Herbaceous layer production and utilization by herbivores under different ecological conditions in an arid savanna of Kenya

Agriculture, Ecosystems and Environment 69 (1998) 55±67

Herbaceous layer production and utilization by herbivores under different ecological conditions in an arid savanna of Kenya George A. Keya* Department of Geography and Geosciences, Trier University, P.O Box 3825, D-54286, Trier, Germany Received 8 July 1997; accepted 5 February 1998

Abstract A three-year study was done to determine herbaceous biomass production and its utilization by herbivores in an arid zone inhabited by a largely nomadic population in northern Kenya. The indicator selected for study was aboveground live standing biomass of grasses, forbs, dwarf shrubs and the total sum biomass of these vegetation categories (total herbaceous layer). Sampling was done along grazing gradients in order to estimate the utilization levels in the arid zone 7 and the semi-arid zone 6. Mean peak standing biomass for grasses, forbs, dwarf shrubs and total herbaceous layer under non-grazed conditions was 184.4, 374.2, 1094.4 and 1504.0 kg/ha in eco-zone 7, respectively. In the zone 6, mean peak standing biomass was 55.3, 98.8, 4259.1 and 4320.1 kg/ha under non-grazed conditions, respectively. Results indicated graminoid removal to be respectively 57.1±99.8% and 24.2±87.2% of mean peak standing aboveground live biomass in the zone 7 and 6. Forb utilization in zone 7 was estimated to be 51.5±99.3%. Mean peak forb standing biomass however, showed a general increase on grazed sites compared to the non-grazed plot in zone 6. The corresponding utilization of the dwarf shrub layer was estimated to be in the range 40.5±80.0% and 76.6±92.3% in zone 7 and 6 respectively. Total herbaceous layer consumption of 39.3±85.3% and 74.0± 90% was estimated for zone 7 and 6 respectively. These ®ndings suggest that herbivores exert considerable control over biomass dynamics of the herbaceous layer in this zone and contribute to degradation in heavily utilised areas. Efforts to understand and tackle the deserti®cation problem in this area must therefore take this fact into account. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Grasses; Forbs; Dwarf shrubs; Offtake; Herbivores; Deserti®cation; Nomads

1. Introduction Although herbivores have been known to have altered Savannas of East Africa (Walter, 1963), much of this knowledge comes from the studies of habitats

*Corresponding author. Tel.: 49 651 2014538; fax: 496512013980; e-mail: [email protected] 0167-8809/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0167-8809(98)00096-6

inhabited by wild large ungulates. In many areas today however, domestic livestock, especially cattle, have effects that override those of indigenous species (Frost et al., 1986). In many of the areas, these effects have not been quanti®ed, more so on the arid communal rangelands inhabited by nomadic pastoralists and their herds of livestock. This has brought into the limelight the question of whether livestock exert any control on biomass dynamics in the arid rangelands and the role,

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if any, they play in the deserti®cation process (Ellis and Swift, 1988). Utilization or offtake can be de®ned as the amount of forage consumed or removed by herbivores. It is expressed as a percentage of the biomass available to the animals and the concept can be applied to a single plant, group of plants, or to range forage as a whole (Cook and Stubbendieck, 1986). The level of forage consumption and the amounts remaining are often linked to stocking rate and carrying capacity of the area under question so that management decisions can be made. `Proper use' concept on the other hand, emphasizes the fact that judicial use of any forage resource should guard against overgrazing. When plants are grazed beyond their recovery potential, new recruitment is curtailed. A 50% offtake is usually associated with the proper range use to guard against overgrazing (Stoddart et al., 1975 and Lusigi, 1984). Overgrazing is more serious in the fragile arid ecosystems where adverse climatic conditions can aggravate the problem. The eventual long term result is the reduced productivity, both signs of land degradation and deserti®cation. The effort of ecologists, land use planners and managers alike, must therefore, focus on determining community primary production viz. the levels of offtake by herbivores and draw the necessary management implications. To the author's knowledge, no previous work on this important subject has been reported in the literature for the study area, except for the work of Lamprey and Yussuf (1981), who estimated that 7% of the primary production in the 200 mm rainfall zone of the study area was eaten by goats and camels. They however concluded that in areas of higher stocking rate, intake exceeded production by 21 percent. This ®gure could de®nitely be higher when all the livestock species are considered. The paucity of data on this subject therefore, calls for further studies to elucidate the impact of herbivory in the study area. This study was therefore undertaken to quantify the levels of forage production and utilization over a wide range of use gradients in an arid Savanna region of northern Kenya inhabited by nomadic pastoralists. The hypothesis was that biomass production and consumption would be in¯uenced by the intensity of use and ecological zone. Because the destruction of the herb layer due to overgrazing is

usually the ®rst step in the deserti®cation of arid lands (Lampey, 1981), the herbaceous layer was chosen for study. 2. Location of study area and sites The study area is located in the South-western district of Northern Kenya (Fig. 1). The area is bounded by four mountain ranges: Mt. Marsabit to the East, Mt. Kulal to the West, Mt Nyiru on the Southwest fringe and the Ndoto ranges on the Southern fringe. The topography of the area is characterised by extensive, very gently undulating plains lying between the volcanic mountains to the East and West and the Southern mountain ranges derived from basement system rocks. Soil type distribution is linked to physiography and the landform (Van Kekem, 1986). The main soils in the study sites are derived from sediments of the basement system rocks (cambic arenosols/luvic yermosols). Rainfall pattern is bimodally distributed between two rainy seasons. Local annual rainfall distribution patterns are a function of physiography. Wettest areas are the volcanic mountains and hills to the east and west of the study area. Here the mean annual rainfall is about 800 mm. Residual basement mountains and hills to the south and south-west receive about 600 mm annually; lowlands well under 300 mm per year. A detailed vegetation description of the study area has been given. It consists of Acacia woodlands and shrublands, dwarf shrublands and annual grassland. The main source of livelihood for the majority of the inhabitants is livestock rearing. Main species of animals kept are camels, sheep, goats and cattle. Numerical estimates from aerial surveys showed that there were 287 040 sheep and goats, 56 810 cattle, 41 400 camels and 1150 donkeys in the study area (Lusigi et al., 1984). Presently, there are no accurate and reliable estimates of livestock stocking rates for this area. Field (1980) estimated a maximum annual stocking rate of upto 43.7 and 41.6 Tropical Livestock Units per square kilometer (TLU/km2) in zone 7 (at Kargi) and 6 (at Ngurunit), respectively (1 TLUˆ 250 kg liveweight). On the other hand, Lamprey and Yussuf (1981) reported an estimate of 4.6 TLU/km2 for the entire area. Other estimates (Lusigi, 1984) reported an annual stocking rate of 4.4 TLU/km2

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Fig. 1. Map showing location of the study area and sites.

for the whole area. The exploitation of the land resource is opportunistic, dependent on rainfall/water and pasture availability. Land is communally used and therefore, common traditional rights of access prevail. Grazing strategies are opportunistic, largely determined by water availability and rainfall. Field (1980) showed that the areas of excessive stocking rates varied from 16.3 to 26.9% of the study area; 32± 37% of these occurring in areas of permanent settle-

ment and 54±63% in areas of permanent water with the remaining representing the fora (mobile camp) animals. Wild ungulates contribute to herbivore pressure in this area. On the basis of seasonality and intensity of use, the range can be divided into two broad land use categories; permanent and seasonal use rangelands (Keya, 1997). The former are grazed on a yearlong (continuous) basis, while the latter are as a rule, used on a

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deferred seasonal basis. Wet season ranges have no permanent sources of water and are therefore used principally during the rainy season. Dry season reserves on the other hand, have permanent water but are located further away from the settlements. These lie in the mountain and transitional zones where perennial vegetation guarantees forage during the dry season. 3. Materials and methods Methods of estimating the amount of forage utilized by livestock were reviewed by Cook and Stubbendieck (1986). The procedures range from being subjective (i.e ocular estimations) to more direct quantitative measurements namely; (a) cage or plot comparisons (b) comparison of weight before and after grazing (c) stubble height class (d) height±weight ratios and (e) stem count techniques. The method adopted by a given research worker depends on the purpose of study, manpower available and the kinds of vegetation. Some methods are rapid, more detailed and accurate than others. The approach on open access rangelands in East Africa has mainly involved comparison of production from permanent exclosures and/or movable cages (non-grazed conditions) to biomass estimates under real grazed conditions on the open range (Owaga, 1980; McNaughton, 1985; Deshmuck, 1986). A drawback to these methods is that growth under real and simulated conditions may distort the calculated utilization (Cook and Stubbendieck, 1986).

In this study, sampling was done in exclosure sites (E1 and E2 in zone 7 and 6 respectively) and test sites on communal range (Fig. 1 and Table 1). Eco-zone 7 is characterised by a mean annual rainfall of less than 300 mm, with the ratio of annual rainfall to annual potential evaporation (r/Eo) of less than 15%. (Van Kekem, 1986). Eco-zone 6 receives a mean of about 600 mm annual rainfall, with r/Eo of 15±25%. Mean annual temperature is >248C and 21±248C in zone 7 and 6, respectively. The study was conducted over a three-year period (1988±1990). The indicator selected for study was aboveground live standing biomass of grasses, forbs, dwarf shrubs and the total sum biomass of these vegetation. Standing aboveground crop biomass provides an easy, quick and reliable method of vegetation analysis which is acceptable by range ecologists (Lusigi et al., 1986). All live standing aboveground biomass in 12- and 20- 1 m2 plots for protected and grazed sites respectively were clipped every two months. Plots estimated to have a biomass of less than 1 g/m2 were not clipped. Repeated clippings on the grazed sites were done at 15 m intervals along 300 m long permanent transects. Clipped samples were separated into vegetation categories (grasses, forbs and dwarf shrubs), bagged and ovendried to constant weight for dry matter determination. Rainfall data were recorded using standard procedures. The sites were selected to (a) re¯ect the spatial heterogeneity of vegetation formations in the study area (b) be representative of the major eco-zones in the area, (c) be easily accessible to the study team and (d) re¯ect the grazing gradients and different levels of

Table 1 Vegetation type and use description of the study sites Eco-zone

Site

Dominant understory vegetation

Dominant season of use

7

Exclo. E1 14/30a 5a/32 14b 14/15a Exclo. E2 75b 17bi 74b 17b

Indigofera spinosa / cliffordiana dwarf shrubs Indigofera spinosa / cliffordiana dwarf shrubs Indigofera spinosa dwarf shrubs Indigofera spinosa / Heliotropium dwarf shrubs Mixed Indigofera spinosa dwarf shrubs Duosperma eremophilum dwarf shrubs Duosperma eremophilum dwarf shrubs Duosperma eremophilum dwarf shrubs Duosperma eremophilum dwarf shrubs Duosperma eremophilum dwarf shrubs

Protected from herbivory Yearlong Yearlong Wet season Wet season Protected from herbivory Wet season Yearlong Yearlong Wet season

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vegetation disturbances found in the study area. Sites on open-access rangeland were utilised principally by cattle, sheep, camels and goats, under control of the nomadic herdsmen (Keya, 1997). Because land is communally owned, open-access traditional grazing rights prevailed on these sites. The approach involved the comparison of aboveground standing live biomass along sites with differentiated grazing histories and intensities (grazing gradients) in zone 7 and 6 respectively (zonation as in Hornetz et al., 1992). Although grazing gradients may be hidden by other types of spatial variability resulting from local differences in site history, plant species etc., the grazing gradient method is capable of ®ltering out the effects of landscape variability and identifying degradation and short term grazing effects (Pickup, 1991). Hence differences in biomass between sites were attributed to the livestock consumption. The following grazing gradients were subjectively differentiated and de®ned based on the history of past and current use; (a) ungrazed (no impact))NG; (b) seasonally grazed (light to moderate impact))SG and (d) yearlong grazed (heavy to very heavy impact))HG. For the comparisons to be valid, only sites within the same eco-zone and to a large extent, having similar edaphic and rainfall regimes were compared. Likewise, only those sites with similar vegetation type and physiognomy were selected for comparison. Thus, in zone 7 and 6 respectively, the non-grazed (exclosure) Indigofera spinosa (NG1) and Duosperma eremophilum (NG2) dominated sites were selected. Two sites for each of the de®ned use categories on open rangeland (Fig. 1 and Table 1) were selected thus: (a) seasonally grazed sites 14/15a and 14b; heavily/yearlong grazed sites 14/30a and 5a/32 representing zone 7 and (b) seasonally grazed sites 75b and 17b; heavily/yearlong grazed sites 17bi and site 74b representing zone 6. The analysis covered the three-year study period (1988±1990). The data therefore represents a mean of three years. Utilization was estimated as the percentage difference in aboveground standing biomass on open-use range sites and the exclosure sites protected from grazing (Cook and Stubbendieck, 1986; Owaga, 1980; McNaughton, 1985; Deshmuck, 1986) in each eco-zone. Rainfall data were obtained from nearby recording stations.

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4. Results 4.1. Rainfall Fig. 2(a) and (b)) shows monthly rainfall distribution pattern for the 1988±1990 study period at the Kargi and Ngurunit recording stations, respectively. The bimodal pattern is clearly visible, with peak monthly rainfall higher in 1989 and 1990 at Kargi and in 1988 and 1989 at Ngurunit. 4.2. Grass layer Fig. 3(a) presents the grass layer production along different grazing gradients in eco-zone 7. The non-grazed site had the highest peak biomass compared to all the other sites. Mean peak standing biomass was 184.4, 68.2±76.9 and 1.2±82.4 kg/ha on non-grazed, seasonally grazed and heavily grazing sites respectively. Thus, 57.1±99.8% difference in mean peak biomass production between non-grazed (NG1) and grazed sites was attributed to removal by livestock. Grass production on grazed sites was mainly contributed by annual grass species. In zone 6, graminoid mean peak standing biomass consi stently dropped from 55.3 kg/ha on the non-grazed site (NG2) to 23.6±24.3 kg/ha and 7.7±41.9 kg/ha respectively on seasonally (SG3 and SG4) and heavily grazed (HG1 and HG2) respectively (Fig. 3(b)). This drop represented an estimated consumption of the mean peak standing biomass in the range of 56.1±57.3% and 24.2±87.2%, respectively. There was virtually no standing biomass on grazed sites in the dry season and well into the second rainy season. 4.3. Forb layer Mean peak standing biomass in zone 7 declined from 374.2 kg/ha on non-grazed site (NG1) site to 5.4±37.4 kg/ha on seasonally and heavily grazed sites (Fig. 4(a)). This represented a 51.5±74.9% and 90± 99.3% difference respectively. In zone 6 however, mean peak forb standing biomass rose from 98.8 kg/ha on the non-grazed plot to 48.4±150 kg/ha and 113.7±150.0 kg/ha on seasonally and heavily grazed sites, respectively (Fig. 4(b). Notable invading species were Heliotropium steudneri, Amaranthus sp.,

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Fig. 2. (a) Monthly rainfall distribution at Kargi (eco-zone 7) during the study period; (b) monthly rainfall distribution at Ngurunit (eco-zone 6) during the study period.

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Fig. 3. (a) Mean graminoid standing biomass along grazing gradients in eco-zone 7. NG1: non-grazed exclosure site; SG1: seasonally grazed site 14/15a; SG2: seasonally grazed site 14b; HG1: yearlong grazed site 14/30a; HG2: yearlong grazed site 5a/32a; (b) Mean graminoid standing biomass along grazing gradients in eco-zone 6. NG2: non-grazed exclosure site; SG3: seasonally grazed site 75b; SG4: seasonally grazed site 17b; HG3: yearlong grazed site 17bi; HG4: yearlong grazed site 74b

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Fig. 4. (a) Mean forb standing biomass along grazing gradients in eco-zone 7. NG1: non-grazed exclosure site; SG1: seasonally grazed site 14/15a; SG2: seasonally grazed site 14b; HG1: yearlong grazed site 14/30a; HG2: yearlong grazed site 5a/32a; (b) Mean forb standing biomass along grazing gradients in eco-zone 6. NG2: non-grazed exclosure site; SG3: seasonally grazed site 75b; SG4: seasonally grazed site 17b; HG3: yearlong grazed site 17bi; HG4: yearlong grazed site 74b.

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Fig. 5. (a) Mean dwarf shrub standing biomass along grazing gradients in eco-zone 7. NG1: non-grazed exclosure site; SG1: seasonally grazed site 14/15a; SG2: seasonally grazed site 14b; HG1: yearlong grazed site 14/30a; HG2: yearlong grazed site 5a/32a; (b) Mean dwarf shrub standing biomass along grazing gradients in eco-zone 6. NG2: non-grazed exclosure site; SG3: seasonally grazed site 75b; SG4: seasonally grazed site 17b; HG3: yearlong grazed site 17bi; HG4: yearlong grazed site 74b.

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Fig. 6. (a) Mean total herbaceous standing biomass along grazing gradients in eco-zone 7. NG1: non-grazed exclosure site; SG1: seasonally grazed site 14/15a; SG2: seasonally grazed site 14b; HG1: yearlong grazed site 14/30a; HG2: yearlong grazed site 5a/32a; (b) Mean total herbaceous standing biomass along grazing gradients in eco-zone 6. NG2: non-grazed exclosure site; SG3: seasonally grazed site 75b; SG4: seasonally grazed site 17b; HG3: yearlong grazed site 17bi; HG4: yearlong grazed site 74b.

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Tribulus terrestris, Solanum dubium and Indigofera sp., all of which are known to be undesirable to livestock. A preponderance of these species in this zone should therefore be considered as a bio-indicator of overgrazing. 4.4. Dwarf shrub layer There was a decline of dwarf shrub productivity with increasing grazing intensity in zone 7 (Fig. 5(a)). Mean peak standing biomass was respectively 1099.4, 361.1±653.9 and 220.6±379.5 kg/ha on non-grazed, seasonally grazed and heavily grazed sites respectively, over the period under consideration. Hence, the difference in mean peak standing biomass amounting to 40.5±67.2% and 65.5±80.0% respectively between the reference non-grazed site and seasonally grazed site on the one hand and the reference site and yearlong/heavily grazed sites on the other, was attributed to removal by herbivores. Dwarf shrub mean peak biomass in zone 6 (Fig. 5(b)) declined from 4259.1 kg/ha on the non-grazed exclosure site, to 547.4±922.1 kg/ha and 328±997 kg/ha on the seasonally and yearlong grazed sites respectively. This corresponds to an estimated removal of 78.3±87.1% and 76.6±92.3% respectively. Despite the overall decline in dwarf shrub biomass, the species Indigofera spinosa and Indigofera cliffordiana tended to increase with the increasing level of herbivory impact (Keya, unpublished). 4.5. Total herbaceous layer Mean total lower layer biomass was similarly affected by the increasing level of utilization (Fig. 6(a). Peak total standing biomass declined from 1504 kg/ha on the exclosure site to 474.6±912.3 and 220.6±699.3 kg/ha on moderately and heavily grazed sites respectively. This amounted to an estimated utilization of 39.3±68.4% and 53.5±85.3% respectively over the period of study. Total lower layer mean peak aboveground standing biomass in zone 6 (Fig. 6(b)) dropped from 4320.1 kg/ha on NG2 to 611.5±1105.6 kg/ha and 432.7±1053.6 kg/ha on seasonally and yearlong grazed sites respectively. This represents a utilization of 74.0±86.0% and 76.0±90% respectively.

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5. Discussion Differences in aboveground standing biomass corresponded to land use and grazing gradients. Seasonally used sites recorded lower offtake rates compared to those used on permanent basis. Offtake levels displayed in this study fall in the range 30±90% for the East African grasslands (McNaughton, 1985). However, this level of utilization remains far greater than the 7.5% (Owaga, 1980) and 4% (Deshmuck, 1986), found for the moist Savanna grasslands in Southern Kenya. The high offtake rates in the present study indicate that herbivores exert considerable control over plant biomass in this ecosystem. In the neighbouring area of South Turkana, 10±12% of the total annual net primary productivity during a good year was estimated to be consumed by livestock (Ellis and Swift, 1988). These authors attributed the low consumption to the low stocking rates. In the present study, higher consumption rates are perhaps an indication of high stocking rates. Allowing for 50% proper use factor, it appears that most of the seasonally used ranges are optimally utilised, particularly with regard to browse (dwarf shrubs). However, those ranges under permanent use appear to be over-used. A virtual absence of graminoid production on grazed sites during the dry season in both eco-zones is a re¯ection of the replacement of perennial grasses by annuals whose production is restricted to wet seasons. It con®rms that grazers rather than browsers are more likely to suffer forage shortages in the dry seasons. The situation in eco-zone 6 was slightly different from that in zone 7 with respect to forb biomass. Although forb biomass decreased consistently with increasing intensity of use in zone 7, the converse was true in the zone 6. The explanation for this anomaly lies in the fact that, although the majority of the forbs encountered in eco-zone 7 are desirable to livestock and therefore expected to decline with increasing grazing intensity, those that increased in eco-zone 6 are known to be undesirable to livestock and therefore expected to increase with grazing pressure. Secondly, plant reactions to grazing events depend upon the ability of individuals to compensate for lost organs and the relative removal on competitive relationships in the canopy (Milchunas and Lauenroth, 1993). Hence, the forb increase in zone 6 could also be due to competitive relationships that act in their

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favour. These authors contended that, because canopy development and aboveground-to-belowground ratios increase with increasing moisture/productivity, a given percentage removal from the canopy will have a greater effect in subhumid than in arid environments both in the intensity of plant interactions in the canopy and percentage removal of total biomass. Results of this study agrees with this assertion, in that greater changes in species composition and estimated biomass removal due to herbivory occurred on sites in the messic and more productive eco-zone 6 compared to the more arid zone 7. 6. Conclusions There is a current debate as to whether herbivory impact is large enough to in¯uence vegetation dynamics in the arid rangelands, particularly in the African rangelands where episodic and unpredictable rainfall events are associated with long term shifts in vegetation and productivity dynamics. Recent schools of thought (Ellis and Swift, 1988; Westoby et al., 1989; Benhke and Scoones, 1992) asserted that long term dynamics are in¯uenced more by abiotic factors such as rainfall, than biotic factors (i.e herbivory). However there is still a broad disagreement over this issue (Hutchinson, 1996). This is no longer an academic debate, as donors and governments are currently engaged in the implementation of programmes that seek to alleviate environmental problems in these areas. The resolution of this debate therefore depends on sound scienti®c facts to determine policies towards the arid rangelands. This study showed that in the arid lands of northern Kenya, livestock herbivory does, to a great extent, in¯uence biomass dynamics of the herbaceous layer. The understanding and tackling of the desert encroachment problem in this region should take this fact into account. Acknowledgements Financial support for this study was provided by the Kenya Agricultural Research Institute (KARI). The paper could probably not have been written without the ®nancial and material support from the German Academic Exchange Service (DAAD). The staff of the

National Arid Lands Research Centre (NALRC), Marsabit, greatly helped during the data collection. In particular, the assistance of I.K Wamugi, A. Ndathi, Diba Guyo, H. Walaga, Adano Wario and Dibayo Iljala is greatly appreciated. This work was originally written within the framework of doctoral studies at the Department of Geography and Geosciences, University of Trier, Germany. References Benhke, R.H., Scoones, I., 1992. Rethinking range ecology: Implications for rangeland management in Africa. Issue Paper No. 33. ODI, pp. 1±43. Cook, C.W., Stubbendieck, J., 1986. Range research: Basic Problems and Techniques. Society for Range Management, Denver, CO, p. 317. Deshmuck, I.K., 1986. Primary production of a grassland in Nairobi National Park. Kenya. J. Appl. Ecol. 23, 115±123. Ellis, J.E., Swift, D.M., 1988. Stability of African pastoral ecosystems: Alternate paradigms and implications for development. J. Range Manage. 41, 450±459. Frost, P., Medina, E., Menaut, J.C., Solbrig, O., Swift, M., Walker, B. (Eds.), 1986. Responses of Savannas to Stress and Disturbance: A proposal for Collaborative Programme of Research. Report of a Workshop organized in collaboration with the Commission of European Communities (CEC). Special issue - 10, IUBS, pp. 1±82. Field, C.R., 1980. A summary of livestock studies within the Mt. Kulal study area. in: Proceedings of a scientific seminar. IPAL. Tech. Report A-3. UNESCO, pp. 89±122. Hornetz, B., JaÈtzold, R., Litschko, T., Opp, D., 1992. Beziehungen zwischen Klima, WeideverhaÈltnissen und AnbaumoÈglichkeiten in Marginalen Semiariden und Ariden Tropen mit Beispielen aus Nord- und Ost-Kenya. Materialien zur Ostafrika-Forschung, Heft 9. GGT, p. 257. Hutchinson, C.F., 1996. The Sahelian desertification debate: A view from the American south-west. J. Arid Environ. 33, 519± 524. Keya, G.A., 1997. Effects of herbivory on the production ecology of the perennial grass Leptothrium senegalense (Kunth.) in the arid lands of northern Kenya. Agric. Ecosy. and Environ. 66, 101±111. Lampey, H.F., 1981. The problem of livestock night enclosures in North-Eastern Africa. Arid Lands Newsletter, University of Arizona, Tuscon, AR, pp. 23±26. Lamprey, H.F., Yussuf, H., 1981. Pastoralism and desert encroachment in northern Kenya. Ambio. 10, 131±134. Lusigi, W.J. (Ed.), 1984 . Integrated resource assessment and management plan for western Marsabit, District, Kenya: Part 1. Integrated Resource assessment. IPAL Tech. Report A- 6. UNESCO, p. 481. Lusigi, W.J., Nkurunziza, E.R., Awere-Gyekye K., Masheti, S., 1986. Range resource assessment and management strategies

G.A. Keya / Agriculture, Ecosystems and Environment 69 (1998) 55±67 for southwestern Marsabit, Kenya. IPAL. Tech. Report D-5. UNESCO, p. 230. Lusigi, W.J., Nkurunziza, E.R., Masheti, S., 1984. Forage preference of livestock in the arid lands of northern Kenya. J. Range. Manage. 37, 542±548. McNaughton, S.J., 1985. Ecology of a grazing ecosystem: The Serengeti. Ecol. Monogr. 55, 259±294. Milchunas, D.G., Lauenroth, W.K., 1993. Quantitative effects of grazing on vegetation and soils over a wide range of environments. Ecol. Monogr. 63, 327±366. Owaga, M.L.A., 1980. Primary productivity and herbage utilization by herbivores in Kaputei Plains. Kenya. Afr. J. Ecol. 18, 1±5. Stoddart, L.A., Smith, A.D., Box, T.W., 1975. Range Management. McGraw-hill, New York, p. 300. Van Kekem, A.J., 1986. Soils of the Mt. Kulal Marsabit Area. MOLD. p. 268.

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Walter, H., 1963. Productivity of vegetation in arid countries, the Savannah problem and the bush encroachment after overgrazing. in: The Impact of Man on the Tropical Environment, The Ninth Technical Meeting, Nairobi, Kenya, 17±20 September 1963. International Union for Conservation of Nature and Natural Resources, Morges (Vaud), Switzerland, pp. 1±10. Westoby, M., Walker, B., Noy-meir, I., 1989. Opportunistic management for rangelands not at equilibrium. J. Range Manage. 42, 266±274. Pickup, G., 1991 Spatial models for identifying land degradation. in: A., Gaston, M., Kernick, H.N., Houerou (Eds.), Proceedings of the 4th International Rangeland Congress. Montpellier, France, pp. 50±53.