Introduction

Protected areas constitute key tools for conserving biodiversity (Marton-Lefèvre, 2014; Watson et al., 2014). The European Union (EU) has set up the largest coordinated network of protected areas in the world—Natura 2000. Covering 18% of EU’s land area, it comprises Special Protection Areas (SPA) and Special Areas of Conservation (SAC) designated respectively under the Birds Directive (2009/147/EC) and the Habitats Directive (1992/43/EEC). EU countries are required to manage Natura 2000 sites to maintain or improve the conservation status of species and habitats listed in these Directives. Therefore, monitoring species populations, particularly those from target species, is essential to evaluate the effectiveness of Natura 2000.

Most research evaluating the effectiveness of the Natura 2000 protected area network has focused on assessing its spatial coverage of biodiversity values (e.g., Cabeza, 2013; Abellán & Sánchez-Fernández, 2015), but much less is known about its effect on species persistence or population trends (e.g., as a consequence of habitat loss or climate change) due to a lack of temporal data (e.g., Pellissier et al., 2013). Most recent studies compare population trends across species with different conservation priority levels (e.g., Sanderson et al., 2016), while rigorous studies based on empirical designs to compare conservation outcomes in areas with and without exposure to conservation policy instruments are still scarce (Miteva, Pattanayak & Ferraro, 2012). Even for taxa with abundant information on population sizes, such as birds, few studies have assessed the effectiveness of the SPA network on the conservation status of target species (Orlikowska et al., 2016).

Farmland birds are of high conservation concern in Europe, showing a steep population decline across the continent mainly due to ongoing intensification of agricultural practices (BirdLife International, 2004; Donald et al., 2006). At the European scale, however, few studies assessed whether SPAs designated in agricultural land are delivering any positive effects on farmland bird populations (Gamero et al., 2017). For example, Pellissier et al. (2013) used a national breeding bird survey to contrast species population trends in the period 2001–2010 in plots located within and outside Natura 2000 sites in France and found no significant differences. They concluded that the network of protected areas was established too recently to allow an assessment of its influence on population trends. In contrast, and at a larger scale, Gamero et al. (2017) used country wide information for a set of 25 EU countries. This study reported that species listed in Annex I of the Birds Directive (species with higher conservation status and for which EU Member States are obliged to implement special conservation measures) had higher population growth rates (during 1981–2012) in countries with a higher proportion of land designated as SPA. They conclude that EU policies seem to generally attenuate the declines of farmland bird populations, but do not reverse them.

The Portuguese farmland bird SPA network was created between 1994 and 2008, and consists of 13 areas covering over 195,000 ha which were delineated based on detailed information on the spatial patterns of occurrence of priority species listed in Annex I. Here, we assess the effectiveness of this SPA network in maintaining the population of a priority farmland bird, the little bustard Tetrax tetrax, a species that has undergone a major decline and breeding range reduction since the beginning of the last century (Iñigo & Barov, 2010). We expected that populations within SPA would show more favourable trends when compared to populations in farmland areas outside SPAs. For this purpose, we compared estimates of breeding population size and density over a decade (2003–2006 to 2016) within and outside SPAs. We further assessed trends in the availability of grasslands comprising fallow lands and extensive pastures, the preferred breeding habitat of the species (Morales, García & Arroyo, 2005; Silva, Palmeirim & Moreira, 2010; Moreira et al., 2012).