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Grazing induced biodiversity in the highland ecozone of East Africa
Agriculture, Ecosystems and Environment 79 (2000) 43–52
Grazing induced biodiversity in the highland ecozone of East Africa Zerihun Woldu a , M.A. Mohammed Saleem b,∗ a
Department of Biology, the National Herbarium, Addis Ababa University, P.O.Box 3434, Addis Ababa, Ethiopia b International Livestock Research Institute (ILRI), P.O.Box 5689, Addis Ababa, Ethiopia Received 15 April 1999; received in revised form 11 August 1999; accepted 22 October 1999
Abstract The species composition of grazing lands can be influenced by livestock and grazing pressure. A study on manure seed bank was conducted in Ghinchi highland Research Site in Ethiopia between 1995 and 1997. The data on species composition and life-form of the plants germinating in pots receiving air dried manure were compared with species composition of experimental plots in natural grassland subjected to varying grazing intensity. There was significant difference among the species composition of grazed and non-grazed grasslands and the manure seed bank (p = 0.01). The life-forms of the species also showed variation. There were more families and species in the natural grassland vegetation than indicated in the manure seed bank. The manure seed bank had more annuals than the natural grassland vegetation. The species composition and life-forms in the manure seed bank showed variation with time and this corresponded with the seasonal variation in the grassland, which had a direct relationship with the rainfall pattern. The study showed that livestock play a major role in maintaining the biodiversity of grassland vegetation by spatial and temporal dispersion of readily germinating seeds in their manure. The use of manure to improve soil fertility should be weighed cautiously against the introduction of weeds into crop fields, although weeds are important feed resource for livestock in land-constrained areas. There is therefore the need for developing manure management practices so that the benefits can be optimised and the undesirable effects can be minimised. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Manure; Species; Life-form; Livestock; Seed bank; Biodiversity
1. Introduction A long history of land clearing and sedentary agriculture has changed the vegetation cover in the east African highlands. The natural vegetation of the central plateau of Ethiopia, for example, may have been dry evergreen montane forest with Juniperus procera and Olea europaea sp. cuspidata as the dominant species (Pichi Sermoli, 1957). This vegetation type ∗ Corresponding author. Tel.: +251-1-613215; fax: +251-1-611892. E-mail address: [email protected] (M.A. Mohammed Saleem)
has disappeared from most parts of the highland except in few remnant patches around holy places and inaccessible areas. Records of early travelers in the fifteenth and sixteenth century (Almeida, 1954; Alvarez, 1970) indicate that the agro-climate conditions of the Ethiopian highland about 500 years ago were similar to the present day conditions except that the density of trees then was higher in the crop fields. The central plateau of Ethiopia is characterised by mixed cereal and livestock agriculture. Nutrient deficient soils, high stocking rates and shortage of animal feeds are common features. Livestock in the highlands account for 80% of the total population and
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about 20% of the agricultural gross domestic product of the country (Mengistu, 1997). The livestock are herded together and grazed on communal pastures, private land and stables depending on the time and season of the year. Cattle are the most important livestock species in the Ethiopian agriculture and they are kept mainly for traction and milk production. Grazing is being expanded to very steep slopes and marginal lands, as more land is cultivated to compensate for the diminishing soil fertility and to meet the ever-increasing food demands. The species composition and productivity of the pasture of the common grazing lands are highly influenced by the species of the livestock, the intensity of grazing, climatic and edaphic factors. The seasonal stocking rate on the central plateau of Ethiopia is very high for the ecological carrying capacity of the grassland (Woldu, 1986). The grassland communities are predominantly of Pennisetum sphacelatum– Commelina africana type and could develop to Andropogon abyssinicus–Hyparrhenia arrhenobasis type if grazing intensity is relaxed (Woldu, 1986). Grasslands in general are stressful environments. Defoliation, uprooting, trampling and desiccation are the important stresses in grasslands under high grazing pressure. The role of livestock on the grassland vegetation can be seen from two main perspectives. The most obvious one is the influence on the species composition and reduction in the above and below ground biomass. The less obvious but equally important is the maintenance and distribution of the biodiversity. Free grazing livestock move to any accessible site and their feeding behaviour or foraging choice may generate a pattern of association between the plant species and the livestock. This relationship helps in seed dispersal which may be manifested in spatial and temporal differences in ground cover and patchiness. Seed morphology may influence whether seeds survive the mastication and the digestive enzymes of the ingesting animal. Those that survive through the animal gut germinate faster as dormancy is broken by the abrasion, scarification and wetting of the digestive system of the animal. On the other hand, seeds retained in the animal gut for a long period may be induced to germinate and be killed while in the gut (Janzen, 1982a, b). Deposition of seeds with manure gives a better chance of survival since a source of nitrogen is readily available for early seedling growth. It
appears, therefore, that the animal gut filters out those species by enhancing their germination while in the gut and by providing favourable conditions when they are released. It is with this understanding that an experiment was designed to study the species composition of manure seed bank at different times of the year and compare these with the species composition of natural grasslands under different grazing pressures. The specific objectives of this study were (1) to determine the species composition and richness of germinating seeds in manure seed bank, (2) to compare the species composition of manure seed bank and those of the natural grassland under different grazing pressure, (3) to assess the role of livestock in the maintenance of the biodiversity of the grassland vegetation through the spatial transfer of seeds in their manure, and (4) to assess the use of manure in improving soil fertility.
2. Materials and methods 2.1. Experimental site Two complementing experiments were conducted at Ghinchi Research Station, a local research base, where the International Livestock Research Institute (ILRI) is working with a consortium of institutions on natural resource management issues. Ghinchi is located in the Ethiopian highlands at 2290 m asl 80 km west of Addis Ababa (9◦ 020 N; 38◦ 070 E). The experiments include manure seed bank pot trials and grazing experiments in six different sites. 2.2. Manure seed bank pot trials For the manure trials, the cattle herd of two farmers were monitored as they grazed different parts of the landscape at different times of the year. Manure was collected each day from the grazing areas, and a composite sample was taken from the weekly collections and air dried. Five replicates were made from each composite sample, and 123.3g of the sample was spread over a pot (18.0 cm — top diameter, 11.5 cm — bottom diameter and 15.5 cm — height) half filled with 1.90 kg of sterilised sand. The plant species growing in the pots were recorded until all seedlings had emerged. The seedlings were identified to the species level as much as possible. The manure experiment in the pots
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was run for two complete years, August 1996–July 1997 and August 1997–July 1998. The plant species and other data for each replicate of the week were entered in access database. Relevant information was extracted using the structured query language. 2.3. Grazing experiments To study the influence of grazing on the grassland vegetation, six sites with varying slopes ranging from 0–8% were selected to represent the area where the livestock of the community grazed. Each site had 10 m × 10 m plots where (a) grazing was totally precluded (NOG), (b) moderately grazed (MDG — 1.8 Tropical Livestock Unit Month — TLUM/ha) and (c) very heavily grazed (VHVG — 4.2 TLUM/ha) (see also Mwendera et al., 1997). The NOG and MDG plots were enclosed by barbed wire fences and there was no fencing around VHVG plots. The door of the fence around MDG was opened for 3 days in a week for the free grazing livestock of the farming community. Grazing took place during all days of the week in VHVG. Data on species composition, per cent cover of three most dominant species and bare ground were collected at 30 days interval from the second week of September to the second week of December. This period is shortly after the summer rain season when plant growth rate is maximal and most plants bear inflorescence. During the long dry period (January–May) and the summer rain period (June–September) most species in VHVG and some in MDG and NOG are not identifiable. Data were collected for three consecutive years from 1995–1997. Only the list of species in NOG and VHVG were considered for the present article. The result which includes MDG will be communicated in another publication. 2.4. Data analysis The weekly manure-seedling data from the five replicates were pooled for each month and a data matrix was constructed for each year with the species in the rows and months on the columns. The species composition of the manure (M) was compared with data on the grazing experiment plots using chi square (χ 2 ). The relative occurrences of the four important grassland families were calculated for each year of
Fig. 1. Monthly average rainfall in Ghinchi Research Station, Ethiopia in 1996 and 1997.
the grazing experiment and manure seed bank data. The relative composition of the annual and perennial species in each experiment was also calculated. Rainfall data of 1996 and 1997 was obtained from Ethiopian Agricultural Research Meteorological Services and the monthly average rainfall of the 2 years were plotted (Fig. 1). Principal Components Analysis (PCA) was performed using Syntax to find the influence of time on the species composition of the manure seed bank. The PCA is an ordination technique which involves extracting the same number of axes of variations as in the rows or columns of the input data matrix in decreasing order of importance. Scatter diagram (scattergram) of the two axes showing the highest variations provides a two dimensional display of the underlying pattern in the original data matrix (see Orloci, 1978).
3. Results and discussion 3.1. Species in manure seed bank and grazing experiment Table 1 gives the occurrence of the species in the manure seed bank during the 12 months of the year in
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Table 1 List of species in manure seed bank (M), no grazing plots (NOG) and very heavily grazed (VHVG)a Species
Manure
NOG
VHVG
Abutilon sp. Acacia abyssinica Ajuga remota Alchemilla fischeri Amaranthus hybridus Anagalis arvensis Andropogon abyssinicus Arthraxon lancifolius Bidens biternata Bidens pilosa Bothriochloa insculpta Brachiaria semiundulata Brassica nepus Carduus chamaecephalus Carum carvi Centaurium tenuiflorum Cerastium octandrum Chenopodium album Chenopodium ambrosoides Chloris pycnothrix Cineraria abyssinica Cirsium vulgare Coleus punctatus Commelina africana Commelina bengalensis Commelina subulata Conyza tigreensis Corchorus trilocularis Corrigiola litoralis Corrigiola sp. Crassocephalum rubens Crepis carbonaria Crotalaria spinosa Cynodon dactylon Cynoglossum coeruleum Cyperus rigidifolius Cyperus teneriffae Datura stramonium Dichrocephala integrifolia Digitaria rivae Dischoriste radicans Echinochloa colona Eleusine floccifolia Eragrostis botryodes Eragrostis tef Eragrostis tenuifolia Eriochloa meyeriana Euphorbia petitiana Evolvulus alisinoides Falkia oblonga Fimbristylis complanata Galinsoga parviflora Galium spurium
1 1 0 0 1 0 0 1 0 0 0 0 1 0 1 0 0 1 1 0 0 0 0 1 1 0 1 0 1 1 1 0 0 1 1 1 1 0 1 1 0 0 1 0 1 1 0 1 1 1 0 1 1
0 0 0 1 1 1 1 1 1 1 0 0 0 1 1 0 0 0 0 0 1 1 1 1 0 0 0 1 0 0 0 0 0 1 0 1 0 0 1 1 1 1 1 1 0 1 1 0 1 1 1 1 0
0 0 1 1 1 0 0 0 1 1 1 1 1 1 0 1 1 0 0 1 1 0 1 1 0 1 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 0 0 1 1 1 1 0
Table 1 (Continued). Species
Manure
NOG
VHVG
Geranium arabicum Gnaphalium luteo-album Guizotia abyssinica Guizotia scabra Haplociadum abyssinicum Hibiscus sp. Hibiscus tridentata Hibiscus trionum Hygrophila auriculata Hyparrhenia arrhenobasis Hyparrhenia filipendula Hyparrhenia hirta Hyparrhenia rufa Hyparrhenia sp. Indigofera sp. Kohautia coccinia Kyllinga appendiculata Lactuca inermis Laggera pterodonta Lathyrus sativus Lens culinaris Leucas marticinensis Lolium perenne Lotus corniculatus Medicago polymorpha Oxygonum sinuatum Panicum sp. Pennisetum clandestinum Pennisetum divisum Pennisetum riparium Pennisetum villosum Persicaria nepalensis Phalaris paradoxa Phyllanthus rotundifolius Plantago lanceolata Poa annua Poa leptoclada Polygala sp. Rumex nepalensis Scirpus inclinatus Scleria clathrata Scleria hispidior Scorpiurus muricatus Sesbania sesban Setaria verticillata Sida rhombifolia Snowdenia polystachya Solanum nigrum Sonchus sp. Sorghum bicolor Sphaeranthus suaveolens Spilanthus mauritiana Sporobolus africanus Tagetes minuta Thesium radicans
1 1 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 1 0 0 1 1 0 1 1 1 0 1 1 1 1 1 1 1 0 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 0
0 1 0 1 0 1 0 1 1 1 1 1 1 0 1 1 1 1 1 0 0 1 1 1 1 1 1 1 1 1 0 1 1 1 1 0 0 1 1 1 1 1 1 0 1 0 1 1 0 0 1 1 1 0 0
0 1 0 1 0 1 1 1 1 1 0 1 0 1 1 1 0 0 0 0 0 1 0 1 1 0 1 1 1 1 1 1 1 1 1 0 0 1 1 1 1 0 1 0 1 0 1 1 0 0 1 1 1 0 1
Z. Woldu, M.A. Mohammed Saleem / Agriculture, Ecosystems and Environment 79 (2000) 43–52 Table 1 (Continued). Species
Manure
NOG
VHVG
Trifolium abyssinicum Trifolium acaule Trifolium burchellianum Trifolium cryptopodium Trifolium johnstonii Trifolium lanceolatum Trifolium multinerve Trifolium rueppellianum Trifolium schimperi Trifolium semipilosum Trifolium simense Trifolium steudneri Trifolium tembense Triticum aestivum Uebelinia abyssinica Verbascum sinaticum Verbena officinalis Veronica abyssinica Veronica anagallis-aquatica Vigna oblongifolia Xanthium spinosum Zea mays
0 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1
1 0 0 0 1 0 0 0 1 1 0 1 1 0 1 0 1 1 0 0 0 0
1 0 0 0 0 0 0 0 1 1 0 1 1 0 0 0 1 1 0 0 1 0
a
0 stands for absence and 1 stands for presence.
1996 and 1997, and the occurrence of species in the NOG and VHVG from 1995 to 1997. The number of species per year in the grazing experiment ranged between 50 and 59 while it ranged between 62 and 64 in the manure experiment (Table 2). The number of families in the grazing areas ranged between 16 and 21 while it ranged between 17 and 22 in the manure experiment when the data of each year
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are considered separately. The number of species increased to 94 and 79; and the number of families increased to 28 and 24 in the grazing areas and manure seed bank, respectively, when the data of the whole period were pooled together. The grazing experiments and the manure seed bank had 37 species (14 families) in common. The manure seed bank species included six annual food crop species. The relative importance of the four major grassland families in Ghinchi grazing area corresponded to the pattern in the grassland communities in the east African highlands in general, and in Ethiopia, in particular (Woldu, 1986). Members of the Poaceae were the highest, followed by those of Asteraceae, Fabaceae and Cyperaceae in that order. In the manure seed bank, however, Poaceae and Fabaceae were equal in importance and were higher than the other two families. The percentage of Asteraceae was higher and the percentage of Cyperaceae was lower than in the grassland communities. The number of species, families and the relative importance are presented in Table 2. The chi-square analysis showed that the composition of species in the M versus NOG, M versus VHVG and NOG versus VHVG are significantly different (χ 2 = 6.13, 5.48 and 25.65, respectively, significant at p = 0.01). The species unique to M, NOG and VHVG and those common to the three treatments are given in Table 3. The results indicate that the species composition of the manure seed bank mostly resembled the species composition of VHVG and less to those of NOG. The relatively higher resemblance between M and VHVG may suggest that the areas where the livestock grazed were identical to VHVG.
Table 2 Proportion of the four important plant families in the grazing area and in the manure during the study period at Ghinchi in the Ethiopian highlands over 3 years Grazing areas
Manure
1995
1996
1997
6
1996
1997
6
Species
50
59
50
94
62
64
86
Families 1 2 3 4
16 Poaceae (32%) Fabaceae (14%) Asteraceae (12%) Cyperaceae (12%)
21 Poaceae (24%) Asteraceae (20%) Fabaceae (11%) Cyperaceae (10%)
17 Poaceae (34%) Asteraceae (20%) Fabaceae (8%) Cyperaceae (6%)
28
18 Fabaceae (26%) Poaceae (23%) Asteraceae (13%) Cyperaceae (5%)
22 Poaceae (23%) Fabaceae (20%) Asteraceae (13%) Cyperaceae (5%)
24
Total
70%
65%
66%
67%
61%
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Table 3 Species unique to manure seed bank (M), no grazing (NOG), very heavily grazed (VHVG) and those common to the three treatments (M–NOG–VHVG) Species occurring only in manure
Species occuring only in NOG
Species occurring only in VHVG
Species occuring in manure, NOG and VHVG
Abutilon sp. Acacia abyssinica Chenopodium album Chenopodium ambrosoides Commelina bengalensis Conyza tigreensis Corrigiola litoralis Corrigiola sp. Crassocephalum rubens Eragrostis tef Euphorbia petitiana Galium spurium Geranium arabicum Guizotia abyssinica Haplociadum abyssinicum Lathyrus sativus Lens culinaris Poa annua Poa leptoclada Sesbania sesban Sida rhombifolia Sonchus sp. Sorghum bicolor Tagetes minuta Trifolium acaule Trifolium burchellianum Trifolium cryptopodium Trifolium lanceolatum Trifolium multinerve Trifolium rueppellianum Trifolium simense Triticum aestivum Verbascum sinaticum Veronica anagallisaquatica Vigna oblongifolia Zea mays
Anagalis arvensis Andropogon abyssinicus Cirsium vulgare Corchorus trilocularis Eriochloa nubica Hyparrhenia filipendula Hyparrhenia rufa Kyllinga appendiculata Lactuca inermis Laggera pterodonta Lolium perenne Oxygonum sinuatum Scleria hispidior Trifolium johnstonii
Ajuga remota Bothriochloa insculpta Brachiaria semiundulata Centaurium tenuiflorum Cerastium octandrum Chloris pycnothrix Commelina subulata Crepis carbonaria Crotalaria spinosa Datura stramonium Hibiscus tridentata Hyparrhenia sp. Thesium radicans Xanthium spinosum
Amaranthus hybridus Commelina africana Cynodon dactylon Cyperus rigidifolius Dichrocephala integrifolia Digitaria rivae Eleusine floccifolia Eragrostis tenuifolia Evolvulus alisinoides Falkia oblonga Galinsoga parviflora Gnaphalium luteo-album Guizotia scabra Hygrophila auriculata Lotus corniculatus Medicago polymorpha Panicum sp. Pennisetum clandestinum Pennisetum divisum Persicaria nepalensis Phalaris paradoxa Phyllanthus rotundifolius Plantago lanceolata Rumex nepalensis Scorpiurus muricatus Setaria verticillata Snowdenia polystachya Solanum nigrum Sphaeranthus suaveolens Spilanthus mauritiana Sporobolus africanus Trifolium schimperi Trifolium semipilosum Trifolium steudneri Trifolium tembense Verbena officinalis Veronica abyssinica
3.2. Proportion of life-forms The pattern of occurrence of annuals and perennials in M clearly follows the pattern of the rainfall (Figs. 1, 2A and B). Although the Ethiopian highland is known for its bimodal rainfall (Gamachu, 1977), the rainfall pattern of Ghinchi in 1996 and 1997 was unimodal. Comparison of the rainfall of the 1996 and 1997 shows that the rainfall in 1996 was evenly distributed while
that of the 1997 was concentrated mainly between July and September and had a long dry period. This seems to have a bearing on the composition of annuals and perennials. The evenly distributed rainfall in 1996 appears to have encouraged the continuous availability and production of seeds from annuals hence their dominance in the manure seed bank (69%) while the relatively more arid climate and high torrential rainfall in the summer of 1997 had discouraged annuals (57%)
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Fig. 3. Proportion of annuals and perennials in the species unique to manure seed bank (M), no grazing (NOG), very heavily grazed (VHVG) and those common to the three treatments (M–NOG–VHVG)
common to M, NOG and VHVG was 40.5% (Fig. 3). The list of species is given in Table 3. 3.3. Temporal patterns of occurrence
Fig. 2. Annuals and perennials in the species composition of the manure seed bank in (A) 1996; (B) 1997.
but encouraged the persistence of perennials through out the year. The composition of annuals in M was 63%, while that of the grassland (NOG, and VHVG pooled together) was 41%. Seventy eight percent of the species unique to M were annuals, while the proportions of annuals unique to NOG and VHVG were 28% only. The proportion of annuals in the species
The seasonal pattern in the samples indicates that the seeds of perennials attain dominance at the peak of the rainfall (July–September) while the seeds of the annuals were dominant during the rest of the year. The PCA of M in 1996 showed three distinct clusters whereas that of 1997 showed four diffused clusters (Fig. 4A and B). The relatively higher annual precipitation and the more even distribution of the rainfall in 1996 may have provided more uniform climatic conditions during most part of the year hence the less number of clusters. It is interesting to note that each month clustered with the month preceding or succeeding indicated that the clusters also follow the seasonal pattern. The life-form of the species in the clusters corresponds to the average precipitation of the months. The number of annuals was higher than the perennials in those clusters where the average rainfall was higher (Fig. 5A and B). The general pattern of the clusters suggests that the species in the
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Fig. 4. Scattergram of the months by data from the manure seed bank (M) showing their relationship to each other as influenced by the species composition of the manure seed bank in (A) 1996; (B) 1997.
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Fig. 5. Annuals and perennials in the ordination cluster of (A) 1996; (B) 1997. The letters in the scattergram correspond to months.
grazing areas matured at different times of the year and therefore had diverse phenological history. 4. Implications Landscapes are made up of mosaics of different habitats arranged in intricate patterns at different
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scales and formed through ecological and cultural processes (Harris, 1984; Forman and Gordon, 1986). This study has shown that the influence of livestock on landscape is expressed through their effect on vegetation. This effect is expressed through the elimination of some species and the dominance of others with a decrease in ground cover and diversity of palatable species. The influence of livestock on botanical composition and species richness depended on stocking rate (see also Mwenedera et al., 1997). Livestock in free grazing systems has a very important but less perceived influence on vegetation as agents of seed dispersal. The species composition of the manure seed bank has been found significantly different from the plant communities in the field. This may have been due to the effect of mastication, difference in hardiness and rates of digestion of the seeds. The higher proportion of annuals than perennials in the manure seed bank indicated that annuals were favoured by the grazing livestock. The higher proportion of members of Asteraceae in the manure seed bank than in the grazing environment suggested that both the feeding preference and the small size of the seeds, which could escape mastication, may have contributed to their proliferation. The study shows that the overgrazed hills and plains in the mixed cereal and livestock complex are reseeded by the livestock through their manure. It can therefore be suggested that manure when used to improve soil fertility can change the weed flora in favour of members of the Asteraceae and Fabaceae. Therefore, although manure can improve soil fertility, it may also introduce noxious weeds such as the members of the Asteraceae which are mainly annuals (Stroud and Parker, 1989). However, in land-constrained situations farmers feed livestock with weeds removed from cropland. The merit of manuring crop fields will have to be evaluated from the standpoint of these concerns. In general, the species diversity of the grasslands of the Ethiopian plateau is maintained by the active and passive influences of the livestock population. However, in the actual situation of the mixed cereal-livestock agricultural system, manure deposited during the dry season is not allowed to recycle as it is used for fuel. The role of manure in maintaining the biodiversity of grazing area can be realized when manure is not collected for fuel.
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5. Conclusions This research has made it possible to obtain the temporal variations within the manure seed bank and comparison of the data with those of the natural grassland. The role of livestock in grasslands may be viewed as important agents in maintaining landscape and species diversity and this can be realized if efforts are made to keep the stocking rate within the limits of the carrying capacity of the grazing areas. Further study on carrying capacity of the natural grasslands, improved methods of producing more livestock feed and detailed experiments on some important species in manure seed bank will complement the findings in this research. References Almeida, M.D., 1954. Some Records of Ethiopia 1593–1664 — C.P. Bekingham and C.W.B. (Huntingford translation and edition). The University Press, Glasgow. Alvarez, F., 1970. Portuguese Embassy to Abyssinia during the years 1520–1527 — Lord Stanley of Adlerley (translation and edition). Burt Franklin Publisher, NY.
Forman, R.T., Gordon, M., 1986. Landscape Ecology. Wiley, NY, Chichester, Brisbone, Toronto, Singapore. Gamachu, D., 1977. Aspects of climate and water budget in Ethiopia. Addis Ababa University Press, Addis Ababa. Harris, L.D., 1984. Habitat fragmentation. In: Meffe, K.G., Carrol, C.R. et al. (Eds.), Principles of Conservastion Biology. Sinauer Associates, Inc., Saunderland, MA, pp. 237–258. Janzen, D.H., 1982a. Differential seed survival and passage rates in cows and horses, surrogate pleistocene dispersal agents. Oikos 38, 150–156. Janzen, D.H., 1982b. Removal of seeds from tropical rodents: influence of habitat and amount of dung. Ecology 63, 1887– 1900. Mengistu A., 1997. Conservation Based Forage Development for Ethiopia. Self Help Development International Institute for sustainable Development, Addis Ababa, Ethiopia. Mwendera, E.J., Mohame Saleem, M.A., Zerihun, W., 1997. Vegetation response to cattle grazing in the Ethiopian highlands. Agric. Ecosystem Environ. 64, 43–51. Orloci, L., 1978. Multivariate Analysis in Vegetation Research, 2nd Edition. Junk, The Hague. Pichi Sermoli, R.C.E., 1957. Una carta geobotanica dell’ Africa Orientale, Etiopia, Eritrea ed Somalia. Webbia 13, 15–132. Stroud, A., Parker, C., 1989. A weed identification guide for Ethiopia. TCP/ETH/4532, Food and Agricultural Organization of the United Nations, Rome. Woldu, Z., 1986. Grassland communities on the central plateau of Ethiopia. Vegetatio 67, 3–16.
Grazing induced biodiversity in the highland ecozone of East Africa Zerihun Woldu a , M.A. Mohammed Saleem b,∗ a
Department of Biology, the National Herbarium, Addis Ababa University, P.O.Box 3434, Addis Ababa, Ethiopia b International Livestock Research Institute (ILRI), P.O.Box 5689, Addis Ababa, Ethiopia Received 15 April 1999; received in revised form 11 August 1999; accepted 22 October 1999
Abstract The species composition of grazing lands can be influenced by livestock and grazing pressure. A study on manure seed bank was conducted in Ghinchi highland Research Site in Ethiopia between 1995 and 1997. The data on species composition and life-form of the plants germinating in pots receiving air dried manure were compared with species composition of experimental plots in natural grassland subjected to varying grazing intensity. There was significant difference among the species composition of grazed and non-grazed grasslands and the manure seed bank (p = 0.01). The life-forms of the species also showed variation. There were more families and species in the natural grassland vegetation than indicated in the manure seed bank. The manure seed bank had more annuals than the natural grassland vegetation. The species composition and life-forms in the manure seed bank showed variation with time and this corresponded with the seasonal variation in the grassland, which had a direct relationship with the rainfall pattern. The study showed that livestock play a major role in maintaining the biodiversity of grassland vegetation by spatial and temporal dispersion of readily germinating seeds in their manure. The use of manure to improve soil fertility should be weighed cautiously against the introduction of weeds into crop fields, although weeds are important feed resource for livestock in land-constrained areas. There is therefore the need for developing manure management practices so that the benefits can be optimised and the undesirable effects can be minimised. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Manure; Species; Life-form; Livestock; Seed bank; Biodiversity
1. Introduction A long history of land clearing and sedentary agriculture has changed the vegetation cover in the east African highlands. The natural vegetation of the central plateau of Ethiopia, for example, may have been dry evergreen montane forest with Juniperus procera and Olea europaea sp. cuspidata as the dominant species (Pichi Sermoli, 1957). This vegetation type ∗ Corresponding author. Tel.: +251-1-613215; fax: +251-1-611892. E-mail address: [email protected] (M.A. Mohammed Saleem)
has disappeared from most parts of the highland except in few remnant patches around holy places and inaccessible areas. Records of early travelers in the fifteenth and sixteenth century (Almeida, 1954; Alvarez, 1970) indicate that the agro-climate conditions of the Ethiopian highland about 500 years ago were similar to the present day conditions except that the density of trees then was higher in the crop fields. The central plateau of Ethiopia is characterised by mixed cereal and livestock agriculture. Nutrient deficient soils, high stocking rates and shortage of animal feeds are common features. Livestock in the highlands account for 80% of the total population and
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about 20% of the agricultural gross domestic product of the country (Mengistu, 1997). The livestock are herded together and grazed on communal pastures, private land and stables depending on the time and season of the year. Cattle are the most important livestock species in the Ethiopian agriculture and they are kept mainly for traction and milk production. Grazing is being expanded to very steep slopes and marginal lands, as more land is cultivated to compensate for the diminishing soil fertility and to meet the ever-increasing food demands. The species composition and productivity of the pasture of the common grazing lands are highly influenced by the species of the livestock, the intensity of grazing, climatic and edaphic factors. The seasonal stocking rate on the central plateau of Ethiopia is very high for the ecological carrying capacity of the grassland (Woldu, 1986). The grassland communities are predominantly of Pennisetum sphacelatum– Commelina africana type and could develop to Andropogon abyssinicus–Hyparrhenia arrhenobasis type if grazing intensity is relaxed (Woldu, 1986). Grasslands in general are stressful environments. Defoliation, uprooting, trampling and desiccation are the important stresses in grasslands under high grazing pressure. The role of livestock on the grassland vegetation can be seen from two main perspectives. The most obvious one is the influence on the species composition and reduction in the above and below ground biomass. The less obvious but equally important is the maintenance and distribution of the biodiversity. Free grazing livestock move to any accessible site and their feeding behaviour or foraging choice may generate a pattern of association between the plant species and the livestock. This relationship helps in seed dispersal which may be manifested in spatial and temporal differences in ground cover and patchiness. Seed morphology may influence whether seeds survive the mastication and the digestive enzymes of the ingesting animal. Those that survive through the animal gut germinate faster as dormancy is broken by the abrasion, scarification and wetting of the digestive system of the animal. On the other hand, seeds retained in the animal gut for a long period may be induced to germinate and be killed while in the gut (Janzen, 1982a, b). Deposition of seeds with manure gives a better chance of survival since a source of nitrogen is readily available for early seedling growth. It
appears, therefore, that the animal gut filters out those species by enhancing their germination while in the gut and by providing favourable conditions when they are released. It is with this understanding that an experiment was designed to study the species composition of manure seed bank at different times of the year and compare these with the species composition of natural grasslands under different grazing pressures. The specific objectives of this study were (1) to determine the species composition and richness of germinating seeds in manure seed bank, (2) to compare the species composition of manure seed bank and those of the natural grassland under different grazing pressure, (3) to assess the role of livestock in the maintenance of the biodiversity of the grassland vegetation through the spatial transfer of seeds in their manure, and (4) to assess the use of manure in improving soil fertility.
2. Materials and methods 2.1. Experimental site Two complementing experiments were conducted at Ghinchi Research Station, a local research base, where the International Livestock Research Institute (ILRI) is working with a consortium of institutions on natural resource management issues. Ghinchi is located in the Ethiopian highlands at 2290 m asl 80 km west of Addis Ababa (9◦ 020 N; 38◦ 070 E). The experiments include manure seed bank pot trials and grazing experiments in six different sites. 2.2. Manure seed bank pot trials For the manure trials, the cattle herd of two farmers were monitored as they grazed different parts of the landscape at different times of the year. Manure was collected each day from the grazing areas, and a composite sample was taken from the weekly collections and air dried. Five replicates were made from each composite sample, and 123.3g of the sample was spread over a pot (18.0 cm — top diameter, 11.5 cm — bottom diameter and 15.5 cm — height) half filled with 1.90 kg of sterilised sand. The plant species growing in the pots were recorded until all seedlings had emerged. The seedlings were identified to the species level as much as possible. The manure experiment in the pots
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was run for two complete years, August 1996–July 1997 and August 1997–July 1998. The plant species and other data for each replicate of the week were entered in access database. Relevant information was extracted using the structured query language. 2.3. Grazing experiments To study the influence of grazing on the grassland vegetation, six sites with varying slopes ranging from 0–8% were selected to represent the area where the livestock of the community grazed. Each site had 10 m × 10 m plots where (a) grazing was totally precluded (NOG), (b) moderately grazed (MDG — 1.8 Tropical Livestock Unit Month — TLUM/ha) and (c) very heavily grazed (VHVG — 4.2 TLUM/ha) (see also Mwendera et al., 1997). The NOG and MDG plots were enclosed by barbed wire fences and there was no fencing around VHVG plots. The door of the fence around MDG was opened for 3 days in a week for the free grazing livestock of the farming community. Grazing took place during all days of the week in VHVG. Data on species composition, per cent cover of three most dominant species and bare ground were collected at 30 days interval from the second week of September to the second week of December. This period is shortly after the summer rain season when plant growth rate is maximal and most plants bear inflorescence. During the long dry period (January–May) and the summer rain period (June–September) most species in VHVG and some in MDG and NOG are not identifiable. Data were collected for three consecutive years from 1995–1997. Only the list of species in NOG and VHVG were considered for the present article. The result which includes MDG will be communicated in another publication. 2.4. Data analysis The weekly manure-seedling data from the five replicates were pooled for each month and a data matrix was constructed for each year with the species in the rows and months on the columns. The species composition of the manure (M) was compared with data on the grazing experiment plots using chi square (χ 2 ). The relative occurrences of the four important grassland families were calculated for each year of
Fig. 1. Monthly average rainfall in Ghinchi Research Station, Ethiopia in 1996 and 1997.
the grazing experiment and manure seed bank data. The relative composition of the annual and perennial species in each experiment was also calculated. Rainfall data of 1996 and 1997 was obtained from Ethiopian Agricultural Research Meteorological Services and the monthly average rainfall of the 2 years were plotted (Fig. 1). Principal Components Analysis (PCA) was performed using Syntax to find the influence of time on the species composition of the manure seed bank. The PCA is an ordination technique which involves extracting the same number of axes of variations as in the rows or columns of the input data matrix in decreasing order of importance. Scatter diagram (scattergram) of the two axes showing the highest variations provides a two dimensional display of the underlying pattern in the original data matrix (see Orloci, 1978).
3. Results and discussion 3.1. Species in manure seed bank and grazing experiment Table 1 gives the occurrence of the species in the manure seed bank during the 12 months of the year in
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Table 1 List of species in manure seed bank (M), no grazing plots (NOG) and very heavily grazed (VHVG)a Species
Manure
NOG
VHVG
Abutilon sp. Acacia abyssinica Ajuga remota Alchemilla fischeri Amaranthus hybridus Anagalis arvensis Andropogon abyssinicus Arthraxon lancifolius Bidens biternata Bidens pilosa Bothriochloa insculpta Brachiaria semiundulata Brassica nepus Carduus chamaecephalus Carum carvi Centaurium tenuiflorum Cerastium octandrum Chenopodium album Chenopodium ambrosoides Chloris pycnothrix Cineraria abyssinica Cirsium vulgare Coleus punctatus Commelina africana Commelina bengalensis Commelina subulata Conyza tigreensis Corchorus trilocularis Corrigiola litoralis Corrigiola sp. Crassocephalum rubens Crepis carbonaria Crotalaria spinosa Cynodon dactylon Cynoglossum coeruleum Cyperus rigidifolius Cyperus teneriffae Datura stramonium Dichrocephala integrifolia Digitaria rivae Dischoriste radicans Echinochloa colona Eleusine floccifolia Eragrostis botryodes Eragrostis tef Eragrostis tenuifolia Eriochloa meyeriana Euphorbia petitiana Evolvulus alisinoides Falkia oblonga Fimbristylis complanata Galinsoga parviflora Galium spurium
1 1 0 0 1 0 0 1 0 0 0 0 1 0 1 0 0 1 1 0 0 0 0 1 1 0 1 0 1 1 1 0 0 1 1 1 1 0 1 1 0 0 1 0 1 1 0 1 1 1 0 1 1
0 0 0 1 1 1 1 1 1 1 0 0 0 1 1 0 0 0 0 0 1 1 1 1 0 0 0 1 0 0 0 0 0 1 0 1 0 0 1 1 1 1 1 1 0 1 1 0 1 1 1 1 0
0 0 1 1 1 0 0 0 1 1 1 1 1 1 0 1 1 0 0 1 1 0 1 1 0 1 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 0 0 1 1 1 1 0
Table 1 (Continued). Species
Manure
NOG
VHVG
Geranium arabicum Gnaphalium luteo-album Guizotia abyssinica Guizotia scabra Haplociadum abyssinicum Hibiscus sp. Hibiscus tridentata Hibiscus trionum Hygrophila auriculata Hyparrhenia arrhenobasis Hyparrhenia filipendula Hyparrhenia hirta Hyparrhenia rufa Hyparrhenia sp. Indigofera sp. Kohautia coccinia Kyllinga appendiculata Lactuca inermis Laggera pterodonta Lathyrus sativus Lens culinaris Leucas marticinensis Lolium perenne Lotus corniculatus Medicago polymorpha Oxygonum sinuatum Panicum sp. Pennisetum clandestinum Pennisetum divisum Pennisetum riparium Pennisetum villosum Persicaria nepalensis Phalaris paradoxa Phyllanthus rotundifolius Plantago lanceolata Poa annua Poa leptoclada Polygala sp. Rumex nepalensis Scirpus inclinatus Scleria clathrata Scleria hispidior Scorpiurus muricatus Sesbania sesban Setaria verticillata Sida rhombifolia Snowdenia polystachya Solanum nigrum Sonchus sp. Sorghum bicolor Sphaeranthus suaveolens Spilanthus mauritiana Sporobolus africanus Tagetes minuta Thesium radicans
1 1 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 1 0 0 1 1 0 1 1 1 0 1 1 1 1 1 1 1 0 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 0
0 1 0 1 0 1 0 1 1 1 1 1 1 0 1 1 1 1 1 0 0 1 1 1 1 1 1 1 1 1 0 1 1 1 1 0 0 1 1 1 1 1 1 0 1 0 1 1 0 0 1 1 1 0 0
0 1 0 1 0 1 1 1 1 1 0 1 0 1 1 1 0 0 0 0 0 1 0 1 1 0 1 1 1 1 1 1 1 1 1 0 0 1 1 1 1 0 1 0 1 0 1 1 0 0 1 1 1 0 1
Z. Woldu, M.A. Mohammed Saleem / Agriculture, Ecosystems and Environment 79 (2000) 43–52 Table 1 (Continued). Species
Manure
NOG
VHVG
Trifolium abyssinicum Trifolium acaule Trifolium burchellianum Trifolium cryptopodium Trifolium johnstonii Trifolium lanceolatum Trifolium multinerve Trifolium rueppellianum Trifolium schimperi Trifolium semipilosum Trifolium simense Trifolium steudneri Trifolium tembense Triticum aestivum Uebelinia abyssinica Verbascum sinaticum Verbena officinalis Veronica abyssinica Veronica anagallis-aquatica Vigna oblongifolia Xanthium spinosum Zea mays
0 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1
1 0 0 0 1 0 0 0 1 1 0 1 1 0 1 0 1 1 0 0 0 0
1 0 0 0 0 0 0 0 1 1 0 1 1 0 0 0 1 1 0 0 1 0
a
0 stands for absence and 1 stands for presence.
1996 and 1997, and the occurrence of species in the NOG and VHVG from 1995 to 1997. The number of species per year in the grazing experiment ranged between 50 and 59 while it ranged between 62 and 64 in the manure experiment (Table 2). The number of families in the grazing areas ranged between 16 and 21 while it ranged between 17 and 22 in the manure experiment when the data of each year
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are considered separately. The number of species increased to 94 and 79; and the number of families increased to 28 and 24 in the grazing areas and manure seed bank, respectively, when the data of the whole period were pooled together. The grazing experiments and the manure seed bank had 37 species (14 families) in common. The manure seed bank species included six annual food crop species. The relative importance of the four major grassland families in Ghinchi grazing area corresponded to the pattern in the grassland communities in the east African highlands in general, and in Ethiopia, in particular (Woldu, 1986). Members of the Poaceae were the highest, followed by those of Asteraceae, Fabaceae and Cyperaceae in that order. In the manure seed bank, however, Poaceae and Fabaceae were equal in importance and were higher than the other two families. The percentage of Asteraceae was higher and the percentage of Cyperaceae was lower than in the grassland communities. The number of species, families and the relative importance are presented in Table 2. The chi-square analysis showed that the composition of species in the M versus NOG, M versus VHVG and NOG versus VHVG are significantly different (χ 2 = 6.13, 5.48 and 25.65, respectively, significant at p = 0.01). The species unique to M, NOG and VHVG and those common to the three treatments are given in Table 3. The results indicate that the species composition of the manure seed bank mostly resembled the species composition of VHVG and less to those of NOG. The relatively higher resemblance between M and VHVG may suggest that the areas where the livestock grazed were identical to VHVG.
Table 2 Proportion of the four important plant families in the grazing area and in the manure during the study period at Ghinchi in the Ethiopian highlands over 3 years Grazing areas
Manure
1995
1996
1997
6
1996
1997
6
Species
50
59
50
94
62
64
86
Families 1 2 3 4
16 Poaceae (32%) Fabaceae (14%) Asteraceae (12%) Cyperaceae (12%)
21 Poaceae (24%) Asteraceae (20%) Fabaceae (11%) Cyperaceae (10%)
17 Poaceae (34%) Asteraceae (20%) Fabaceae (8%) Cyperaceae (6%)
28
18 Fabaceae (26%) Poaceae (23%) Asteraceae (13%) Cyperaceae (5%)
22 Poaceae (23%) Fabaceae (20%) Asteraceae (13%) Cyperaceae (5%)
24
Total
70%
65%
66%
67%
61%
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Table 3 Species unique to manure seed bank (M), no grazing (NOG), very heavily grazed (VHVG) and those common to the three treatments (M–NOG–VHVG) Species occurring only in manure
Species occuring only in NOG
Species occurring only in VHVG
Species occuring in manure, NOG and VHVG
Abutilon sp. Acacia abyssinica Chenopodium album Chenopodium ambrosoides Commelina bengalensis Conyza tigreensis Corrigiola litoralis Corrigiola sp. Crassocephalum rubens Eragrostis tef Euphorbia petitiana Galium spurium Geranium arabicum Guizotia abyssinica Haplociadum abyssinicum Lathyrus sativus Lens culinaris Poa annua Poa leptoclada Sesbania sesban Sida rhombifolia Sonchus sp. Sorghum bicolor Tagetes minuta Trifolium acaule Trifolium burchellianum Trifolium cryptopodium Trifolium lanceolatum Trifolium multinerve Trifolium rueppellianum Trifolium simense Triticum aestivum Verbascum sinaticum Veronica anagallisaquatica Vigna oblongifolia Zea mays
Anagalis arvensis Andropogon abyssinicus Cirsium vulgare Corchorus trilocularis Eriochloa nubica Hyparrhenia filipendula Hyparrhenia rufa Kyllinga appendiculata Lactuca inermis Laggera pterodonta Lolium perenne Oxygonum sinuatum Scleria hispidior Trifolium johnstonii
Ajuga remota Bothriochloa insculpta Brachiaria semiundulata Centaurium tenuiflorum Cerastium octandrum Chloris pycnothrix Commelina subulata Crepis carbonaria Crotalaria spinosa Datura stramonium Hibiscus tridentata Hyparrhenia sp. Thesium radicans Xanthium spinosum
Amaranthus hybridus Commelina africana Cynodon dactylon Cyperus rigidifolius Dichrocephala integrifolia Digitaria rivae Eleusine floccifolia Eragrostis tenuifolia Evolvulus alisinoides Falkia oblonga Galinsoga parviflora Gnaphalium luteo-album Guizotia scabra Hygrophila auriculata Lotus corniculatus Medicago polymorpha Panicum sp. Pennisetum clandestinum Pennisetum divisum Persicaria nepalensis Phalaris paradoxa Phyllanthus rotundifolius Plantago lanceolata Rumex nepalensis Scorpiurus muricatus Setaria verticillata Snowdenia polystachya Solanum nigrum Sphaeranthus suaveolens Spilanthus mauritiana Sporobolus africanus Trifolium schimperi Trifolium semipilosum Trifolium steudneri Trifolium tembense Verbena officinalis Veronica abyssinica
3.2. Proportion of life-forms The pattern of occurrence of annuals and perennials in M clearly follows the pattern of the rainfall (Figs. 1, 2A and B). Although the Ethiopian highland is known for its bimodal rainfall (Gamachu, 1977), the rainfall pattern of Ghinchi in 1996 and 1997 was unimodal. Comparison of the rainfall of the 1996 and 1997 shows that the rainfall in 1996 was evenly distributed while
that of the 1997 was concentrated mainly between July and September and had a long dry period. This seems to have a bearing on the composition of annuals and perennials. The evenly distributed rainfall in 1996 appears to have encouraged the continuous availability and production of seeds from annuals hence their dominance in the manure seed bank (69%) while the relatively more arid climate and high torrential rainfall in the summer of 1997 had discouraged annuals (57%)
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Fig. 3. Proportion of annuals and perennials in the species unique to manure seed bank (M), no grazing (NOG), very heavily grazed (VHVG) and those common to the three treatments (M–NOG–VHVG)
common to M, NOG and VHVG was 40.5% (Fig. 3). The list of species is given in Table 3. 3.3. Temporal patterns of occurrence
Fig. 2. Annuals and perennials in the species composition of the manure seed bank in (A) 1996; (B) 1997.
but encouraged the persistence of perennials through out the year. The composition of annuals in M was 63%, while that of the grassland (NOG, and VHVG pooled together) was 41%. Seventy eight percent of the species unique to M were annuals, while the proportions of annuals unique to NOG and VHVG were 28% only. The proportion of annuals in the species
The seasonal pattern in the samples indicates that the seeds of perennials attain dominance at the peak of the rainfall (July–September) while the seeds of the annuals were dominant during the rest of the year. The PCA of M in 1996 showed three distinct clusters whereas that of 1997 showed four diffused clusters (Fig. 4A and B). The relatively higher annual precipitation and the more even distribution of the rainfall in 1996 may have provided more uniform climatic conditions during most part of the year hence the less number of clusters. It is interesting to note that each month clustered with the month preceding or succeeding indicated that the clusters also follow the seasonal pattern. The life-form of the species in the clusters corresponds to the average precipitation of the months. The number of annuals was higher than the perennials in those clusters where the average rainfall was higher (Fig. 5A and B). The general pattern of the clusters suggests that the species in the
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Fig. 4. Scattergram of the months by data from the manure seed bank (M) showing their relationship to each other as influenced by the species composition of the manure seed bank in (A) 1996; (B) 1997.
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Fig. 5. Annuals and perennials in the ordination cluster of (A) 1996; (B) 1997. The letters in the scattergram correspond to months.
grazing areas matured at different times of the year and therefore had diverse phenological history. 4. Implications Landscapes are made up of mosaics of different habitats arranged in intricate patterns at different
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scales and formed through ecological and cultural processes (Harris, 1984; Forman and Gordon, 1986). This study has shown that the influence of livestock on landscape is expressed through their effect on vegetation. This effect is expressed through the elimination of some species and the dominance of others with a decrease in ground cover and diversity of palatable species. The influence of livestock on botanical composition and species richness depended on stocking rate (see also Mwenedera et al., 1997). Livestock in free grazing systems has a very important but less perceived influence on vegetation as agents of seed dispersal. The species composition of the manure seed bank has been found significantly different from the plant communities in the field. This may have been due to the effect of mastication, difference in hardiness and rates of digestion of the seeds. The higher proportion of annuals than perennials in the manure seed bank indicated that annuals were favoured by the grazing livestock. The higher proportion of members of Asteraceae in the manure seed bank than in the grazing environment suggested that both the feeding preference and the small size of the seeds, which could escape mastication, may have contributed to their proliferation. The study shows that the overgrazed hills and plains in the mixed cereal and livestock complex are reseeded by the livestock through their manure. It can therefore be suggested that manure when used to improve soil fertility can change the weed flora in favour of members of the Asteraceae and Fabaceae. Therefore, although manure can improve soil fertility, it may also introduce noxious weeds such as the members of the Asteraceae which are mainly annuals (Stroud and Parker, 1989). However, in land-constrained situations farmers feed livestock with weeds removed from cropland. The merit of manuring crop fields will have to be evaluated from the standpoint of these concerns. In general, the species diversity of the grasslands of the Ethiopian plateau is maintained by the active and passive influences of the livestock population. However, in the actual situation of the mixed cereal-livestock agricultural system, manure deposited during the dry season is not allowed to recycle as it is used for fuel. The role of manure in maintaining the biodiversity of grazing area can be realized when manure is not collected for fuel.
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5. Conclusions This research has made it possible to obtain the temporal variations within the manure seed bank and comparison of the data with those of the natural grassland. The role of livestock in grasslands may be viewed as important agents in maintaining landscape and species diversity and this can be realized if efforts are made to keep the stocking rate within the limits of the carrying capacity of the grazing areas. Further study on carrying capacity of the natural grasslands, improved methods of producing more livestock feed and detailed experiments on some important species in manure seed bank will complement the findings in this research. References Almeida, M.D., 1954. Some Records of Ethiopia 1593–1664 — C.P. Bekingham and C.W.B. (Huntingford translation and edition). The University Press, Glasgow. Alvarez, F., 1970. Portuguese Embassy to Abyssinia during the years 1520–1527 — Lord Stanley of Adlerley (translation and edition). Burt Franklin Publisher, NY.
Forman, R.T., Gordon, M., 1986. Landscape Ecology. Wiley, NY, Chichester, Brisbone, Toronto, Singapore. Gamachu, D., 1977. Aspects of climate and water budget in Ethiopia. Addis Ababa University Press, Addis Ababa. Harris, L.D., 1984. Habitat fragmentation. In: Meffe, K.G., Carrol, C.R. et al. (Eds.), Principles of Conservastion Biology. Sinauer Associates, Inc., Saunderland, MA, pp. 237–258. Janzen, D.H., 1982a. Differential seed survival and passage rates in cows and horses, surrogate pleistocene dispersal agents. Oikos 38, 150–156. Janzen, D.H., 1982b. Removal of seeds from tropical rodents: influence of habitat and amount of dung. Ecology 63, 1887– 1900. Mengistu A., 1997. Conservation Based Forage Development for Ethiopia. Self Help Development International Institute for sustainable Development, Addis Ababa, Ethiopia. Mwendera, E.J., Mohame Saleem, M.A., Zerihun, W., 1997. Vegetation response to cattle grazing in the Ethiopian highlands. Agric. Ecosystem Environ. 64, 43–51. Orloci, L., 1978. Multivariate Analysis in Vegetation Research, 2nd Edition. Junk, The Hague. Pichi Sermoli, R.C.E., 1957. Una carta geobotanica dell’ Africa Orientale, Etiopia, Eritrea ed Somalia. Webbia 13, 15–132. Stroud, A., Parker, C., 1989. A weed identification guide for Ethiopia. TCP/ETH/4532, Food and Agricultural Organization of the United Nations, Rome. Woldu, Z., 1986. Grassland communities on the central plateau of Ethiopia. Vegetatio 67, 3–16.