Modelling trade offs between ecological and economic performances in grazed grasslands: importance of temporal stocking density
Résumé
Recent changes in grassland management have led to increasing threats on farmland birds populations (Newton, 2004). Through its impact on sward structure, grazing can promote suitable habitats for ground nesting birds. However it can also have negative side effects on birds dynamics through nest trampling. We developed a cost effectiveness approach to asses the effect of nest trampling by cattle on the economic and ecological performances of grazed grasslands. Several grazing strategies satisfied the ecological criterion. However, the best ecological strategy was the worst on the economic viewpoint. The best economic strategy ensuring ecological objectives in the long term implied a 0.5 LU.ha-1 stocking constraint in April and May. This strategy allowed an economic performance of 235.8 €.ha-1 and was consequently 27% less profitable than the best reachable economic performance. The initial grazing peak in spring was split in two. A first peak appeared in March and another one in June. This illustrates a need to anticipate grass growth so as to maintain suitable habitats during the breeding months. The trade off between economic and ecological performances is highly dependant on the stocking density sequences. A maximal economic performance seems to be sorely compatible with ecological outcomes. The impact of cattle trampling on waders is all the more important since nesting occurs during the two months where grass growth is the strongest. Even if it implies lower economic performances, limited grazing is compatible with ecological goals and is essential to produce an optimal habitat for waders. These results are useful to reflect on agri-environment schemes aimed at both economic and ecological objectives.