Natural Manuring By Elephants In The Tsavo National Park, Kenya
MetadataShow full item record
Elephants 1n the Tsavo (East) National Park make up about 70% of the large mammal biomass and have been estimated to consume approximately 10% of annual primary production. In the first part of this thesis the role of the elephant in recycling nutrients is examined by recourse to studies of this species that have been carried out 1n East and Central Africa. Particular attention is paid to feeding, defaecation, damage to woodland and to aspects of behaviour that determine the distribution of these activities within the study area. The remainder of the thesis is concerned with a single pathway of material, that taken by undigested food following its deposition on the surface of the ground as dung. In Part II, the amount of material entering this pathway is quantified (Chapter 2) and its distribution is examined by a number of methods. The amount of dung produced by elephants in the study area (the part of Tsavo East lying south of the Galana river) is estimated to be 600 metric tons per day, equivalent to 53.5 metric tons per km2 per year. The distribution of elephant dropping s in part of the study area was examineUy waOlking transects (Chapter 3~. The results showed that there were gradients in the density of droppings across both axes of the area, a north-south gradient that opposed a rainfall gradient and an east-west one that coincided with that of rainfall but opposed a gradient in the degree of vegetation damage. The distribution of droppings on the transects was essentially random although a slight clumping effect close to paths was evident. However, due to their infrequent occurrence, some features that were expected t9 cause accumulations of dung were not adequately sampled by the transects, including sources of permanent water and shade trees. A separate study (Chapter 4) was made of the effect of shade trees on the distribution of droppings, a study that revealed that almost 30 times as many droppings were deposited under shade trees as on the same area of ground in the open. The number of droppings found in the open was less than half of that expected on the assumption of random distribution, suggesting that factors caus1ng localised dung concentration also lead to depletion in the .areas away from these features. Part III deals with the removal of dung from the site of its deposition on the ground. Previous studies of dung breakdown are reviewed in the·first chapter (Chapter 5), followed by a description of three different programmes of work designed to measure the rate of dung removal under different seasonal conditions (Chapter 6). The first two programmes, carried out during the wet season, showed that rainfall was the dominant factor determining rate of removal, and that the majority of removal was effected by dung beetles within 24 hourS'of dung deposition. The final programme examined the rate of removal of dung that was fresh in the dry season, during a 500- day period. Dung beetles were found to remove little or none of these droppings, instead removal was carried out, mostly by termites, at a mean rate of 0.155% per day. Studies of removal rate were followed by further investigation of the biological agents involved. Firstly dung beetles were investigated by monitoring the numbers of 56 species groups over the period of a year (Chapter 7). The results showed that all groups, except those that were too rare to be satisfactorily analysed, were significantly and positively correlated with rainfall, a finding that confirmed the seasonality of the::r action, suggested by the.studies of removal rate. In the same chapter, beetle numbers were converted to biomass and a similar relationship with rainfall was found for the biomass of all species combined. Characterist: that determine the part played in dung removal by individual species' of beetle were discussed, and it was concluded that only a small proportion of the total number of speC1es were responsible for the majority of removal by beetles. During this study a large collection of beetles was made, these have since been identified and a taxonomic list of the species found in elephant dung B included. In Chapter 8 the action of termites in the removal of dung is described and quantified in terms of the rate at which a number of defined stages were reached. It was concluded that the demonstrated rate of removal (0.155% per day) during the first 500 days declined subsequently, and that the action of rainfall is important in hastening the final stages of breakdown. In the final chapter of Part III the results of the studies of dung removal are employed in the provision of a summary of dung removal during 1973 and 1974, for the area in which the studies of removal were carried out. 25.0% and 20.2% of dung was removed by beetles in the two years, reflecting differences in total rainfall. The remainder of the dung was removed at a mean rate of 4.7% per month until none remained. The summary demonstrates that the mean amount of dung on the ground during 1974 was equal to the amount deposited in 7.45 months, or 62% of one year's production. At anyone time the droppings on the ground will be made up of some from each of at least 16 previous months, the oldest being more than 21 months· of age. Part IV (Chapter 10) describes some simple techniques employed to investigate the effects of dung on soil Composition and plant growth. Soil samples from below individual droppings and from under the canopy of shade trees were analysed. In the former samples calcium and potassium, and in the latter, seven of the eight elements that were determined (including carbon, nitrogen, potassium and phosphates) were shown to occur at higher concentrations in soil from the sites of dung deposition than in control samples. Measurements of grass growth under shade trees at the beginning of the wet season revealed that there was eight times as much growth in the area of maximum dung deposition (and maximum soil nutrient levels) as in control areas.