A nationwide collapse of a priority grassland bird related to livestock conversion and intensification

Abstract

Grassland birds are among the most threatened and fastest declining terrestrial vertebrate species in Europe, principally due to agricultural intensification and transformation. The little bustard is a priority grassland bird under the European Directive (2009/147/CE) that led to the classification of a network of Special Protected Areas (SPAs) in Portugal. A third national survey carried out in 2022 reveals a worsening of an ongoing population collapse at a national scale. The population declined by 77% and 56% compared to the previous surveys in 2006 and 2016, respectively. We found that the little bustard has greatly disappeared outside SPAs, while the remaining breeding population concentrated within the protected area network is showing a steep decline at a rate of 9% a year. This decline is now twice as fast when compared to the period 2006–2016. Analysis of the variation of the breeding densities between 2006 and 2022 at 49 survey sites revealed that those that initially had higher bustard densities and shifted toward a higher proportion of cattle among the total stocking rate experienced steeper declines. Areas where the density of roads increased also experienced declines over the course of the study period. Agricultural areas converted to or dominated by beef production likely relate to low breeding success and mortality of nesting females in fodder crops. Still, major habitat conversion outside SPAs to permanent crops led to overall habitat destruction, which contributed to the species decline and range contraction. Other threats are likely acting synergistically such as fragmentation, climate change and anthropogenic mortality. The extinction of the little bustard in Portugal is expected in the short term if no conservation actions are put in place.