Okavango Herbivore Research

The Okavango Herbivore Research Project started in 2005. This multi-disciplinary project aimed 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

This project was completed in 2009 and formed Hattie's PhD project.

The major findings from this study included:

  • Habitat availability and within habitat grass characteristics in the Delta were spatially and temporally variable.
  • The population demographics of herbivore species were largely similar to those found in other protected areas in southern Africa. A female sex bias was found in all species as well as reasonable juvenile recruitment. These factors were relatively stable between regions and years during the course of the study.
  • The availability of floodplains and grasslands was the most important determinant of space-use at larger scales; herbivore density increased, home range size decreased and relative land-use increased with their increasing availability.
  • Zebra followed a selective, energy maximising strategy throughout the dry season, perhaps due to a short resource-limiting season and lack of other temporal constraints. Selectivity varied with season but not region; with preference for patches that improved intake rate during the more resource-limiting flood season, and patches which improved diet quality during the more productive hot dry season.
  • The Okavango Delta is not a closed system and medium sized herbivores do move in and out of the system.

Outputs
Spatial heterogeneity in a dynamic wetland: determinants of herbivore distribution in the Okavango Delta and their relevance to conservation (PhD thesis). pdf
Bartlam-Brooks, H.L.A., Bonyongo, M.C. & Harris, S. (2013) How landscape scale changes affect ecological processes in conservation areas: external factors influence land use by zebra (Equus burchelli) in the Okavango Delta. Ecology and Evolution, 3 (9), 2795-805. DOI: 10.1002/ece3.676 pdf