Elephants, drought and the woody vegetation in Namibia's Etosha National Park

Abstract

Across southern Africa savanna elephants (Loxodonta africana) influence biological diversity (vide Cumming et al. 1997 and Western & Maitumo 2004 for more references), especially when confined and when occurring at relatively high densities. Under such conditions, their foraging and feeding habits may reduce tree densities and so transform woodlands into mixed woodlands and even grasslands (e.g. Ben-Shahar 1998; Mosugelo et al. 2002). Studies investigating interactions between elephants and their environment (n = 229) have increased significantly over the last 40 years. Most of them are descriptive, only 157 of them providing supporting data. Most (40) of the 69 sites included in these studies were only sampled once - only 21% of them incorporated controls but study design did not include replication and independence of sampling sites. Only half of all of these studies concluded that elephants impacted negatively on their environment. Since most studies on elephant-plant interactions suffer from poor scientific design, interpretations of the influence of elephants on their natural environment is confounded by lack of statistical rigour. This also seems to be affected by the educational background of the scientists that have conducted the studies. Extended droughts, rather than elephants, inducing mortality amongst woody plants, may confound the interpretation of the apparent influences of elephants on vegetation. We therefore opted to investigate how woody vegetation, at known distances away from water sources utilised by elephants, responded to a near 20-year long drought across the Etosha National Park in northern Namibia. The elephant population is confined to our study area and remained relatively stable at about 2000 individuals over the 17-year period of below-average rainfall represented by our study. Based on satellite tracking data we know that these elephants spend more than 80% of their roaming activities within an eight km radius of water. We therefore reasoned that mortality amongst trees further away from water represent that induced by the drought. Mortality amongst trees <8 km from water would then represent mortality induced by the drought and elephant trampling and browsing. We used a 17-year span of fixed-point photographs taken on four occasions and at about six-year intervals to deduce the influence of distance from water on the survival of all woody plants at 150 fixed points across the Park. Photographs were taken in four different directions at each of the points. Our evaluation of the photographs showed that only 26 woody plants were recruited over a period of 17 years across all these locations. Seventy nine percent of the 1676 woody plants that we could recognise survived over the same period. Mortality rate decreased with increasing distance from water and variability in mortality was higher close to water than further away. Based on these observations we conclude that elephants may have been instrumental in causing a decline in woody plant survival within a radius of eight km from waterpoints across Etosha. In total these affected areas represent about 42% of the terrestrial area of the Park. A continued decline in the survival of woody vegetation around water points may further compromise the survival of many plant and animal species. Thus, the artificial provisioning of water points may have negative consequences for biodiversity conservation in the long term. We suggest that management actions, such as the periodic closure of selected water points, be instituted to modulate woody species survival.