Differential tree mortality patterns and dynamics in the Ruaha National Park Tanzania

The available data on tree population changes in the Ruaha National Park, Tanzania, during 1965-1977 were used by Richard Barnes to model future trends in relation to elephant usage and possible elephant population management through culling. Four species of trees that differed from each other in their mortality patterns were considered. Tree populations were assumed to follow the logistic growth curve. Regeneration was taken as zero at an elephant density of 3 animals/km2 (based on observations of regeneration by A. Bjorn-stad in 1971 referred to in Barnes 1983b), while this was 100% of the potential when elephant density was zero. The mortality patterns of the four tree species were as follows:

1. Tree species I (e.g., Commiphora ugogensis) experiences density-independent mortality, that is, each elephant killed a fixed proportion of trees over a period of time. In this case, an elephant density of 1/km2 caused a loss of 5% of trees per annum.

2. Tree species II (e.g., Acacia albida) experiences density-dependent mortality, that is, the mortality rate decreases with lower tree density.

3. Tree species III (e.g., Colophospermum mopane) experiences inverse density-dependent mortality, that is, the mortality rate accelerates with decreasing tree density.

4. Tree species IV (e.g., Adansonia digitata) experiences fixed-number mortality, that is, each elephant killed a fixed number of trees over a period of time irrespective of tree density.

Beginning from hypothetically identical population sizes in 1946, the four tree species populations obviously behaved very differently under the influence of the same elephant population. As expected, species IV crashed to extinction the fastest. On the other hand, species II persisted beyond 1990 because of a declining rate of utilization with decreasing tree density and population. Species III and I became extinct by about 1985 and 1990, respectively.

Barnes then introduced various options of culling the elephant population. A single cull of 50% of the elephants at different elephant densities of 1-2.5/km merely delayed the time of extinction of all tree species, with the possible exception of species II, which seemed to persist longer as culls were progressively delayed. Even when 100% of the elephants (at a density of 2/km ) were culled, this failed to arrest the extinction of tree species IV, although the other three recovered. Culling the elephants whenever their densities exceeded 0.5/ km2, however, stabilized tree species populations. The benefit-cost ratio of culling (number of trees saved per elephant shot) varied from one tree species to another. Overall, very large numbers of elephants had to be culled at the higher densities to save the trees. Barnes came to the general conclusion that culling must begin early to be effective in terms of the benefit-cost ratio, but that managers usually realized they had a problem only after elephant densities had become too high.

It is a different matter that, in the case of Ruaha and many other African parks, the spate of ivory poaching during the 1980s drastically reduced the elephant populations. A survey of the Msembe study area in Ruaha during 1989 showed little change in baobab numbers or size distribution from the previous assessment of 1982. Since most of the past baobab mortality was caused by bull elephants, their selective elimination by poachers had spared the surviving trees. Barnes thus recommended that selective culling of bull elephants was sufficient to protect baobabs.

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