Protected Area Effectiveness Meta-Analysis

Abstract

Protected areas (PAs) are the cornerstone of global biodiversity conservation strategy, yet their effectiveness in retaining species richness, population abundance, and ecosystem integrity relative to unprotected lands remains contested, with individual studies reporting outcomes ranging from substantial biodiversity benefits to negligible effects. This systematic meta-analysis synthesised evidence from 412 primary studies published between 1990 and 2023, encompassing 6,847 PA-unprotected site comparisons across 94 countries, six major biomes, and five taxonomic groups. Random-effects meta-analytic models estimated a pooled effect size of Hedges' g = 0.62 (95% CI: 0.54-0.70, p < 0.001) favouring PAs over unprotected controls for species richness, and g = 0.84 (95% CI: 0.73-0.95, p < 0.001) for vertebrate population abundance. PA effectiveness was significantly moderated by governance quality (beta = +0.38 per unit increase in governance score, p < 0.001), enforcement index (beta = +0.29, p < 0.001), PA age (beta = +0.014 per year, p < 0.001), and area size (beta = +0.18 per log-unit increase in km2, p < 0.001). Strictly protected IUCN Category I-II reserves outperformed multi-use areas (Categories V-VI) by a margin of g = 0.41 (p < 0.001). Substantial heterogeneity was detected across studies (I2 = 74.3%), partially explained by biome, taxonomic group, and PA governance covariates. Publication bias was modest (Egger's test: p = 0.18) after trim-and-fill correction. These results confirm that well-governed, adequately enforced PAs deliver measurable biodiversity benefits, while highlighting governance quality and enforcement as the critical determinants of conservation outcomes.