Cemeteries are key urban green spaces with multifaceted societal and ecological importance. Their biodiversity is shaped by unique environmental and cultural factors. They can potentially protect rare and endangered species, yet their conservation value compared with other urban green spaces remains largely unexplored. We sought to fill this gap by systematically reviewing literature to investigate the conservation value of cemeteries relative to other urban green spaces (botanical gardens, institutional premises, natural remnants, and parks) by comparing species richness and proportions of native and unique species. We analyzed data from 70 papers covering 50 cities in 27 countries with linear and binomial mixed-effects models at both site and city level. Cemetery conservation value was similar to urban parks, except for the proportion of unique species, for which parks had significantly higher proportions (21.9% vs. 14.2%, p < 0.001). Cemeteries hosted slightly higher proportions of native species at the city level than botanical gardens (99.7% vs. 99.6%, p < 0.001) and institutional green spaces (96.3% vs. 94.1%, p = 0.034) and proportions comparable to parks and natural remnants (p > 0.05). They also had similar or higher values than institutional premises in species richness and unique species proportions (p > 0.05) and a higher site-level proportion of native species (p < 0.001). In contrast, species richness (slopes = −0.11 and −0.25, respectively) and unique species proportions (4.4% and 6.9%, respectively, p < 0.001 for both) were lower in cemeteries than in remnants of natural areas and in botanical gardens. The conservation value of cemeteries and parks was similar for animals, but parks had a higher value for plants. Overall, cemeteries were generally at least as valuable as some other green spaces for urban biodiversity and mostly native biota. Their religious and cultural significance suggests they will remain intact in the long term; thus, it is essential to prioritize and further promote their biodiversity in conservation and sustainable urban design plans.
