The Role of Elephants in Habitat Dynamics and Its Effects on Other Mammalian Species in Mwea National Reserve

Abstract

The vegetation of Mwea National Reserve can be categorized into three distinct types characterized by distinctive woody plant species. These vegetation types are the woodland, bushland and the wooded grassland as previously defined by Van de Weg and Mbuvi (1975) and Chebures (1989). These vegetation types, however, can be further classified in relation to dominance and co-dominance of two woody species and structural composition of woody species common to all vegetation types. This latter classification was found important in determining vegetation dynamics, which is critical to future management of the reserve. The woodland vegetation type was split into two vegetation types namely the Acacia mellifera bushed woodland and Commiphora africana bushed woodland while the other vegetation types remained as previously described. However, there have been evident changes in vegetation types since their classification by Chebures (1989) where for instance the wooded grasslands were showing massive invasion by woody species common 111 the woodlands while the previously cultivated areas are now under the thick bushland. The elephant population density in the reserve is currently high at 0.88±0.31 krn" but has not effected serious ecological changes through vegetation destruction. The elephants are only responsible for approximately 20% impact on woody plants in the reserve. Elephants seriously browsed on 7% of these woody plants while approximately 13% of the plants were slightly browsed. Proportions of dry biomass off-take and preference ratios on woody species show that Acacia ataxacantha and Grewia bicolor were the two most preferred woody species. Elephants, however, show disproportionate utilization of woody plants within height classes both within and between vegetation types. The height class I-3m high and height class >3m high were mostly preferred to height class <1m. The mean seasonal growth rate of woody plants was high as evidenced by changes in coppice heights of selected woody species utilized by elephants, especially for Acacia brevispica (94.51±6.26cm) and A. ataxacantha (93.S1±6.26cm). The elephants, however. did not show preference for emerging coppices as food items. Elephants were similarly xv not found to browse or impact on previously browsed woody species by the elephants in the reserve with the exception of Grewia virosa and G. bicolor. Elephant use of vegetation types has no cascade effect on habitat use by other mammalian species in the reserve. For instance, there were no significant associations (p>O.05) between elephants and other mammalian species during the wet season and the dry season. The bushbuck, however, showed close association with the elephant during the dry season while the dik-dik, the sunni and the waterbuck were closely associating with the elephant during the wet season. Similarly, very few animals showed seasonal preference for various habitats in the reserve. All vegetation types were important to all species of wildlife found in the reserve in that the species had ubiquitous distribution. Very few wildlife species showed preference for a particular vegetation type in any season. It was only the elephant that showed avoidance of the wooded grassland habitat among species that were not recently introduced into the reserve. Pasture conditions within the wooded grasslands are poor, judged from high presence of invasive herb species and low diversity of grass species due to lack of a regular burn. Burned wooded grassland areas were found to have improved in species composition and biomass compared to unburned wooded grasslands. Low reserve occupancy by mammalian species, thick vegetation, poor infrastructure and poor road connectivity are a hindrance to promotion of sustainable tourism activities in the reserve. To maintain and evaluate the ecological integrity of Mwea National Reserve. a number of characteristic indicators are suggested for future monitoring and intervention management. They include changes in woody species density, diversity, woody species structural composition, and change in numbers of indicator species for plants. Mammalian species succession, distribution and change in their physical conditions among others are good indices to consider in evaluating the ecological health of the reserve, These ecological indicators will help the management to monitor and determine ecological changes, the direction they are taking and when to undertake management interventions.