A. Grazing effects on wetlands structure and botanical composition

It is clear up to now that livestock have multiple effects on wetlands which can be beneficial if overgrazing is avoided. Important also is the system of grazing. Planned grazing systems adapted to the condition of each wetland seem to be more effective in harmonizing, livestock husbandry and wetlands than continuous grazing (Duncan and D'Herbes, 1982, Skovlin, 1984). Also, the kind of livestock species should be considered as well as their special breed. Sheep and goats do not like the water and they often develop worm and foot problems. Cattle, on the contrary, freely walk in the water as well as horses. According to Gordon et al. (1990), some cattle and horse breeds have developed special morphological adaptations for grazing in water, such as inflated homs, broad hooves or small bodies so that they can easily walk in wetland habitats.
    Grazing as a management tool in wetlands has received little scientific attention in the past. Apparently because of the detrimental effects of overgrazing, on the environment, livestock have been removed out from most wetlands and other natural reserves, including forests and hostile policies to livestock husbandry have been developed (Papanastasis, Livestock -grazing in wetlands is an old and traditional practice in Greece. Most of these ecosystems evolved with the presence of farm animals including buffaloes, thus leading to a peaceful coexistence
    However, this coexistence has already been broken because of the expansion of cultivated fields in the old marshlands, which led to the reduction of the available grazing lands and finally, to the overgrazing of the remaining wetlands. In the meantime, buffaloes were replaced by cattle in the majority of wetlands due to socioeconomic changes .
    The impact of livestock on wetlands can be aggravated or mitigated depending on the management practices applied. Livestock grazing affects the canopy and species composition of wetland vegetation, but its impact depends on its intensity. If it is severe, plant reproduction is prevented and complete elimination of vegetation may result with adverse effects on the aquatic environment.
    Riparian vegetation, in particular, stabilizes the stream banks, regulates the temperature in the water, reduces sediment and nutrient transport and, if woody, it can remove nutrients from sub-surface flow and store them. Overgrazing of such a vegetation results in the reduction of shade and cover which may raise water temperature and eliminate the sensitive fish species (Platts, 1979). Also, it may cause soil and stream bank erosion, increased soil water evaporation and a rise in water temperature which will increase, with the additional light, the growth of algae and, therefore, lead to the destruction of fish habitats (Skovlin, 1984).

B. Influence of grazing on birds habitat
    As far as the effects of livestock on wetland birds are concerned, they depend on where a particular bird feeds or nests and on the grazing season (Skovlin, 1984). In general, the effects may be direct or indirect (Tsougraids, 1995). Direct effects mainly refer to trampling of nests by large animals, such as cattle and rarely to egg consumption by sheep while a kind of positive interaction is developed between livestock and some bird species, such as Bubuicus ibis and Molothrus spp. Deferment of grazing during, the reproduction period can overcome damaging effects of livestock on nests.
    Indirect effects are caused by changes in the structure and composition of vegetation. In general, livestock grazing increases the number of invertebrates, birds and vertebrates of the open habitats, while it decreases the species of the closed habitats. A case in point is the reedbeds, which are dominated by Phragmitus atistralis and cover larger areas in several wetlands. These communities are poor habitats for both fish and most species of wetland birds. A reduction in cover of these macrophytes in the freshwater marshes of Camargue in Southern France was found to increase the population of the two duck species Anas crecca and A. strepera by 2 to 11 times. On the other hand, overgrazing of macrophytes, such as Scirpus maritimus, may result in the reduction of another duck species (Anas platvrhynches) which feeds on its seeds (Duncan and D'Herbes, 1982).
    Among all livestock, water buffaloes are the most adapted species to the wetland environment. It is a grazer like cattle, but it can stay longer in the water and utilize more efficiently wetland vegetation, especially emergent macrophytes like Phragnites australis. Also, it is not affected by worm and foot problems. However, little research information, if any, is available on the role of water buffaloes in wetlands as compared to the other livestock species.

B. Grazing in Greek wetlands
    According to a recent inventory carried out by the Greek- Biotope / Wetland Centre (Zatidis and Mantzavelas, 1994), there are 408 wetlands in Greece co%-enna an area of about 200,000 ha. Of those, 114 are currently grazed by livestock with low (43%), medium (25%) and hi-h (32%) stocking rates, while 112 were evaluated as having low (38%), medium (26%) and hich (36%) value for livestock (Tsouaralds, 1995). These data suggest that about one third of the total number of Greek wetlands are important for livestock and most of them are currently grazed but with variable stocking rates. 'Ms means that there are both overgrazed as well as under grazed wetlands.
    A characteristic case of overgrazed wetland is the delta of Evros river, which is a Rarmar site
    with international importance for wetland birds. In this particular wetlands large numbers of cattle graze freely the whole year round, thus, creating severe problems to the function of the ecosystem and damage the breeding grounds of several species of birds. An opposite case is the National Park of Prespa with the Prespa lake which is also a Ramsar site with international importance for breeding of wetland birds. In this particular site, the function of the lake ecosystem is being threatened by the expansion of reedbeds, fens and bogs caused among other reasons (e.g. entrop@cation, cessation of bumina, thatching etc.) by the reduction of _grazing activities, too. In both wetlands, buffaloes do not exist, although they were there 30-40 years ago.
    In order to stop the expansion of reedbeds in lake Prespa and improve the habitats for fish and wildlife, a project was recently (1997) initiated to re-introduce water buffalo, so that a sustainable management of Prespa ecosystem is implemented. In this project, the effects of buffalo grazing on various plant communities are recorded and their impact on fish and bird populations are evaluated. Results so far indicate that buffalo controls reeds and other emergent macrophytes of the take much better than cattle that farmers currently use, moreover
    farmers started to get attracted by this interesting animal realizing its better adaptation to the, Prespa environment than cattle. It is believed that the information collected from this project together with supporting policy measures on behalf of the Government will encourage farmers to promote buffalo as a farm animal for increased economic and environmental sustainability of wetland ecosystems in Greece.

References
Duncan, P. and J.M. D Herbes. 1982. The use of domestic herbivores in the management of wetlands for water birds in the Camargue, France. In: Management of Wetlands and their Birds (D.A. Scott, ed.). IWRB, Slinibrid-,e, p. 51-57.

Georgoudis, A. 1996. Buffalo The environment and the quality. 1lth International Symposium on Buffalo Products. Paestum, Italy. p. 145-150.

Gordon, I., P. Duncan, P. Grilles and T. Lecompte. 1990. The use of domestic herbivores in the conservation of the biological richness of European Wetlands. Bull. Ecol. 21:49-60.

Krostitz. W.1992. Present situation and future role of the riverine buffalo: A world review. Proc. Int. Symp. On prospects of buffalo production on Mediterranean - MeddEast. Cairo 9-12 November 1992.

Papanastasis, V.P 1984. . Forestry and livestock grazing: a policy perspective. In: policy Analysis for Forestry Development. Proceedings of IUFRO Conference in Thessaloniki, Greece, vol. 1. Thessaloniki, p.479-488.

Platts. W.S.1979. Livestock grazing and riparian / stream ecosystems an overview. The Vegetation as a Resource in Wetlands. Proc. Forum Grazing and Riparian / Stream Ecosystems (O.B. Copa. ed.).

Skovlin, 1984. Impacts of grazing on wetlands and riparian habitat: a review of our knowledge. In: Developing Strategies for Rangeland Management. National Research Council . National Academy of Sciences, Westview Press, p. 1001-1103.

Tsougrakis, I. 1995. Livestock grazing in wetlands and its effects on the avifauna: a literature review. Greek- Biotope / Wetland Center, Greece. 27 PP (in Greek; English summary).
 

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