Influence of protected areas and socioeconomic development on litter contamination: A global analysis

Anthropogenic litter (AL) contamination is a pervasive global issue, particularly in aquatic environments. Although protected areas (PAs) are designed to conserve biodiversity, they are not immune to AL contamination, and their effectiveness in mitigating it remains poorly assessed. Here, we conducted a systematic analysis of AL levels in aquatic environments worldwide over the last decade, focusing on protection status and management levels, by compiling 6049 records from 298 articles. AL contamination was seven times higher in unprotected areas, which were predominantly categorized as dirty or extremely dirty, whereas most PAs were classified as very clean, indicating partial effectiveness in reducing AL accumulation. Plastics accounted for 68.0 % of all litter, and cigarette butts were the most common item (11.2 %). There was an edge effect, with higher AL contamination observed near the PAs boundaries, underscoring their vulnerability to external anthropogenic pressures. In both protected and unprotected areas, AL levels initially rose with increasing economic development. However, in unprotected areas AL declined at higher development levels, showing an inverted U-shaped relationship and suggesting reduced contamination in more developed contexts. These findings highlight the role of PAs in reducing AL contamination and underscoring the urgency of advancing the global sustainable development agenda. Nevertheless, economic development alone may not be sufficient to reverse the transboundary problem of global litter contamination, emphasizing the need for integrated strategies across the entire production chain.