Aim: We investigated taxonomic and functional beta diversity of bird communities inhabiting Mediterranean olive groves subject to either intensive or extensive management of the ground cover and located in landscapes with different degrees of complexity.

Location: Andalusia, southern Spain.

Methods: We partitioned taxonomic and functional beta diversity into its two additive components, turnover and nestedness. We also explored the contributions of single sites to overall beta diversity (LCBD) and separated the effects of species replacement (turnover) and richness difference (nestedness) in order to identify ecologically unique sites -keystone communities- within the metacommunity. In a further step, we employed abundance- and functional-based indicator species analyses to characterize bird assemblages. 

Results: Taxonomic beta diversity increased with landscape complexity. Although both taxonomic and functional differences among assemblages were driven mainly by species replacement (regardless of management or landscape type), the contribution of trait replacement to the total functional beta diversity was much lower, suggesting that species performing similar functions replace each other between sites. There were no differences in LCBD between management types or categories of landscape complexity, but the contributions of sites to beta diversity decreased as the percentage cover of olive groves increased. Species richness was also important in explaining variation in LCBD as species-poor sites tended to contribute the most to the local-to-regional beta diversity. However, some farms displayed high values of LCBD due to the existence of a high replacement component, indicating that some species recorded in these sites were scarce elsewhere. The indicator species analyses revealed that the woodchat shrike Lanius senator may constitute an excellent indicator of biodiversity in this agro-forestry-system. 

Main conclusions: Our results show that agricultural expansion promotes biotic homogenization and exemplify how the identification of both keystone species and communities can represent a powerful tool for the management of anthropized landscapes.

Bird censuses carried out in the framework of the LIFE project 'Olivares Vivos' (https://olivaresvivos.com)

Surveys were conducted using point census stations. At each count point, all individuals detected -seen or heard- within a 100-m radius were identified and counted for 5 min.

We set six stations in small olive farms (<25 ha) and 10 in large olive farms (>25 ha). To avoid repeated counts between neighbor census stations, they were located at least 200 m apart in small olive groves and at least 300 m apart in large olive groves. Surveys were carried out monthly in two slots (from April to June 2016, and from September 2016 to March 2017) under favorable weather conditions by a team of skilled ornithologists. 

Monthly data (abundances) were merged into a single plot x species quantitative matrix used as input in beta-diversity analyses.  

Bird abundance data (97 species) for each olive farm (plot). Each farm is catalogued according to the level of landscape complexity (high; intermediate; low) and the type of management of the ground cover (extensive or intensive).