The ongoing biodiversity crisis necessitates increased land and sea protection. While global commitments exist to expand protected areas (PAs), evidence of their effectiveness is mixed, and causal links are rarely established. This research addresses this gap by comprehensively evaluating the impact of statutorily designated PAs in the UK on bird populations. The UK boasts extensive citizen science-based biodiversity monitoring, providing a valuable dataset for this analysis. Birds are a well-studied taxon, with many species of high conservation concern protected under national (SSSI) and European (Natura 2000) legislation. These designations often overlap, with SSSIs aiming for geographical representation and SPAs/SACs targeting specific species or habitats. While PA effectiveness can be measured through Common Standards Monitoring (CSM) for designated features, broader benefits are harder to quantify. This study aims to determine whether PAs are associated with improved bird occurrence, abundance, colonization, persistence, and population trends, while controlling for environmental factors and exploring potential mechanisms like breeding success. Specifically, the study investigates whether the effectiveness of PAs varies with designation reasons (SPAs vs. SACs) and whether rare, declining, or habitat specialist species benefit most. Finally, the study examines whether PA communities are more diverse or better adapted to climate change.

Existing literature on protected area effectiveness presents a mixed picture. While some studies show positive associations between PA extent and biodiversity trends (species diversity and population abundance), others find no such relationship. The impact of PAs is further complicated by the potential confounding effects of land use and habitat type, making it difficult to isolate the effect of protection itself. Furthermore, the variation in species responses to PAs remains largely unexplained. Studies have explored the role of PAs in facilitating species adaptation to climate change, but with high inter-species variability. The lack of causal link testing between PAs and conservation outcomes highlights the need for comprehensive assessments like the present study.

This study analyzed data from three major citizen science programs in the UK: two national bird atlas surveys (1988–1991 and 2007–2011) providing data on species occurrence, colonization, and persistence; the annual British Trust for Ornithology (BTO)/Joint Nature Conservation Committee (JNCC)/Royal Society for the Protection of Birds (RSPB) Breeding Bird Survey (BBS) (1994–2019) for abundance and population trends; and a constant effort mark–recapture program (CES) (1990–2019) for breeding success. Data were analyzed at a 1 km (abundance, productivity) or 2 km (occupancy) scale. The proportion of protected area (PA) coverage within each survey square was calculated for SSSIs, SPAs, and SACs. Generalized additive mixed models (GAMMs) were used to assess the relationships between population metrics and PA extent, controlling for land cover, elevation, climate, and human population density. A statistical matching approach was employed as a complementary analysis. The study examined species-level responses and then explored how responses varied with species traits (rarity, decline status, habitat specialization) using phylogenetically weighted regressions. Finally, community-level analyses examined the effects of PA extent on species richness, diversity, evenness, community specialization index (CSI), and community temperature index (CTI).

The study found that many bird species occurred more frequently and abundantly in areas with greater PA extent, particularly in SPAs. While positive associations were found for colonization and persistence in PAs, no significant effect was observed on abundance trends. The strongest positive associations with PA extent were observed for rare, declining, and habitat specialist species. Higher productivity in PAs was linked to more positive abundance trends, particularly in SPAs. Community analyses revealed that areas with greater PA coverage supported more specialist and cold-dwelling species, with reduced rates of increase in CTI, suggesting a role in mitigating climate change impacts. However, overall species richness was generally lower in areas with more PA.

The findings strongly support the effectiveness of the UK's PA network in conserving bird species, especially those most in need of protection. The results highlight the importance of targeted PA designation, as evidenced by the stronger positive effects of SPAs (bird-focused) compared to SACs. The observed positive associations between PA extent and species occurrence, abundance, and range dynamics, even after controlling for environmental factors, indicate a direct effect of protection. The link between increased productivity and positive population trends in PAs provides a plausible mechanism for these observed effects. The study’s findings support the global ambition to increase PA coverage to 30% by 2030, emphasizing the need for strategic targeting and effective management of PAs to maximize conservation benefits.

This comprehensive assessment demonstrates the positive impact of protected areas on UK avifauna, particularly for rare and declining species. The findings strongly support the global initiative to increase protected area coverage. Future research should focus on refining PA management strategies to optimize their effectiveness and further investigate the interplay between habitat quality, PA management, and species responses.

While the study controlled for several environmental variables, some confounding factors might remain. The complex interactions between PA management practices and species responses were not fully explored due to data limitations. The analysis focused primarily on breeding birds and may not fully reflect the impacts of PAs on non-breeding populations or migratory species. The temporal scale of the study (three decades) may limit the detection of long-term trends or subtle changes.