Semi-natural habitat complexity affects abundance and movement of natural enemies in organic olive orchards

Olive orchards have been shown to be able to improve the abundance of natural enemies due to the establishment of adjacent vegetation and ground cover in recent years. Yet evidence regarding the positive effects that such semi-natural habitats provide to the presence and movement of the trophic guilds of the arthropod community is lacking. In this study we assess the effects that plants, both as individual species and as semi-natural habitat assemblages, have on the abundance, presence, and movement of the arthropod community in organic olive orchards. We collected 97 families of arthropods from the canopy of olive trees and the foliage of plants in the ground cover and adjacent vegetation. We analysed the data in relation to habitat complexity. Our results show that the abundance of natural enemies is higher in areas with more complex semi-natural habitats. Parasitoids were able to colonize the olive trees, irrespective of the area or type of vegetation. Predators and parasitoids occurred in the ground cover and adjacent vegetation, but not in the orchard. The adjacent vegetation mainly acted as an important sink for natural enemies when the ground cover withered in June-July, and thus, ground cover and adjacent vegetation may serve as a source of parasitoids and predators for colonizing olive trees. Overall, the density of the natural enemies in organic olive orchards is better enhanced by complex stands of ground cover and natural adjacent vegetation, which gives support to the complex-habitat hypothesis.