show increased phenotypic plasticity (meas
ured as CV) under increasing toxicant exposure. A further analysis of the c
omponents of variability for life-history responses of the field population
showed that increases in phenotypic plasticity with exposure levels were e
xplained by increased levels of genetic variability.
4. These results support the conclusion that as the tolerance of a field po
pulation is strongly influenced by genetic factors, the use of genetically
homogeneous laboratory populations has limited relevance in predicting long
-term responses of field populations to toxic chemicals. However, this conc
lusion must remain tentative until further supporting evidence is obtained
from this and other species.