Conservation of endangered species - Ecological Applications: Toward a Sustainable World - page 123

modeling package: RAMAS - STAGE ). Multiple simulations, each lasting 1000 years,
were then run for every matrix to determine the population growth rate and the

probability of extinction. Sensitivity analyses, similar to those performed on the
koala populations, showed that population performance was not more sensitive to
any particular demographic parameter, indicating that a combination of growth,
fecundity and survival all contribute to population performance.
Figure 5.6 shows the median population growth rate

λ

for the 16 populations,
grouped into cases where particular management regimes were in place. This was
done both for years when recruitment of seedlings occurred and for years when
seedling recruitment did not occur. All sites where

λ

is greater than 1.35 when
recruitment took place are managed by burning and some by mowing as well; none
of these was predicted to go extinct during the modeled period. On the other hand,
populations with no management regime, or whose management does not include
fi re, have lower values for

λ

and all except two have predicted extinction probabili-
ties (over 1000 years) of from 0.10 to 1.00.
The obvious management recommendation i s to use pre scr ibed burning to provide
opportunities for seedling recruitment. Low establishment rates of seedlings may
be due to rodents or ants eating fruits or to competition for light with other plants
- burnt areas probably reduce one or both of these negative effects. While manage-
ment regime was by far the best predictor of persistence, it is also worth noting that
populations with higher genetic diversity also had higher median values for

λ

λ .

5.4.3 Ethiopian
wolves - dogged by
disease

It is not unusual for disease to contribute signifi cantly to the population dynamics
of a species but epidemiology has rarely been incorporated into population viability
simulations. An overly simplistic approach would be to incorporate disease simply
as an additional mortality factor. However, epidemiology theory makes clear that
the size of epidemics and the intervals between them are acutely dependent on

Fig. 5.6 Median rates of
population increase ( λ )

of Silene regia popula-
tions in relation to
management regime, for
years with seedling
recruitment (black
circles) and without
(open circles).
Unburned management
regimes include just
mowing, herbicide use
or no management.
(After Menges & Dolan,
1998.)

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

Fire and
mowing

Fire No fire

Management regime