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

Understanding 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.