|
Martin uses a population model to examine age specific mortality
and the effects of natural mortality on an elephant population.
Age specific mortality
Age-specific mortality is set by means of a 'template'. It
is only necessary to specify the central mortality for the
population and the curves for juvenile mortality and senescence
are adjusted automatically. The mortality for each age class
is derived by multiplying the number in the template by the
central mortality of 0.5%.
| Age |
1 |
2 |
3 |
4 |
5 |
6 - 42 |
43 |
44 |
45 |
46 |
47 |
48 |
49 |
50 |
| Template |
16 |
8 |
4 |
2 |
1 |
1 |
1 |
2 |
4 |
8 |
16 |
32 |
64 |
100 |
| Mortality |
8 |
4 |
2 |
1 |
0.5 |
0.5 |
0.5 |
1 |
2 |
4 |
8 |
16 |
32 |
50 |
The mortality for males in the age classes 20-25 years is
doubled.
The rate of growth for an elephant population with a stable
age distribution is slightly less than 5%. If all mortality
is set to zero (apart from the animals which die at the age
of 50 years), the maximum growth rate rises to 5.7%. The various
recorded cases in the literature where elephant populations
appear to have increased at up to 7% per annum (e.g. Hall-Martin
(1980) - Addo National Park) are invariably in situations
where a stable age distribution has not been achieved. Although,
in theory, a fecundity of one calf every 3 years is possible
such a rate is likely to be an episodic event. Synchrony of
calving among females following a drought could also give
the effect of a very high rate of increase for a single year.
However, averaged over four years the result is no different
to that which would be obtained with a fecundity of 0.25.
Response of an elephant population to changes in natural
mortality
Once natural mortality exceeds the threshold at which the
population can maintain itself, it is of more interest to
express the decline as a 'half-life' i.e. the time it take
the population to halve.
| Natural mortality % |
0 |
0.25 |
0.5 |
0.75 |
1 |
1.25 |
1.5 |
2 |
2.25 |
2.5 |
| Rate of population growth % |
5.70 |
5.11 |
4.56 |
3.99 |
3.42 |
2.84 |
2.26 |
1.09 |
0.00 |
Decline |
| Half-life (years) |
150 |
100 |
50 |
25 |
10 |
9 |
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
| Natural mortality % |
2.5 |
2.6 |
2.8 |
3.5 |
5.4 |
5.8 |
6.1 |
7.0 |
8.1 |
9.8 |
11.8 |
15.4 |
21.5 |
36.6 |
Table 3a Effects of changes in overall mortality on population
growth rate
The effect of varying juvenile mortality independently of
adult mortality is examined below. The specified mortality
in the first row is for animals under one year old. Mortality
is halved for each subsequent age class up to 5 years old.
The adult mortality has been set at 1%.
| Juvenile mortality % |
5 |
10 |
15 |
20 |
25 |
30 |
35 |
40 |
45 |
50 |
| Rate of population growth % |
4.35 |
3.95 |
3.51 |
3.07 |
2.62 |
2.16 |
1.65 |
1.17 |
0.40 |
-0.12 |
Table 3b: Effects of changes in juvenile mortality on population
growth rate
It is apparentthat an elephant population can tolerate very
high levels of juvenile mortality - it is only when mortality
reaches 50% that the population begins to decline (Table 3b).
The same is not true for adult female survival. A mortality
of more than 2.5% causes the population to decline. These
results are used later in this study to examine particular
Namibian elephant subpopulations.
|
