Contrasting predation services of predator and omnivore diversity mediated by invasive ants in a tropical agroecosystem

Invasive natural enemies are known to either strengthen or weaken the suppression of herbivorous arthropods. However, the impact of invasive species on the predation service provided by natural enemy diversity remains largely unexplored. Here, we tease apart the roles of invasive ants as providers of a predation service and a potential disservice, i.e. reducing the diversity of natural enemies. In mango orchards on Reunion Island, we evaluated the predation service in 20 open fields by simultaneously monitoring the predation on bait eggs and arthropod communities in two strata: the ground surface and the mango tree canopy. Our results show that the predation on bait eggs was limited to the ground surface. This stratum is dominated by three invasive omnivorous ants: Pheidole megacephala and Solenopsis geminata strongly increased the predation rate of bait eggs, whereas Brachymyrmex cordemoyi was responsible for only a small decrease in predation rate. Predation rate was positively related to predator species richness, and was negatively related to omnivore species richness. The negative relationship between the predation rate and omnivore species richness is caused by the most dominant invasive ant, P. megacephala, which reduces omnivore richness and seems to strongly prey on eggs. This study demonstrates, for the first time, the distinct influence of the diversity of two trophic groups on the predation service and how these effects can be mediated by invasive ant species. (Résumé d'auteur

Seven newly recorded species and families of spiders from Reunion Island (Malagasy region) (Araneae, Araneomorphae)

To date, 24 spider families have been identified on Reunion Island. But published data on spider fauna mainly concerned natural habitats and highlands. Studies of predatory arthropods in agroecosystems provided the opportunity to conduct a thorough investigation of spider diversity. Seven species and families of spiders are newly recorded for Reunion Island: Creugas gulosus Thorell, 1878 (Corinnidae), Oedignatha scrobiculata Thorell, 1881 (Liocranidae), Ero aphana (Walckenaer, 1802) (Mimetidae), Palicanus caudatus Thorell, 1897 (Miturgidae), Oecobius marathaus Tikader, 1962 (Oecobiidae) and Orthobula impressa Simon, 1897 (Phrurolithidae) and Tropizodium peregrinum Jocqué & Churchill, 2005 (Zodariidae). (Résumé d'auteur

Arthropod diversity is governed by bottom-up and top-down forces in a tropical agroecosystem

International audienceUnderstanding the factors underlying biodiversity patterns is crucial to develop sustainable agroecosystems conserving diversity and the services it provides. The aim of our study was to identify multi-trophic interactions between arthropod trophic guilds in a tropical agroecosystem, while taking the effects of farming practices and landscape complexity into account. To do so, we conducted an experiment in 10 mango orchards on Reunion Island during three consecutive years. In each orchard, we monitored arthropod diversity in two different plots: one plot which maintained customary farming practices and one plot where conservation biological control practices were applied. We used structural equation models to identify the variables that affected the abundance and diversity of different arthropod trophic guilds in two strata in mango orchards: the surface of the ground vs. the mango tree canopy. Links were found to be weak at the trophic guild abundance level on both the surface of the ground and in the mango tree canopy. Conversely, biodiversity mediated complex bottom-up and top-down interactions, including diversity cascades, which differed significantly between the strata. A remarkable difference in the forces affecting herbivore and predator diversity was observed. Herbivore diversity was controlled by top-down forces on the ground, whereas predator diversity was controlled by bottom-up forces in the canopy. These results demonstrate that biodiversity depends on both top-down and bottom-up effects in the tropical agroecosystem community studied here. Interaction directions indicate that conservation biological control based on diverse plant community in ground cover can be an effective lever to foster parasitoid diversity, but not for predatory diversity

Genetic Analysis of Platform-Phenotyped Root System Architecture of Bread and Durum Wheat in Relation to Agronomic Traits

International audienceRoots are essential for water and nutrient uptake but are rarely the direct target of breeding efforts. To characterize the genetic variability of wheat root architecture, the root and shoot traits of 200 durum and 715 bread wheat varieties were measured at a young stage on a high-throughput phenotyping platform. Heritability of platform traits ranged from 0.40 for root biomass in durum wheat to 0.82 for the number of tillers. Field phenotyping data for yield components and SNP genotyping were already available for all the genotypes. Taking differences in earliness into account, several significant correlations between root traits and field agronomic performances were found, suggesting that plants investing more resources in roots in some stressed environments favored water and nutrient uptake, with improved wheat yield. We identified 100 quantitative trait locus (QTLs) of root traits in the bread wheat panels and 34 in the durum wheat panel. Most colocalized with QTLs of traits measured in field conditions, including yield components and earliness for bread wheat, but only in a few environments. Stress and climatic indicators explained the differential effect of some platform QTLs on yield, which was positive, null, or negative depending on the environmental conditions. Modern breeding has led to deeper rooting but fewer seminal roots in bread wheat. The number of tillers has been increased in bread wheat, but decreased in durum wheat, and while the root-shoot ratio for bread wheat has remained stable, for durum wheat it has been increased. Breeding for root traits or designing ideotypes might help to maintain current yield while adapting to specific drought scenarios