Regulation of large herbivore populations

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Populations at carrying capacity decline when resources are in short supply and increase when they are abundant. There is a time lag between changes in resource availability and the response of the population, to the extent that populations are continually over-shooting and under-shooting carrying capacity.

The time delay is short for small species (e.g. duiker and warthog) and longer for large species (e.g. elephant and hippo). In closed (fenced) management systems, carrying capacity over-shoots can cause substantial medium-term damage to the vegetation, resulting in a reduction in the carrying capacity of the system.

Strategies for managing large herbivore populations

A principal goal of wildlife management is to minimise the negative impacts of these over-shoots on the food resource. Given the above, three strategies for the management of the large herbivore populations here at Malilangwe are recognised:

  1. No management. This is applied to species that are strongly constrained by resources or predation.
  2. Proportional harvest. This is applied to the medium-sized species, not strongly constrained by resources or predation. Commercial off-takes are possible, but levels of predation must be taken into account.
  3. Fixed population. This is applied to the larger species (e.g. elephant and hippo), which respond slowly to changes in resources and are not constrained by predation. They can therefore easily over-shoot carrying capacity, causing long-term habitat degradation. Off-take is adjusted to meet a resource determined carrying capacity.

With the proportional harvest strategy, population management commences once the population exceeds 90% of estimated carrying capacity and then a constant proportion is removed each year, with the rate depending on the objective and levels of predation.

In the case of the fixed population strategy, population size can be allowed to approach the agreed carrying capacity prior to harvesting. On achieving this level, the population is reduced below carrying capacity by a number determined by practicality (family unit or saleable group), and then allowed to increase to carrying capacity again.

White rhinos are particularly vulnerable to forage shortages caused by drought or competition from other grazing herbivores. This is because, unlike other herbivores, they cannot supplement their diet with browse when grass is in short supply. White rhinos here at Malilangwe are in competition for food with buffalo, elephants and zebra. Therefore, to prevent carrying capacity over-shoots, and make more grazing available to a growing white rhino population, 1 414 buffalo, 53 elephants, 215 zebra and 35 white rhinos have been translocated off our Reserve. In addition, we have translocated 8 black rhinos to Botswana and 10 black rhinos to neighbouring Gonarezhou National Park.

Elephant capture

Animals are first immobilised by darting from a helicopter and are then transported on a low-bed truck to ‘wake up’ crates on the transport truck.