A co-viability model of grazing and bird community management in farmland
Résumé
In European grasslands, livestock grazing is a key driver of habitat quality. However, the specific grazing regimes that favour habitat quality and ensure biodiversity conservation remain largely unknown. This paper develops a dynamic model to predict how livestock grazing may be used to sustain a bird community without penalizing cattle feeding. The model focuses on a grassland grazed by cattle which is also the breeding habitat of three wader species. It integrates the dynamics of a sward, controlled through grazing, and that of three bird populations. Viability theory and dynamic programming are used to reveal viable grazing strategies that ensure the creation of two levels of habitat quality (homogeneous and heterogeneous) whilst ensuring cattle feeding. Viable grazing strategies are ranked through an economic criterion and referred as ecologic or economic grazing. Then using viability population analysis, we assess the extinction risk of the wader community in both levels of habitat quality. Globally, grazing had a more positive effect on the wader community than did the no-grazing scenario. Both levels of habitat quality could be generated through both grazing strategies corresponding to contrasting temporal distributions of livestock densities. Grazing strategies resulted in differential community viability. Ecologic grazing always ensured wader community maintenance, whatever the level of habitat quality targeted. This was not the case for economic grazing which reduced the wader community to two species. Species response showed two contrasting patterns, with a more dramatic decline for lapwings than redshanks in both the no-grazing and heterogeneous habitat quality scenarios. This may lead to conservation problems because of the relationship between population size and probability of extinction observed for lapwings.