A systematic review of the effectiveness of protected areas at reducing threats to biodiversity

Background Protected areas (PAs) are central to global biodiversity conservation efforts, yet their effectiveness in reducing threats is context-dependent and poorly understood. This systematic review evaluates how well PAs mitigate anthropogenic threats to biodiversity by synthesizing findings from peer-reviewed and grey literature. Specifically, we assess how threat levels change in relation to protection status, over time, and in response to specific management interventions. The review aims to evaluate threat levels inside PAs relative to appropriate comparators or interventions, identify variation across threat types, and highlight key factors associated with both successful and unsuccessful conservation outcomes. Methods We conducted a comprehensive systematic review, screening 5,687 unique articles and including 105, that assessed changes in threat within at least one PA compared to other locations, time periods, or types of interventions. A descriptive, narrative synthesis was performed to evaluate the strength of study designs and the influence of methodological differences on reported outcomes. The synthesis focused on three key dimensions: (1) the proportion of studies reporting reduced threats inside PAs over time or relative to comparators; (2) effectiveness across different threat types; and (3) contextual and management-related factors influencing PA performance. Review findings Our review revealed three major findings: Firstly, PAs on average had lower threat levels compared to unprotected areas, but effectiveness was mixed. Most studies reported increasing threats both inside and outside PAs. Nonetheless, threats tended to be lower within PAs boundaries. Secondly, the threat reduction potential depends on the type of threat. PAs were more effective at reducing land-use and land-cover change (LULCC), fire, and hunting. Studies frequently reported lower levels of deforestation, agricultural expansion, and infrastructure development within PAs. However, PAs were less effective in addressing threats like invasive alien species, pollution, and recreation, likely due to external origins or indirect impacts that extend beyond formal boundaries. Thirdly, PA outcomes were shaped by multiple interrelated factors. Effectiveness was generally lower in areas with high baseline threats, high human population density, and easy access. In contrast, strong governance, adequate funding, robust enforcement, large PA size, and alignment with national development goals and governance were associated with more positive conservation outcomes. Conclusions Despite rising human-induced threats across landscapes, PAs demonstrated better performance than unprotected landscapes in reducing threats to biodiversity. However, their success is highly dependent on the context, including the type of threat, biophysical characteristics, and socio-economic context where PAs are located. This review highlights the urgent need to enhance the effectiveness of existing PAs and offers important insights for the planning and implementation of future ones. The potential of PAs to reduce threats is not solely determined by their design or location but also by how well they are integrated into the local socio-ecological systems in which they operate. Strengthening governance, improving management strategies, and ensuring the meaningful involvement of local stakeholders are essential to ensure long-term conservation outcomes. Additionally, this review underscores the need for standardized methodologies, consistent threat assessment metrics, and rigorous evaluation frameworks to better understand what drives PA effectiveness and to support evidence-based decision-making.