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 agri- culture has changed the vegetation cover in the east
African highlands. The natural vegetation of the cen- tral 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: m.saleemcgiar.org M.A. Mohammed Saleem
has disappeared from most parts of the highland ex- cept in few remnant patches around holy places and
inaccessible areas. Records of early travelers in the fifteenth and sixteenth century Almeida, 1954; Al-
varez, 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
0167-880900 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 8 0 9 9 9 0 0 1 4 1 - 3
44 Z. Woldu, M.A. Mohammed Saleem Agriculture, Ecosystems and Environment 79 2000 43–52
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 live- stock 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 compen- sate 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 influ-
enced 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 Andro- pogon 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 graz- ing 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 im-
portant is the maintenance and distribution of the biodiversity. Free grazing livestock move to any ac-
cessible 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 en- zymes 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 ni- trogen 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 ex-
periment was designed to study the species composi- tion 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 de- termine the species composition and richness of germi-
nating 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