Can ecological engineering restore Mediterranean rangeland after intensive cultivation? A large-scale experiment in southern France

Ecological restoration has been identified as one of the possible ways to replace biodiversity loss. While ecological engineering methods can successfully restore some ecosystem attributes, restoration is generally incomplete, mainly due to lack of target species propagule dispersal, unsuitable abiotic conditions and negative biotic interactions, especially after intensive cultivation. How best to restore reference ecosystems is therefore a vital research objective, not only on a small experimental scale but also on a large scale. This study aimed to determine which ecological engineering techniques are the most suitable for large-scale restoration of a low-productive species-rich ecosystem after intensive cultivation. Experiments were carried out at La Crau in southern France, within a 357ha land rehabilitation project whose aim was to recreate a herbaceous sheep-grazed habitat. We investigated: (i) covering species seeding, (ii) topsoil removal, (iii) hay transfer, and (iv) soil transfer as methods to restore a steppe plant community in this rehabilitated area, using the last remaining French Mediterranean steppe as a reference ecosystem. Species-richness, diversity and composition of vascular plant communities were monitored over three years. The rehabilitation made it possible to recover a large area dominated by grasses but with vegetation different from that of the steppe. Hay transfer was successful in transferring some target species, but the number of target species did not significantly increase by the end of the third year. Covering species seeding seemed to provide a suitable area for target species colonization, but probable competition with grasses needs to be monitored. The greatest similarity in richness, diversity, and composition with the reference ecosystem was obtained with topsoil removal and soil transfer, which made it possible to recover the species-richness and, to some extent, the composition of the steppe. Our findings show that current ecological restoration and ecological engineering techniques can lead to at least partial restoration of some ecosystem attributes. However, they also underline the importance of in situ conservation of natural habitats rather than restoring them after their destruction.