Biodiversity in riverbank techniques for erosion control: assessment of animal and plant species diversity along a naturality gradient
Résumé
Riverbanks are characterised by high levels of both species-richness and anthropic pressure. Erosion controls on riverbanks often involve installations to protect human investments (i.e. buildings, public amenitiesetc.). However, whether such installations can accommodate natural biodiversity has not been well assessed and subsequently seldom taken into account in the choice of technique. The aim of this study was to assess animal and plant species diversity on the natural gradient of various riverbank protection systems, from entirely stone riprap, through combined constructions (mixing riprap and bioengineering), to purely vegetative bioengineering structures. We compared plant species diversity and animal taxonomic diversity above ground (beetles) and below ground (mites) on five installations in each category. Vegetation was sampled along three longitudinal transects using the point contact method. Fauna were sampled in the soil, air and herbaceous layers by trapping (yellow bowl trap, pitfalls and Berlese-Tullgren extractor). The fifteen sites sampled were located in the Rhône-Alpes region (South East France). 148 plant species, 78 beetle genera and 8 mite families were recorded. We found a significantly lower animal and plant diversity in riprap constructions than in the other two types. Surprisingly, diversity was higher, even if not significantly, in combined works than in purely vegetative ones. Furthermore, riprap techniques were more subject to exotic invasions than techniques using bioengineering. This study gives new insights into the animal and plant biodiversity of riverbank protections against erosion, ranging from entirely mineral to purely vegetative constructions. It also provides useful information to help practitioners choose techniques in relation to their future impact on biodiversity.
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