With the exception of Asante Sana and the NCA, dietary quality followed the rainfall pattern observed at each site. Body condition on the other hand seemed to be correlated with the rainfall recorded roughly three months before recorded body conditions. Various authors have reported responses in forage resources to changes in rainfall (e.g. Sinclair 1974; Coe et al. 1976; Phillipson 1975; Owen- Smith 1990), with values of nitrogen positively correlated with rainfall (McCullagh 1969b; Sinclair 1974; Erasmus et al. 1978; Hall-Martin et al. 1982; Van der Waal
et al. 2003), and values of fibre negatively correlated with rainfall (Hall-Martin et al. 1982). Rutherford (1984) showed that browse and grass/forb species had
different responses to rainfall. Whereas grasses and forbs responded rapidly to changes in rainfall, browse species were slower to respond, generally responding to the previous season’s rainfall. This was supported by Hoffman & Cowling (1990), who showed that grasses of the eastern Karroo showed marked responses to seasonal and short term rainfall, whereas shrubs of the sub-tropical thickets had much less dramatic responses to rainfall. As herbivores select for the forage item highest in nutrition based on availability (Owen-Smith 2002), it would be expected that they would selectively feed on the flush of rapidly growing, nutritious grasses and forbs directly after rains, thus explaining the correlation between rainfall and increased dietary quality. This is supported by the elephant dietary study of Davis (2005), who showed that herbaceous plants
became the dominant component in the diet during summer, which was also the period with the highest recorded rainfall. The delayed response in elephant body condition could be a result of the slower response of browse species to rainfall. Browse has been shown to contribute between 39 – 54% of the elephant diet in succulent thicket (Davis 2005; Wolmarans 2006; Landman et al. 2008), and as such greatly contributes to the nutrition and condition of elephants. Alternatively, it could represent a delay in the response of body condition to increased dietary quality (Malpas 1977).
As herbivores are ultimately limited by their food resources (Sinclair 1974, 1975; Skogland 1983; Mduma et al. 1999), their body condition is linked to the quality and quantity of available forage (Price & White 1985; Brochus et al. 1988; Choquenot 1991; Pettorelli et al. 2005). Numerous studies have demonstrated a decrease in overall condition of herbivores in areas of high density due to a decrease in the availability of forage (Ruffner & Carothers 1982; Choquenot 1991; Swihart et al. 1998; Mduma et al. 1999; Van der Waal et al. 2003; Keyser et al. 2005; Stewart et al. 2005). Rainfall also directly affects primary productivity, with a decrease in rainfall being associated with a decrease in forage availability (Sinclair 1974; Coe et al. 1976; Phillipson 1975; Owen-Smith 1990; Mduma et al. 1999). This is supported by my findings, which illustrated a correlation between dietary quality (i.e. faecal protein and faecal NDF) and rainfall, as well as a decrease in elephant body condition with increasing elephant density, and emphasises these factors as drivers of body condition through their effect on resource availability in this landscape. This supports my hypothesis that elephant body condition will vary as a function of primary productivity, with lower primary productivity (due to rainfall and elephant density) being associated with lower body condition scores.
My study has shown that, for elephant populations occurring in the Eastern Cape, variables that affect condition cannot be considered in isolation, as they interact with one another in different ways. It is this interaction between rainfall, elephant
density and forage quality that ultimately drives the observed variations in body condition over space and time.
Although variation in dietary quality and rainfall was observed within all reserves, the much more dramatic response in body condition observed in the AMC, as compared to other reserves, demonstrates that the high density in the AMC has resulted in the decrease in available resources in this reserve (as shown by Penzhorn et al. 1974, Barratt & Hall-Martin 1991, Lombard et al. 2001), causing an increase in competition for the limited resources. However, the individual responses are not yet shown at the population level, as is evident by the lack of density dependent effects on population performance (Gough & Kerley 2006). A study by McGregor & Butler (2008) found that mortality due to hypothermia in Angora goats was related to body condition, with a significantly increase in mortality rates experienced when body conditions decreased below a specific point. This was found to be confounded by goat density, with areas of high goat density experiencing higher mortality rates (McGregor & Butler 2008). This suggests that, although body conditions in the AMC are currently not low enough to induce density dependence, a prolonged period of inadequate food supply (i.e. drought) may result in an increased mortality rate, which would be exacerbated by the high elephant density within the park. The findings of McGregor & Butler (2008) that show that increases in mortality occur when a threshold is crossed suggests that, when such mortalities are observed, the resources will already have been overutilized. This concurs with Gough & Kerley’s (2006) postulated catastrophic expression of density dependence through a collapse of resources in the AMC, rather than a gradual increase in mortality and decline in fecundity.
With the exception of one or two observations, the lack of any significant difference between energy stressed and non-energy stressed individuals in all reserves but the AMC indicate that, in these reserves, the increased energy requirements of these life stages (i.e. lactation, old age and weaning) are met by sufficient quantity and quality of forage. This is in contrast to the AMC, where not
only energy stressed, but also non-energy stressed individuals are in poorer condition than other reserves, and again highlights the fact that the high density over time of this population has already negatively affected their resources.
The above results thus support the hypotheses that 1) elephant body condition will vary across sites as a function of primary productivity, and 2) that elephant body condition will vary across sites as a function of dietary quality.