Partitioning beta diversity to untangle mechanisms underlying the assembly of bird communities in Mediterranean olive groves

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.