Impact of the African elephant (loxodonta African) on woody vegetation in a savanna ecosystem in Laikipia, Kenya

Abstract

The impact of elephant utilization on woody vegetation was investigated on Mpala Research Center and Conservancy, in Laikipia savanna ecosystem. Elephant sightings were recorded using hand-held GPS devices and were used to develop a use intensity map. Elephant behaviour was recorded. Feeding was recorded using scan sampling while reaction to observers was recorded using Wittemeyer's reaction index. 80 transects of 100xlOm each were sampled for woody species abundance, damage levels and types (total trees sampled n= 2945). Elephant presence across the conservancy was found to vary spatial- temporally, with the major influencing factor being water availability (p=0.003). Mpala was found to be a iIry season grazing area, with Elephant populations increasing during the dry season and drastically reducing at the onset of the rainy season. Human activity through livestock bomas was found not to affect elephant presence significantly (p=0.08). Elephant population was found to consist mainly of transient families and a few resident families which spent more than 50% of their time on the conservancy. Resident elephant families exhibited calmer reaction indices to observers while transients exhibited more agitated reaction indices (p=0.03). Acacia mellifera ((M. Vahl) Benth) exhibited the most feeding hits during the dry season while Acacia brevispica (Harms) exhibited the highest feeding hits during the wet season. There was a significant difference in damage levels across use intensities (p< 0.0(1) with damage levels increasing with increasing use intensity. Acacia brevispica was the most abundant woody species (32%, n= 940) and also had the highest numbers of individuals with general damage (83%). Acacia nilotica ((L) Delile) exhibited the highest percentage of mortalities (53%), which coupled with low reproduction rates in the area, made it the species at highest risk of overutilization and possible extirpation. The most common type of plant injury was crown damage for all the woody species apart from Acacia drepanolobium (Sjostedt), which exhibited high levels of stem breakage. There were low levels of debarking which were attributed to the low stem circumferences of trees sampled. Less than 10 %) of trees sampled were dead, indicating an unlikelihood of imminent complete degradation and conversion to grassland. However, the positive relationship between use intensity and damage suggests that an increase in elephant population on the conservancy would lead to an eventual change in vegetation composition, community structure and savanna ecosystem functioning. Further studies should be conducted to assess coppicing rates of the various tree species on Mpala. This, in addition to the recruitment and survivorship data, will help give an idea of the net effect of elephant herbivory on individual species.