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African elephants are a large management policy concern in sub-Saharan African countries. The study was aimed at assessing the impact of elephants on the immediate surroundings of pumped watering holes in Botswana's Chobe National Park. Picture imaging software was used to enhance and mark pictures taken by students from Kings College London in 1995, 1996, 2001 and 2006 in the park. Findings suggest that approximately 32.4% of the trees were lost by 2006. It also concluded that elephants have a negative impact on vegetation and supports the need for joint elephant culling efforts in the Chobe River riparian states.
Key words: Chobe, pumped pans, human-elephant conflict
1. INTRODUCTION AND LITERATURE REVIEW
Elephant (Loxodonta africana) populations are a large management policy concern for woodland conservation in most of Sub-Saharan African countries (Coe et al., 1976) like Botswana. Coupled with persistent drought and high frequency of fires, elephants have contributed significantly to loss in vegetation due to over-browsing (Ben-Shahar, 1996).
As protected species under the Convention on International Trade in Endangered Species agreement [CITES], elephants are a major tourist attraction; bringing in foreign exchange to the region. Interestingly, about 17% of Botswana's land area is within a designated National Park or Game Reserve with a significant number of people living in the designated reserves. Elephants are therefore an essential material resource for the native people living in the designations (IUCN/ CBNRM Support Programme, 2002).The implication is that the destruction caused to vegetation directly or indirectly affects humans.
1.1 Chobe National Park (NP)
Chobe NP is located in northern Botswana and covers an area of about 10,566 km2. It is located in the transition area between the arid/ nutrient-rich savannahs of the Kalahari of Botswana and moist/ nutrient-poor savannahs of South-western Zambia (Huntley & Walker, 1982; Kamuhuza et al. 1997), as shown in figure 1 below.
The main source of water supply in the park is the Chobe River, which supports evergreen woody plants and thicket as well as floodplain grass habitats. Chobe NP has the largest elephant population in Africa (Skarpe, et al., 2004). Aerial surveys indicate that the population was 120,000 (dry season estimate) in 2000 with an annual growth rate of 5.7% (IUCN/ CBNRM Support Programme, 2002).
Chobe NP has an average of 400 mm [in the south] 650 mm [in the north east] of rainfall per annum that predominantly occurs between November and March (Mosugelo et al., 2002). It was designated a National Park in 1967 in direct response to declining elephant population.
Three main factors have been identified as responsible for the decline in the elephant numbers. First, outbreaks of diseases [rinderpest and anthrax] led to the death of many elephants. Secondly, rampant poaching fed the ivory trade in the early 19th century. Although poaching generally declined since 1989, illegal ivory trade still exists. Lastly, persistent droughts have resulted in further decline in the population. These factors not only necessitated the designation of Chobe as a National Park, but also the need to protect the elephants.
1.2 Specific Study Site
There are both natural [non-pumped] and artificial [pumped] pans in the Chobe NP. Pumped pans were established by the Botswana's Department of Wildlife and National Parks [DWNP] to provide water supplies following severe drought periods in Botswana.
For this study, pictures of vegetation around the pumped pans [Tambiko, Nogatsaa, Tchinga, Kabunga and Poha] that lie south of the Chobe River in the Nogatshaa region of Chobe NP were reviewed. The location of these pans is shown in figure 2 below.
2 ELEPHANT CHARACTERISTICS AND BEHAVIOUR
African elephants are gregarious large herbivorous animals of the family elephantidae. Female and male adult elephants measure up to 3 m and 3.6 m, respectively, at the shoulder. Elephants live in families of 10 individuals but can congregate into clans of 6-70 members led by a large female (WWF, 2009).
Elephant densities close to water sources can be as high as 7-10 individuals kmË‰Â² especially towards the end of the dry season, but this is much lower for the drier mopane regions (Ben-Shahar, 1996). The movement of elephants is linked to the availability of food and which in turn is linked to availability of water, related to rainfall the region receives. During the rainy season, the foraging niche becomes large and densities can drop to as few as 0.5 individuals km-2 (Gibson et al., 1998). Elephants are essentially grazers in the wet season and turn to browsing in the dry season (Stokke & du Toit, 2000). The average consumption of an adult bull elephant is between 100 kg and 300 kg (Nelleman et al., 2002).
Elephants use their tusks for digging, stripping tree bark and defending themselves against predators. Therefore, the distinguishing indicator of elephant damage to mature woody plants is the height which corresponds with the average crest of the scapula (Stokke & du Toit, 2000).
2.1 Vegetation in Chobe NP
There are three main woody vegetation types in Chobe NP, namely; Acacia erioloba [camel thorn], Biakiaea plurijuga [Zambezi teak] and Colophospermum mopane [mopane]. The prolific elephant population in the park have resulted in high utilisation rates of mopane as a food source in most of Southern Africa. This has resulted in the alteration of the vegetation structure and decline of species diversity (Ben-Shahar & Macdonald, 2002).
2.2 Elephants in the Park
Emphasis has been placed on the role of the elephant on the unstable state of the savannah eco-system (Caughley, 1976). Several studies have shown that the increasing number of elephants can have a marked impact on mature woody species as the adult bull will eat the unpalatable woody vegetation that the adult female elephants and calves, and smaller ungulates do not (Van de Koppel & Prins, 1998). This suggests that elephants can potentially cause maximum damage to their habitats.
Elephant damage is identified by the characteristic knocking over and up-rooting of trees as well as stripping of tree branches. Stripping results in the reduction of the vegetation biomass. Figure 3 below shows the characteristic effects of elephant damage caused to woody vegetation.
Against this background, conflicts have arisen as to whether or not the savannah woodland can sustain the increasing elephant population in Chobe NP and whether intervention is necessary.
Some argue that land users like pastoral farmers have had their grain stores, crops and water supplies damaged by elephants in search of food and water. The policy prescription, therefore, is to control the elephant population. Others argue that intervention is unnecessary as the herbivore-plant model would automatically regulate elephant populations to manageable numbers (Caughley 1976). Yet others dismiss the link between elephant populations and vegetation loss (Kalwij et al., 2008) proposing more stringent measures of raising the elephants into the CITES I category like in Kenya and India. Such stringent policy measures would potentially threaten the livelihood of the local people (IUCN/ CBRNM Support Programme, 2002). An attempt at resolving this conflict underpins the purpose of this study.
3 AIM OF STUDY
The main purpose of the study was to assess the impact of elephants on the immediate surroundings of pumped watering holes. The findings of the study contribute to the debate about whether or not a culling exercise in the Chobe NP is necessary. The assumption of study was that the trees that were no longer there had been knocked over or uprooted by elephants.
4 METHODOLOGY AND STUDY DESIGN
4.1 Imagery and Aerial Surveys
Imagery and aerial surveys are established methods used in several studies of elephant population and their effect on vegetation. For example, Yang and Prince (1998) used satellite imagery to study spatial vegetation cover in Zambia. Verlinden and Masogo (1997) also used the method to study suitability of habitat in the Kalahari for ungulates and ostriches. Gibson et al. (1998) used aerial surveys to establish the trends of elephant populations in northern Botswana. Although effective, these methods are very expensive.
4.2 Procedure Adopted
In order to achieve the stated aim of the study, a desk review of images [pictures] was conducted. The study comprised reviewing pictures of the vegetation around the pumped pans. The pictures used were taken by Kings College London students in 1995, 1996, 2001 and 2006. Pictures were taken in January [rainy season], August and September [dry seasons].
Picture- editing software [IrfanView] was used to enhance, adjust and mark reference trees. Fifty catalogued pictures [1995-2001] were examined and matched with non-catalogued pictures from 2006. Images at different focal lengths were used in order to compare as many angles and landmarks as possible.
Trends in tree numbers for the four cardinal points [north, east, south and west] of each pan were recorded and compiled to monitor changes in the vegetation around the pans over time.
Processing of the data included matching the non-catalogued  with the catalogued pictures [1995, 1996, 2001] and counting trees that were definitely there [âˆš], probably not there [?], and definitely not there [X] by 2006, using different colour markings for each tree in time, as shown in figure 4 below.
a) 1995 b) 1996
c) 2001 d) 2006
The images show three main reference trees for one site [single cardinal direction]. By 2006 there is one tree that is definitely not there and one that is probably not there. The tree that was not there is knocked over, characteristic of destruction caused to woody vegetation by elephants.
A similar protocol was followed for all the other pumped pans for all the cardinal points. The total counts of the trees were compiled in Microsoft Excel 2007.
5 RESULTS AND DISCUSSION
A total of 136 reference trees were counted for all sites from all cardinal points at the beginning of the study period in 1995. Table 1 below shows the total number of tress counted in the pictures reviewed at the pans for the duration of the survey. It also shows the trend of trees numbers between 1995 and 2006 for each site.
By the end of the study period in 2006, a total of 92 trees were left standing i.e. the initial total count was reduced by 44. This represents a percentage decrease of 32.4% in the number of trees over the study period. Figure 5 below shows this trend for the study period.
Both the table and figure above show a definite reduction in the number of trees that were left standing at the pumped pans. Where there was no definite loss in count, there was evidence of the diminishing of the tree foliage caused by elephant stripping. The results also showed that there was more destruction to trees at Nogatsaa, Tambiko, Tchinga and Kabunga in the north compared to Poha in the south. Gibbons (2007) attributed this to the dry season distribution/ range of elephants as illustrated in figure 6 below.
The figure shows that the elephant abundance is not uniform throughout the Park [outlined in yellow]. Poha [18Â°24'46.41"S, 24Â°54'44.72"E] is more likely to lie within the area with a dry season elephant abundance of 0.27; whilst Tambiko [18Â°16'32.36"S, 25Â°1'26.44"E], for example would more likely have a probability of 0.54 dry season abundance. The potential foraging niche for elephants varies with time and this is strongly correlated with the amount of rainfall the area receives (Gibbons, 2007 & Nelleman et al., 2002). These studies also confirmed that elephant migration is largely a function of availability of water and good quality forage.
During the rainy season, elephants will potentially disperse over a larger foraging niche and during the drier seasons will congregate in search of water ending up at pumped pans. This will result in a similar pattern of vegetation loss as is seen at the Chobe River front.
The interaction between the elephant populations and the savannah ecosystem has been described as complex with multiple stable states (Van de Koppel & Prins, 1998; Skarpe et al. 2004). The increase in elephant-numbers results in the loss of woody vegetation. The savannah turns into grassland state; limiting the numbers of large herbivores it can support. When grasslands prove unattractive habitats for them, they will migrate to more suitable habitats and smaller ungulates will take over the habitat until the woodland state can be restored. The elephants will cause similar damage to new habitats and the cycle of destruction continues.
There is therefore a clear link between the elephant population and the destruction to trees in the immediate surroundings of the pumped pans contrary to arguments advanced by Kalwij et al. (2008). These results would suggest that there is a need for a culling effort in the park. One would conclude, considering the herbivore-vegetation model (Caughley, 1998), that the strongest argument for culling is to resolve the human-elephant conflict. The underlying reason for supporting this argument is that when herbivores damage vegetation in one area, they migrate to other areas (including other countries). This allows the vegetation in previous habitats to recover. Therefore, culling should not be an ecological necessity.
Given that elephants that cause damage to vegetation in Chobe NP, migrate and are therefore likely to cause similar damage in other countries; culling should be carried out as a joint effort among the riparian states of the Chobe River between which these elephants migrate. The caveat should be that as a CITES II protected animal; culling should be firmly guided by the international rules.
The main limitations of the study were that the images used were captured in different seasons of the year. This had the obvious effect of changing the appearance of reference trees. It also made the landmarks in the foreground look different. Further, marker pegs were removed by elephants resulting in variations in the angles and distances from which pictures were captured over time.
6 CONCLUSIONS AND RECOMMENDATIONS
The study revealed evidence of destruction to trees around the pumped watering holes. The trees were either not there or diminished in size by the end of the study period. One can conclude that the elephant population is having a negative impact on the vegetation. This finding supports the argument that culling is imperative, as a way to resolve the human-elephant conflict rather than an ecological management necessity. The culling should be a joint effort among the riparian states of the Chobe River, between which the elephants migrate and should be within international rules.
The method used in this study, though having some limitations, is scientifically sound to utilise in assessing the effect of the elephants to vegetation around pumped watering holes.
For more consistent results, future studies would benefit from capturing images for the dry season when the damage is clearly evident and browsing impact of the elephant is at its highest due to the reduced foraging niche.
Studies would also benefit from using a Global Positioning System (GPS) to ensure consistency of position and distances from which pictures are captured over time.
Comparative studies should be conducted in riparian states between which the elephants migrate to establish whether they cause similar damage to the environment in those countries.