Conservation of endangered species - Ecological Applications: Toward a Sustainable World - page 125

Fig. 5.7 (a) Proportion
of 1000 wolf population
simulations that went
extinct in 50 years
using baseline
demographic param-
eters in habitat patches
of different areas,
exposed to different
levels of disease
incidence in the
adjacent domestic dog
population. The
likelihood of extinction
increases with disease
incidence but is smaller
for populations in
larger areas. (b) Effect
of percentage of wolves
vaccinated on popula-
tion persistence in
models with baseline
disease incidence.
Vaccination of just 20-
40% of wolves seems
suffi cient to prevent the
largest epidemics.
(After Haydon et al.,
2002.)

(a)

1.0
0.9
0.8
0.7
0.6
0.5
0.4

Area (km 2 )

250

100
75
50
25

0 0 0 0 0100 120 140

Disease incidence in adjacent domestic dog
population (% of baseline)

(b)

1.0

0.9

0.8

Area (km 2 )
100
75
50
25

0.7

0.6

0.5

0 20 40 60 80 100

Percentage of wolves vaccinated

capacity 25 individuals) did 2% of simulated populations go extinct in 50 years (see
probabilities of persistence when disease incidence is zero in Figure 5.7a).
When exposed to the baseline disease incidence (i.e. 100% on the horizontal axis
of Figure 5.7a), the 50-year probability of extinction for the largest populations
increases to 0.08-0.09, with populations showing one or two crashes down to as
low as 33% of carrying capacity. However, under this scenario, populations remain
reasonably viable until habitat size drops below 100 km 2 . For the 25-km 2 case,
extinction probability increases to 0.46. As disease incidence in the dog population
declined from 140% of baseline to 20%, the 50-year extinction probabilities also
fall, in the case of the 250-km 2 habitat from 0.13 to 0.04.
The potential value of management intervention to vaccinate wolves is illustrated
in Figure 5.7b. With 20% vaccination in the 250-km 2 population, percentage popula-
tion extinction declines from 0.10 to zero. In the smallest population, on the other
hand, 40% vaccination is needed to reduce extinction risk from 0.46 to 0.10. Hay-
don's team concluded that direct vaccination of as few as 20-40% of wolves might
be suffi cient to eliminate the largest epidemics and to protect populations against
the very low densities that make recovery unlikely.
Finally, when they applied a sensitivity analysis to the 250-km 2 case by looking
at the effects of slightly perturbing various parameters, an intriguing result emerged.
Population viability proved particularly sensitive to the process of recruitment of
females to packs. When females cannot be recruited because none or few are present
as fl oaters, pack extinction rates double. Clearly occasional female recruitment is
essential, but paradoxically when the size of the fl oater pool is large this imparts an
additional source of disease transmission and overall population viability declines.