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
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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.
Publisher
University of Nairobi
Subject
Elephants in Habitat DynamicsRights
Attribution-NonCommercial-NoDerivs 3.0 United StatesUsage Rights
http://creativecommons.org/licenses/by-nc-nd/3.0/us/Collections
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