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Growth rate -
Longevity
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First conception -
Age-specific
mortality -
Density dependence
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Fecundity
Reproduction and Population Dynamics
The southern savanna buffalo breeds seasonally from January
to April in southern Africa with the majority of births
occurring in January and February. In East Africa where
a double rainy season occurs, the seasonal pattern of breeding
is less marked. The gestation period is 330-346 days (Smithers
1983) indicating that in a typical savanna habitat most
conceptions take place shortly after the grass sward biomass
has peaked in the February of the previous year. The sex
ratios within buffalo populations are very close to unity
(Pienaar 1969, Taylor 1985). For any large mammal population,
the parameters which determine the population growth rate
are:
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Growth rate
A spreadsheet computer model
was used to examine various rates of population growth
expected under different fecundity regimes, with the average
fecundity being set by rainfall.
| Annual Rainfall (mm) |
200 |
300 |
400 |
500 |
600 |
700 |
| Average Fecundity (calves/year) |
0.288 |
0.343 |
0.397 |
0.452 |
0.507 |
0.563 |
| Expected Population Growth Rate %/year |
-0.48 |
1.18 |
2.71 |
4.13 |
5.41 |
6.61 |
| Population doubling time (years) |
- |
59.2 |
25.9 |
17.2 |
13.2 |
10.9 |
Breeding Males: An assumption of the model is
that there will always be sufficient adult males with
which to breed - an assumption which may not be satisfied
if, for example, sport
hunting quotas are too high. Taylor (1985) found no
males older than 10 years in breeding herds so it can
be assumed that once sport hunting starts to affect the
male age classes below 10 years old female conception
may be reduced.
Under Namibian rainfall conditions it can be expected
that in the wetter east of the Caprivi (rainfall 600-750mm)
buffalo populations will be able to increase rapidly with
doubling times of 10-13 years. In the west of the Caprivi
(rainfall 500-600mm), populations could double in 13-17
years. In those areas in the main body of the country
where annual rainfall is of the order of 300-400 mm growth
rates are likely to be low (<3% per annum). Where rainfall
is less than 250mm buffalo are unlikely to survive.
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Longevity
Although buffalo in captivity may live as long as 25 years,
very few animals in the wild survive to an age of 20 years.
Taylor (1985) found no specimens older than 18 years in
his Matusadona study.
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Age at first conception
| Age |
0-2 years |
2-3 years |
3-4 years |
4-5 years |
5-6 years |
6 years + |
| Proportion conceiving |
0 |
10% |
32% |
79% |
100% |
- |
| Proportion giving birth |
0 |
0 |
10% |
32% |
79% |
100% |
| Adapted from Taylor (1985, pp312-313) |
The age at first conception for buffalo females is slightly
dependent on environmental factors: where the nutrition
regime is high, it tends be to be slightly earlier and where
food is limiting it may
occur later. Female body weight may be a more significant
criterion determining the first conception than age: Taylor
(1985) found that about 50% of females became pregnant when
their body weight reached 350kg - which corresponds roughly
to an age of 3.5 years.
Taylor (1985, pp374) found that cows at Matusadona maintained
a high level of fertility until the fourteenth year of life
but notes that other authors have generally found that fecundity
begins to decline after the eleventh year (e.g. Patterson
1979).
Age specific mortality
The age specific mortality schedule for the population
model in this study is based on Taylor
(1985, p 441) for all age classes above one year. Because
the sex ratios in buffalo populations are very close to
unity the same mortality schedules can be used for both
males and females. Mortality for two-year old animals has
been set at 5%, a constant mortality of 3% is maintained
from 3 - 11 years old and the mortality from 12-18 years
is such that in a population of 1,000 animals one animal
survives to an age of 18.
| AGE |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
| Mortality % |
?? |
5 |
3 |
3 |
3 |
3 |
3 |
3 |
3 |
3 |
3 |
3 |
10 |
15 |
20 |
40 |
60 |
80 |
| Survival |
?? |
0.95 |
0.97 |
0.97 |
0.97 |
0.97 |
0.97 |
0.97 |
0.97 |
0.97 |
0.97 |
0.95 |
0.90 |
0.85 |
0.80 |
0.60 |
0.40 |
0.20 |
|
Figure 3: Relationship between juvenile mortality
and population growth rate
Figure 7: Crude Carrying Capacities for Buffalo
in Southern Africa
|
Martin examined the effect of different juvenile mortalities
on the population over a range from 10% to 70% mortality
in the first year of life, using a nominal fecundity of
0.5 calves per adult female per year. If the mortality in
the first year of life is higher than 50% the population
does not increase and for values of 60-70% the population
declines (Figure
3).
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Density Dependence
Sinclair (1977) shows that buffalo populations are regulated
by their food supply - which is ultimately regulated
by rainfall and soil fertility.
The regulation acts mainly to increase buffalo mortality:
fertility does not appear to be greatly affected although
it can be expected that the age at first conception would
tend to occur slightly later as it is dependent on the
physical condition of females. For the purposes of the
population model, density
dependence has been ignored but it has been taken into
account for carrying capacities for different areas (Figure
7).
