Nyala and Bushbuck

Ecological separation of Greater Kudu, Nyala and Bushbuck at Londolozi

Greg Seymour was the joint winner of the prize for the best field project in the fourth edition of CC Africa's Ecological Journal published in 2002. In this edition, Greg published his research on cheetah, leopard and a comparison of three closely related antelope species at Londolozi. It was this last project in particular that highlighted his diligence and scientific rigour and, in the fourth in our series highlighting field projects conducted by rangers at CC Africa lodges across Africa, we present a shortened version of this research conducted over 2000 and 2001.

Crucial for the management of large herbivores is the management of their food resources. This of course necessitates an understanding of their dietary preferences. African ungulates are of course not ecologically isolated and a certain degree of overlap in diet and habitat use occurs even in unrelated species. The purpose of this study was to determine the diet and habitat selection at Londolozi of the Greater Kudu, Nyala and Bushbuck. All three belong to the genus Tragelaphus and are closely related. The degree of competition and overlap between the three species was also investigated. In order to achieve these aims, it was necessary to define the various habitats at Londolozi and the nine types - Marula-Combretum Savannah; Contour Seep Lines; Mixed Tree Savannah; Calc Brack Thicket; Drainage-line Thicket; the Sand River; Knob Thorn Turf Savannah; Turf Grassland; Clearings - were based on earlier studies. Observations of all three species occurred over a complete seasonal cycle and included records of habitat type and time spent feeding on particular plant species. The Kulczynski similarity index was used as a measure of competition.

Dietary and habitat selection

The dietary selection of all three species was divided into the dry season, the wet season and the ‘pre-rain flush'. Different plant species were utilised in different proportions during the three periods with grass notably being taken by all three species during the pre-rain flush. The diet of Kudu was comprised primarily of just four woody species - Knob Thorn Acacia nigrescens, Red Bushwillow Combretum apiculatum, Russet Bushwillow C. hereoense and Buffalo-thorn Ziziphus mucronata - which in the wet and dry seasons comprised 20-30% of the diet, and as much as 45% during the pre-rain flush. Plants in the herb layer, known as forbs, were even more favoured than the woody component making up about 50% of the diet in the wet and dry seasons, declining to 15% in the pre-rain flush. Nyala similarly also selected Knob Thorn, Russet Bushwillow and Buffalo-thorn as important species (about 20% of diet in all seasons), but showed more variation than Kudu, especially with regard to grass, as this species is a mixed feeder and both browses and grazes. As with Kudu, forbs contributed up to 50% of the diet during the dry season. Bushbuck showed similar trends to these two ungulate species with 20% of the diet across all seasons being comprised of Knob Thorn, Russet Bushwillow, Fig species and Buffalo-thorn. Like Nyala, the White-berry Bush Flueggea virosa and the Transvaal Gardenia Gardenia volkensii was also favoured. Bushbuck showed less seasonal variation than Kudu or Nyala and, particularly with regard to forbs, showed consistently high utilisation through the year.

The three species also showed marked similarities in habitat selection, but despite some overlap, a clear picture of preference emerged for each species. Kudu, for example, favoured Mixed Tree Savannah and Marula-Combretum Savannah during all seasons, with Calc-Brack thickets being heavily utilised only during the dry season. In contrast, Nyala, while utilising Mixed Tree Savannah quite markedly throughout the year, showed a much higher preference than Kudu for both Calc-Brack and Drainage-line Thicket. In addition, Nyala displayed a marked movement into the Sand River vegetation during the dry season. Bushbuck showed far the highest degree of habitat specialisation and throughout the year focused on Drainage-line and Calc-Brack Thicket and the vegetation associated with the Sand River.

Competition (habitat and dietary overlap) between the three species

From the results of this research, Kudu and Nyala have a dietary overlap of more than 73% during the dry season at Londolozi, although this drops to only 16% during the wet season. Kudu and Bushbuck overlap by more than 76% in the wet season and by as much as 44% in the pre-rain flush, and Nyala and Bushbuck overlap by almost 87% in the dry season, although this is below 22% in the wet season. Clearly there is substantial dietary overlap with several key plant species being utilised by all three antelope. Some of this competition might be offset by the different body sizes and browse niches of Kudu, Nyala and Bushbuck, but this is insufficient to counteract all competition. How then do they manage to exist within the same ecosystem?

This is to some extent explained by habitat selection. In the wet season when there is very little dietary overlap (16%) between Kudu and Nyala for example, habitat overlap is significant (52.2%). When dietary overlap is at its most marked (73.7%) between these two species, habitat overlap (63.6%), although still high, is lower than the dietary overlap would suggest. This distinction is even more marked between Kudu and Bushbuck. In the wet season dietary overlap is at its highest (76.5%), but habitat overlap is at its lowest (24.1%). The same applies to Nyala and Bushbuck who have low dietary overlap in the wet season (21.8%), but share much the same habitat types (54.1% overlap). In other words, when the antelope are feeding on the same plant species as each other, they do so in different habitat types.

Nonetheless, there is still considerable overlap - primarily in the dry season when Bushbuck and Nyala have a 86.9% overlap in dietary selection and a 89% overlap in habitat selection, suggesting that during these periods Nyala, as the larger animal, may have an adverse affect on Bushbuck. This is not apparent however, and may be explained by the additional variable of predation with higher predation of Nyala, at the hands of Lion, Leopard, Spotted Hyena, Cheetah and Wild Dog, taking place. Bushbuck, inhabiting slightly more closed parts of the relevant habitat types (Sand River; Drainage-line Thicket), are preyed on mainly by Leopard alone and therefore figure less often in the overall prey sample.

Conclusions

All three species showed significant dietary changes in relation to season. This was most marked in Nyala as mixed feeders. An analysis of dietary and habitat overlap highlighted some minor areas of competition between Kudu and Bushbuck and Kudu and Nyala, but not to same degree of that between Bushbuck and Nyala which in the dry season, perhaps because of reduced resources, showed considerable dietary and habitat overlap. Without the factor of predation, this level of competition might adversely impact on the Bushbuck population and prove unsustainable.

-Greg Seymour-

Posted: Mammals by CC Africa, Date: 21 November 2006