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Herbivore Population Monitoring

Identifying herbivore abundance and distribution is an essential component of understanding the interactions between intraguild species. Firstly, it provides the necessary background information needed to determine prey preference and hence the potential for resource competition. Secondly, the presence and distribution of predators are influenced by the availability and spatial distribution of their prey.

The study area lies in a semi-arid ecosystem characterized by five distinct transitional habitat types; Colophosperum mopane woodland, mixed forest dominated by Acacia spp. (Acacia erioloba and Acacia tortilis), riperian woodland, open grasslands and floodplains created by the extremities of the Okavango. To determine herbivore distribution within these different habitat types, several transects were randomly allocated per habitat type. Transects are driven in an open vehicle at 15-20 km/hr with one person driving and one observer positioned in the back of the vehicle. For all animals sighted during a transect, the species, number of individuals, sex and age-class (where possible), perpendicular distance from the vehicle, GPS coordinates and vegetation characteristics are noted. For accuracy, the distances are measured to the nearest metre using a laser rangefinder and once sighted, animals are counted using. All data collected are analysed in Distance 5.0 (Thomas et al., 2006).

To account for seasonal variation in prey abundance and distribution, the herbivore survey is conducted four times a year; in the middle of the wet season (mid-January), at the end of the wet season (mid-April), in the middle of the wet season (mid-July) and at the end of the dry season (mid-October).

Prey abundance Impala were the most abundant prey species in the study area with an average density estimated at 11.8±4.4 individuals/km². Other recorded species were red lechwe (2.9±2.1 individuals/km²), steenbok (1.7±0.6 individuals/km²) and zebra (1.3±1.0 individuals/km²). Lechwe, being water-dependent ungulates, were found only in swamp vegetation types, where they were more abundant than impala. The least abundant species were warthog and larger species such as wildebeest, kudu, giraffe, buffalo, elephant and tsessebe.