Large Herbivore Population Regulation

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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 offtakes are possible, but levels of predation must be considered. Population management using this strategy 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.
  3. Fixed population. This strategy is applied to larger species (e.g. elephant and hippo), which respond slowly to changes in environmental conditions and are not strongly regulated by predation. Consequently, populations may reach levels where the risk of habitat degradation or other undesirable ecosystem impacts becomes unacceptable. Rather than managing to a fixed carrying capacity, populations are maintained within limits that represent an acceptable level of ecological risk. These limits are determined through scientific research and long-term monitoring of key indicators, including habitat condition, vegetation structure, and species performance. Population management interventions, such as harvesting or translocation, are implemented when monitoring indicates that the upper risk threshold is being approached or exceeded. Population size is then reduced to a lower operational level before being allowed to increase again within the acceptable range.

Translocations

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 as well as 10 black rhinos and 71 white rhinos to other protected areas in Zimbabwe.