Food preferences and food availability are two major determinants of the diet of generalist herbivores and of their spatial distribution. How do these factors interact and eventually lead to diet differentiation in co-occurring herbivores? We quantified the diet of four grasshopper species co-occurring in subalpine grasslands using DNA barcoding of the plants contained in the faeces of individuals sampled in the field. The food preferences of each grasshopper species were assessed by a choice (cafeteria) experiment from among 24 plant species common in five grassland plots, in which the four grasshoppers were collected, while the habitat was described by the relative abundance of plant species in the grassland plots. Plant species were characterised by their leaf economics spectrum (LES), quantifying their nutrient vs. structural tissue content. The grasshoppers' diet, described by the mean LES of the plants eaten, could be explained by their plant preferences but not by the available plants in their habitat. The diet differed significantly across four grasshopper species pairs out of six, which validates food preferences assessed in standardised conditions as indicators for diet partitioning in nature. In contrast, variation of the functional diversity (FD) for LES in the diet was mostly correlated to the FD of the available plants in the habitat, suggesting that diet mixing depends on the environment and is not an intrinsic property of the grasshopper species. This study sheds light on the mechanisms determining the feeding niche of herbivores, showing that food preferences influence niche position whereas habitat diversity affects niche breadth.

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Résumé. — Les auteurs montrent l'intérêt floristique et écologique des zones humides du Jura méridional qui sont en relation étroite avec la plaine inondable du Haut-Rhône à l'aval de Seyssel (Ain). A partir d'un exemple pris dans le Jura savoisien (la Méline et les Lacs de Chevelu), les groupements végétaux présents sont analysés, décrits et cartographiés (à 1/5000 environ). Leurs rôles de «derniers conservatoires régionaux» d'espèces et d'associations spécialisées sont soulignés. Les relations de la végétation avec les facteurs du milieu (substrat, hydrologie) et les activités humaines anciennes sont mises en évidence et permettent d'évaluer les changements affectant la végétation et d'insister sur la nécessité d'un entretien des milieux par une activité agricole de type «traditionnel » et nécessairement subventionnée.