Mortality
A simple population model using the key
reproductive parameters was developed. Under average
conditions, reedbuck, waterbuck, lechwe and puku populations
could be expected to increase at a rate of slightly over 10% per
annum when they are below ecological carrying capacity. This is
not a particularly high growth rate and the reason for this is the
assumed high juvenile mortality (33% in the first year of life).
With mortality less than 20% and with a slight increase in the assumed
female fecundities, the species populations would grow at over 20%
per annum.
Martin examined the extent to which the given regime of adult female
and juvenile mortality, which was derived for average
conditions would have to change in order to throw the 'generic'
population into a decline (Table 3). In the two tables, each age-specific
female survival rate has been changed by the proportion indicated.
| Change in adult female survival % |
+2 |
+1 |
0 |
-2 |
-4 |
-6 |
-8 |
-10 |
-12 |
| Rate of population growth % |
11.9 |
11.0 |
10.2 |
8.4 |
6.7 |
4.9 |
3.10 |
1.30 |
Decline |
Table 3a: Effects of changes in adult female survival on
population growth rate
|
| Change in juvenile survival % |
+30 |
+20 |
0 |
+10 |
-10 |
-20 |
-30 |
-40 |
-50 |
| Rate of population growth % |
16.5 |
14.5 |
12.5
|
10.2 |
7.8 |
5.3 |
2.5 |
0 |
Decline |
Table 3b: Effects of changes in juvenile survival on population
growth rate
|
It is apparent that the population can tolerate very large increases
in juvenile mortality: at a 50% mortality (a change of about 20%
in the present juvenile survival), the population continues to increase
albeit at a low growth rate (5.3%). The same is not true for adult
female survival. An overall decrease of 10% in the existing schedule
of age-specific survival rates of adult females brings population
growth close to zero. The linkage
between rainfall and adult mortality would not have to be particularly
pronounced to produce major swings in the population.
|
