Chemistry Reference
In-Depth Information
Further examination of Figure 5.9 reveals a difference between sparrow hawks and
kestrels. Considering dieldrin levels of 3 ppm and above, the kestrel has proportion-
ally more individuals in the category of 9 ppm and above than has the sparrow hawk.
If the foregoing theory is correct, sparrowhawks, then, showed a higher incidence
of deaths due to sublethal toxicity than kestrels and, conversely, kestrels showed a
higher proportion of deaths due to direct acute poisoning than sparrowhawks. This
suggests that sparrowhawks are more susceptible to sublethal effects than kestrels.
This could be explained on the grounds that the sparrowhawk depends on greater
maneuverability to catch its prey in the air than the kestrel, which simply drops on
its prey from a hovering or perching position. The former species, therefore, may be
more susceptible to neurotoxic disturbances than the latter.
These data for the sparrow hawk have been incorporated, together with other data
for the species, into a population model (Sibly et al. 2000). Population changes in the
sparrow hawk that occurred in Eastern England during 1963-1975 were predicted
reasonably well by a model that assumed that all birds with dieldrin levels of 3 ppm
or more died due to poisoning by the insecticide. The substantially smaller propor-
tion of dead birds having 9 ppm or more, which were presumed to have died from
direct poisoning, was not large enough to predict the rate or scale of the population
decline that actually occurred or, for that matter, the delay in recovery until the resi-
due level in livers fell below 1 ppm (Walker 2004).
In comparison to the situation in birds, there is far less evidence of dieldrin hav-
ing had harmful effects in the field in mammals. It has been suggested that this is
a reflection of the fact that mammals tend to be more reclusive and therefore more
difficult to observe, catch, or count (Shore and Rattner 2001). That said, at the time
when cyclodienes were widely used in Western Europe and North America, there
were a fair number of reports of mammals being poisoned by them on agricultural
land. Such animals included predators such as the fox (
Vulpes vulpes
) and badger
(
Meles meles
), which had evidently acquired lethal doses from their prey.
Retrospectively, there has been considerable interest in the decline of the otter
(
Lutra lutra
) in Britain. Chanin and Jefferies (1978) examined hunting records and
concluded that a very marked and widespread decline of this species coincided in
time and space with the introduction of dieldrin. Unfortunately, there is very little
residue data for otters during that time, reflecting the point made earlier about the
reclusiveness of many mammals. That said, there is strong circumstantial evidence
to support the suggestion of these authors that dieldrin was the single most impor-
tant cause of this decline. Apart from the coincidence in time and space, there is
plenty of evidence for the existence of high dieldrin levels in British rivers at this
time. Substantial levels of dieldrin were reported in freshwater fish and considerably
higher concentrations in piscivorous birds such as the heron (
Ardea cinerea
) at this
time. In a study at a heronry in Lincolnshire (1965-1968), young herons contained
mean dieldrin residues in liver of 4.5 ppm and 3.05 ppm at two colonies, individual
values ranging from 0.1 to 28.2 ppm (Prestt 1970). Fish sampled at the same her-
onries contained mean residue levels in liver of 0.70 ppm (roach), 0.90 ppm (eel),
and 1.11 ppm (bream). These figures indicate that the young herons bioaccumulated
residues in their livers some four times the levels in their food. Other data from that
time showed that adult herons found dead over the whole of Britain contained a mean
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