Environmental Engineering Reference
In-Depth Information
Table 5.4
Modeled
percentages of ungulate
carcasses with lethal
levels of diclofenac
required to cause
population declines at
rates observed for long-
billed vultures (LBW)
or white-backed
vultures (WBW) in
India and Pakistan
between 2000 and
2003. For each
population, results are
given for four feasible
adult survival rates (
S
0
)
in the absence of
diclofenac, and three
values of the interval
between vulture feeding
bouts in days,
F
. This
approach has much in
common with the
sensitivity analyses
described in Sections
5.4.1-5.4.3 (From Green
et al., 2004.)
Percentage of carcasses with lethal level
F
S
0
=
0.90
S
0
=
0.95
S
0
=
0.97
S
0
=
0.99
LBV India
2
0.132
0.135
0.137
0.138
3
0.198
0.202
0.205
0.208
4
0.263
0.271
0.273
0.277
WBV India
2
0.339
0.347
0.349
0.350
3
0.508
0.521
0.526
0.533
4
0.677
0.693
0.699
0.700
WBV Pakistan
2
0.360
0.368
0.372
0.376
3
0.538
0.551
0.558
0.564
4
0.730
0.734
0.743
0.751
Wisconsin, USA, to discuss the plight of the peregrine falcon (
Falco peregrinus
)
(Risebrough, 2004). In their case, it turned out that DDE (a derivative of the agri-
cultural insecticide DDT) had moved through the food web, accumulated in falcon
tissue and caused eggshells to become thin and easily crushed during nesting,
causing populations in Britain and the USA to plummet.
What about the decline of the vultures? It took a few years to determine a common
element in the deaths of otherwise healthy birds - each had suffered from visceral
gout (accumulation of uric acid in the body cavity) followed by kidney failure. Fur-
thermore, vultures dying of visceral gout were found to contain residues of the drug
diclofenac, whereas birds dying of other, known, causes did not (Oaks et al., 2004);
and carcasses of domestic animals treated with diclofenac proved lethal to captive
vultures. Diclofenac, a nonsteroidal anti-infl ammatory drug developed for human
use in the 1970s, came into common use in the past decade as a veterinary medicine
in Pakistan and India. Thus a drug with evident agricultural benefi t to domestic
mammals proved lethal to birds that fed upon them.
Given the relatively small numbers of dead mammals that had been treated with
diclofenac, was the associated vulture mortality suffi cient explanation for the popu-
lation crashes? This is the question addressed by Green et al. (2004) by means of a
simulation population model. On the basis of population surveys and knowledge of
demographic rates for both white-backed and long-billed vultures (see Section 5.1),
they built models to predict population behavior in a similar way to the studies I
discussed in Section 5.4. But Green's team also posed the specifi c question: what
proportion of carcasses would have to contain lethal doses of diclofenac to cause
the observed population declines?
Table 5.4 provides the answers for a range of feasible scenarios. Pre-diclofenac
adult annual survival rates are unknown, but information for related species sug-
gests that annual survival will lie somewhere in the modeled range of 0.90-0.99 (i.e.
90-99% of adults survive each year). Moreover, vultures vary in feeding frequency
and this has a bearing on the outcome: models were run for intervals between
feeding bouts of 2, 3 and 4 days.
You can see that at a maximum (for the Pakistani white-backed vultures when
adult survival is set at 0.99 and feeding interval is 4 days) only 0.751% or, in other
words, 1 in 133 carcasses need to be dosed with diclofenac to cause the observed
population decline. At a minimum (for Indian long-billed vultures when adult
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