Z. Woldu, M.A. Mohammed Saleem Agriculture, Ecosystems and Environment 79 2000 43–52 51
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
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 elim- ination of some species and the dominance of others
with a decrease in ground cover and diversity of palat- able species. The influence of livestock on botanical
composition and species richness depended on stock- ing 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 Aster-
aceae in the manure seed bank than in the grazing environment suggested that both the feeding prefer-
ence 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 ma-
nuring 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.
52 Z. Woldu, M.A. Mohammed Saleem Agriculture, Ecosystems and Environment 79 2000 43–52
5. Conclusions
