To increase or not to increase? The impact of terrestrial protected areas on tree cover and the risk of species extinction.

Over the past two decades, protected areas (PAs) have been central to biodiversity conservation. Global initiatives like the 2030 Agenda and Aichi Targets have spurred the expansion of marine and terrestrial PAs. Despite extensive research on PA, evaluation of their effectiveness has examined deforestation, poverty, and species impacts, mostly using discrete treatment methods; however, research using continuous treatment approaches, particularly with biodiversity indices set by the Convention on Biological Diversity (CBD) and considering IUCN protection categories, remains limited. This study addresses this gap by applying a novel difference-in-differences method with heterogeneous and continuous treatment to assess the impact of PAs on the Red List Index (RLI) and Tree Cover Loss (TCL). We use a panel of 144 countries from 2001 to 2020 to estimate average and dynamic effects for strictly PA (IUCN categories I-II) and other IUCN categories. Results show no statistically significant average effect of PA (all, strict, not strict, or any individual category) on RLI and TCL. However, statistically significant dynamic effects were identified. Species extinction risk (RLI) decreases by 0.68 percentage points 8 years after a 1 % increase in strict PA. Also, TCL decreases by 1.52 percentage points after 17 years, when category IV PAs increase by 1 %. These results illustrate the need for robust evaluation frameworks in conservation and question the effectiveness of percentage area conservation targets. It also provides some evidence of the differential effects on biodiversity of distinct PA categories.