A Mediterranean silent spring? The effects of olive farming intensification on breeding bird communities

Combining biodiversity conservation with the agricultural production needed to meet the rising world food demand is a global challenge. This is a case in point for olive farming in the Mediterranean region, where high-yielding intensive and super-intensive orchards are fast expanding, often replacing biodiversity-rich but low-yielding traditional orchards. Here we test the hypothesis that production intensification reduces bird diversity within olive orchards, and that bird responses to such intensification are largely mediated by species traits. Breeding birds were counted in southern Portugal (2014-2016) at 78 orchards covering the entire intensification gradient. Using hierarchical joint species distribution modelling, we found that the intensification-level explained a large proportion of bird community variation (35%), while accounting for the significant contributions of orchard features (16%) and landscape context (16%). Species richness declined from traditional (29 species), through intensive (25), to super-intensive (20) orchards. The occurrence probability of many species also declined significantly from traditional to intensive (7 species; 22% of all species considered) and to super-intensive (14 species; 44%) orchards, while prevalence in more intensive orchards increased only for linnet and goldfinch (6%). Cavity-nester insectivores were the most affected functional group, declining steeply along the intensification gradient. Our results suggest that production intensification, and particularly the shift towards super-intensive systems, is reshaping and greatly simplifying breeding bird communities associated with olive orchards, which may become dominated by generalist granivores. This is a warning signal of widespread negative impacts on bird diversity that may be occurring due to olive farming intensification in the Mediterranean. Efforts are needed to reduce such impacts, which in the case of birds would require actions targeted primarily at cavity-nester insectivores.