Threshold responses of birds to agricultural intensification in Mediterranean olive groves

Abstract

Olive groves are the most representative crop of the Mediterranean basin. This agroforestry system is undergoing a strong transformation in recent decades as a result of the agricultural intensification process prevailing in the Northern Hemisphere. Although some authors have suggested that farmland biodiversity responds non-linearly to the complexity of agricultural landscapes, few studies have used community thresholds to identify potential tipping points. Here, we examined the existence of synchronous responses in bird abundance across a gradient of decreasing agricultural intensification in Spanish olive groves. Our study system comprised 25 sites, each consisting of a pair of farms: one with intensive management and the other one with extensive management of the herbaceous cover. We explored whether bird abundances exhibit non-linear threshold responses to ant abundance and tree density at a local (field) scale, plant diversity (both herbaceous and woody plants) at a local (farm) scale, and to proportion of natural habitat at a landscape scale using Threshold Indicator Taxa ANalysis (TITAN). We found a higher incidence of positive responses with decreasing levels of intensification, and these were not restricted to certain guilds or avian families. Few indicator species showed a significant negative response. Thresholds detected were not sharp but rather gradual changes along the environmental gradients. Although they do not necessarily support evidence of ecological thresholds characterized by rapid changes in species abundance or distribution, these patterns can help identify optimal change points for management decisions. Specifically, our results indicate that reaching a minimum threshold of ~85 and 15 species of herbaceous and woody plants, respectively, per unit of surface can entail a gain in terms of bird diversity for olive farms with virtually no detrimental effects. Overall, this study shows that the adoption of agri-environmental measures like the maintenance of ground cover and patches of natural habitat has a positive impact on different species inhabiting this woody crop, and this effect occurs at different spatial scales.