The ecology of large herbivores in the Miombo woodlands of South East Tanzania

Abstract

The studies described in this thesis arose from an ecological survey of part of the eastern side of the Selous Game Reserve in south east Tanzania from 1967 to 1975* The intensive study area contained two major, ecologically distinct, habitats, an area of tall grass miombo woodlahd and a smaller area of short grass scattered tree grassland. Several species of large grazing ungulate utilised thefce habitats, namely: buffalo, hartebeest, impala, sable, warthog, wildebeest and zebra* Seasonal changes in the environment such as the six month drought and widespread annual fires affect the pattern of habitat utilisation exhibited by each species. This thesis examines such utilisation and shows that each species has developed its own ecological and feeding strategies in order to maximise nutrient inputs and reproductive success in what is seasonally a harsh environment In many ways the miombo (Brachystegia) woodlands remain the least studied of the East African habitats This lack of data on the miombo in general and the Selous Game Reserve in particular means much of the thesis is taken up by descriptions of the physical and biotic environment of the study area. Following an introductory chapter the thesis is divided into three parts. Part one, with three chapters, describes the physical environment: geology, soil, climate, water and prehistory; the vegetation: community description, distribution and species composition, floristics and vegetation dynamics; and range factors of productivity and forage nutrient content Part two contains chapters on animal numbers, distribution patterns, feeding and nutrition and a chapter examining growth and reproduction as indices of population performance Part three concludes the thesis with a chapter reviewing the miombo woodlands and the Selous as a wildlife habitat and a final chapter discussing management policy for the Selous Game Reserve. The study area is low lying (100 - 700m asl) with a single rainy season from late November to early May (average 760mm p.a.) Despite a long hot dry season, surface water is relatively plentiful Habitat distribution is primarily controlled by geology and soil type with major modifications by fires and past human activity. The upland Karoo and post karoo sandstones give rise to infertile acidic red sandy soils which carry a climax-thicket formation These have been degraded into woodland by fire and settlement activity Lower down the drainage slope the flat Pleistocene river terrace of grits and sands has produced a hard alkaline sodic soil carrying short grass scattered tree grassland The drainage sump of recent alluvial clay carries tall floodplain grasses The vegetation is diverse, collections to date totalling over 1200 vascular plants including 150 grass species. The woodland vegetation shows strong affinities to the Zambezian flora,and the thicket, riverine and alkaline communities show affinities to the East African coastal flora o The thesis contains species lists and a vegetation community map. Fire has opened up the thickets producing tall grass woodlands of various types. Grasses, mainly Andropogonae, are coarse tussock perennials with very low nutritive values when dry* No burning allows the spread of woody and climbing species which engulf the grass layer and rapidly reduce grazing values* Late dry season fires reduce woody elements and plant diversities and leave coarse grass growth thoughout the dry season* Cooler early dry season fires maintain mixed woodland communities and a mixture of grass species and, where water tables allow, rapidly produce a flush of nutritious regenerating grass growth* Crude protein content of such growth can reach 20% although standing crop is low* The alkaline soils carry a short grass sward of mixed annual perennial species of high nutritive values in the rains* There is no flush after burning and dry season grazing values are low* Chapter 5 discusses techniques of animal census in detail* A vehicular ground sample census using fixed systematically placed transects was used for common ungulates* Rainy season densities on the short grass habitats are as follows: Impala 28*7/km2, wildebeest 27,2/km2, zebra l6*2/km^ and warthog 9*4/km2# Wildebeest and impala densities have remained unchanged for six years, zebra and warthog densities have increased significantly* Less detailed data are given for other species* Distribution data have been collected at several different scales from Tanzania wide, to the Selous, to the study area, to the different facets of a miombo valley-ridge catenary system* It is ‘shewn that 'salile *nd'‘L±ch’fcnrteins .'S* : ns hartebeest are restricted to tall grass woodland habitats, wildebeest and impala must have access to short grass areas in the rains and that buffalo, zebra and warthog, are more habitat tolerant but achieve higher densities on short grass habitats . Within the study area virtually all impala, warthog, wildebeest and zebra concentrate on the short grass areas in the rains, moving into the woodlands following the onset of burning Within the woodland habitat wildebeest and impala concentrate on the green flush of the valleys and avoid the unburnt areas and ridge tops. Sable and hartebeest are distributed throughout all categories and warthog and zebra are intermediate showing some selection of valley and green areas The onset of the November rains signals a return to the short grasslands Studies of feeding behaviour showed buffalo, hartebeest and wildebeest to be total grazers whilst sable and impala took significant proportions of browse in the dry season and warthog diet changed to perennial monocot roots and rhizomes. Feeding studies concentrated on wildebeest and impala and much data was obtained from post morterm analysis. By assuming that extrapolation of data from other large ungulate studies was unlikely to cause large relative errors it was possible to make estimates of protein and energy intakes and of theoretical nutrient needs for these two species. the rainy season both species had inputs greater than During theoretical requirements for maintenance, i e i positive nutritional balance. they were in As the dry season progressed T both male and female wildebeest which remained on the dry short grass habit entered a period of negative nutritional balance• However wildebeest moving to fresh flush in the woodlands were shown (albeit on less evidence) to maintain a positive balance. Impala at the onset of the dry season changed their diet to nutrient rich browse components and so maintained a positive nutritional balance. These two ! strategies of habitat change and dietary change have resulted in peak nutritional inputs at different times of year. Impala have peak inputs in September and wildebeest in i December periods. Onset of parturition coincides with these peak The nutritional load of gestation is minimal compared to that of lactation. Calving season therefore allows lactation and wearing of calves at the nutritional optimum time of year. Other species show different strategies, which allow a separation of niche space reducing competition for food resources Chapter eight shows that individual growth rate, body condition and fecundity rates are as high or higher than other populations in Africa., and that populations here are well adapted to the ecological conditions of this part of the Selous Game Reserve. This chapter discusses why the wildebeest population is stationary when nutrition is adequate, condition is good and fecundity is high. Predation by lion is put forward as a major limiting factor* The concluding chapters look at the miombo woodlands throughout Africa as wildlife reserves and suggest that only when tall grass woodlands are adjacent to other habitats such as short grassland or floodplains do animal densities reach levels high enough to warrant major conservation inputs* The importance of fire in providing dry season graze is stressed and fire management polices for the Selous are discussed* Past, present and future human impacts on the wilderness values of the Selous are examined and a plea is made for definite land use policy statements for south east Tanzania and the reserve*