Okavango Herbivore Research

The Okavango Herbivore Research Project started in 2005. This multi-disciplinary project aims to increase our understanding of what factors are critical in determining the structure of the Okavango Delta's large herbivore populations.

Despite its renowned high densities of large mammals and its status as one of the few intact inland Delta systems in the world, wildlife research on the Okavango Delta remains fragmented and in many fields inadequate. Aerial surveys conducted by Botswana's Department of Wildlife and National Parks suggest that populations are unstable, with some species populations apparently increasing whilst others decrease. The underlying factors causing the perceived population instability in the Okavango Delta was not known. Therefore research was needed to acquire in-depth understanding of the population demographics, assembly patterns and movement strategies of key large-bodied herbivores found within the Delta. Combined, such information is crucial in explaining population trends and structure and increasing our understanding of the ecosystem regulatory factors.

The project has three specific aims

1. Determine the present state of the Delta's herbivore population through population dynamics

2. Investigate the relationship between resource characteristics and herbivore distribution

3. Investigate how resources and other environmental factors influence animal movement of differing scales.

Research was focussed on the Moremi Game Reserve and adjacent Wildlife Management Areas. This region provides a good cross section of the habitats within the Delta utilized by herbivores, being composed of seasonal floodplains, open grasslands and dry acacia, mopane and riparian woodland.

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Methodology

The project used a wide variety of field techniques to collect a broad range of data.

The population demographics of all key herbivore species was recorded seasonally during driven transects. The age and sex of all individuals sighted was determined by identifying sex and age species-specific morphological differences (e.g. horn growth, size, muscular development) for each species.

Herbivore distribution and its relationship to habitat characteristics was investigated through monthly driven herbivore transects and vegetation sampling. Strip and Point herbivore transects were used to ensure that all population estimates were habitat specific despite the highly heterogeneous habitat distribution within the delta. To enable resource characteristics and quality to be related to population density and movement, habitat surveys were undertaken at all census sites. These surveys determined the predominant grass and herbaceous species, their structural characteristics, mineral content and palatability.

Home range and movement strategies were investigated by deploying GPS collars onto specific animals. This part of the study focussed on Burchell's zebra chosen due to their being a populous, dry-land grazer that has the capability to move large distances. Fourteen zebra were collared. GPS collars automatically record an animal's position at a pre-determined time schedule, providing detailed information of movement and habitat selection without constantly disturbing the animal. Animals were darted with immobilising drugs from a stationary vehicle by a wildlife vet, once immobilised the collar was fitted to the upper neck and various morphological measurements were taken. Animals were then given an antidote. All study animals coped with immobilisation well. All collars were removed once sufficient information had been gathered on each zebra.

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Findings

Field work has now been completed for most aspects of this project. The data is presently being analysed and findings will soon be published.

Initial observations suggest that:

- Herbivore population structure does not in general support the idea that herbivore populations are universally unstable, surveys may instead be observing herbivore movement around the system

- Herbivore distribution is closely linked to the availability of specific habitat combinations rather than individual habitats.

- Movement strategies are closely linked to available resources and flood dynamics, zebra home range size and seasonal habitat use varies greatly across the Delta.

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