Benefits of organic olive farming for the conservation of gleaning bats

Abstract

Current intensification and expansion of agricultural lands are some of the main anthropogenic processes driving the global decline of biodiversity. Organic farming is generally regarded as a better compromise between production and ecosystems and biodiversity preservation. However, while this practice is gaining popularity worldwide, conventional agriculture is still the main approach, hindering the conservation of many taxa. Bats are poorly studied and generally negatively affected by conventional farming. Their high mobility and long lifespan make them excellent ecological indicators in agroecosystems. We assessed the effect of different crop treatments (conventional and organic olive groves, and sparse coniferous forest as a control) on bat activity, at both guild and species level. In addition, we evaluated whether bat activity was influenced by the abundance of the olive fruit fly Bactrocera oleae, the major insect pest of olive groves worldwide. Bats were surveyed acoustically during autumn 2014 in all treatments using passive ultrasound detectors. In parallel, pheromone traps for B. oleae were used to monitor pest insect abundance. Our results show that aerial hunting bats were significantly more active in sparse coniferous forests compared to the other treatment areas. On the contrary, gleaning bats (those generally more threatened and vulnerable to habitat degradation) showed higher activity rates in organic olive groves. Due to their higher manoeuvrability and slow flight, gleaning species are generally well-adapted to forage in structurally complex and cluttered habitats such as olive groves. A significant negative relation was found between the density of the olive fruit fly and the gleaning bats activity, which were expected to prey on the pest and hence show some positive relation with its density. The reasons are unclear and further research with molecular techniques would be needed to better understand the ecological interaction, if any, between bats and the pest. Organic olive groves, usually more stratified than the conventional ones, are characterized by the presence of spontaneous herbaceous cover and higher diversity of arthropods (either beneficial or deleterious), which favours bat activity. Organic practices should be further prioritised in the agri-environment schemes of the European Union and those of its individual members. If organic farming is not widely implemented, agriculture intensification and the expansion of monocultures may put bat populations at stake, as well as compromise ecosystem quality and the conservation of biodiversity.