Local and landscape effects on the functional biodiversity in mango orchards on reunion island
Résumé
Mango production, which is an important part of tropical areas' economy, is at a phytosanitary and technical dead-end on Reunion Island. The use of chemical insecticides doesn't limit the losses in economy, caused by harmful insects (Diptera, Heteroptera, Homoptera and Thysanoptera). The inefficiency of these energizing and chemical inputs, dictate plant protection evolution towards agro-ecological practices. One of them is to create habitats in crops, in order to promote diversity and abundance of pests' natural enemies. Management of functional biodiversity requires the understanding of the ecological processes involved and the capacity to identify the factors governing predatory arthropod communities in agro-ecosystems. The integrative approach of this study is to characterize the richness and diversity of arthropod predators in mango orchards, depending on various factors: plant species richness in the crop, farming practices and landscape context. The study focuses on epigeal arthropod predators, which are involved in the cycle of several pest species, such as Cecidomyiidae and Tephritidae whose last instar is characterized by a fall to the ground, so as to achieve their pupal stage in the soil. Twenty-four plots were considered, divided into the mango production basins of Reunion Island. Characterization of the composition and properties (richness and diversity) of predatory arthropod communities was conducted by sampling, using pitfall traps. Vegetation adventitious inventory within the orchards permitted to quantify plant species richness. Plots were pooled (fuzzy partitioning) according to their agricultural practices and landscape context. The measures of 17 parameters, corresponding to the nature and frequency of the plant treatment, as well as the weed vegetation management, have identified four practices intensity regimes. The landscape has been mapped to a circle of 400 m radius around each plot; the 11 measured parameters, characterizing the landscape structure and heterogeneity, have distinguished three types of landscape contexts. Richness and evenness of predatory arthropod communities were correlated and compared to these three scales of factors. Trappings revealed that predator communities consisted mainly of ants (Hymenoptera: Formicinae) and of 11 spider families (Araneae). Analysis highlighted the fact that these communities had: (1) a diversity positively correlated with the richness of the vegetation, (2) a greater richness in more moderate cultural practices, (3) a species richness higher when landscape context is heterogeneous, with fragmented mango orchards. To our knowledge, this is the first simultaneous detection of local and landscape factors effects on terrestrial arthropod predators' diversity in a tropical agro-ecosystem. (Texte intégral)