One island, three countries: aquaculture ponds explain the cross-border gap in mangrove loss on Borneo

Borneo's mangroves share one coastline, climate and sediment regime, yet they are governed by three countries. Using the annual Global Mangrove Watch v4.1 series and open data, this study asks why mangrove in Indonesian Kalimantan is lost faster than in Malaysian Borneo and Brunei, and tests three explanations: protected-area performance, conversion pathways and El Niño sea-level fall. Between 2010 and 2020, Kalimantan lost 1.23% of its mangrove a year, against 0.43 to 0.74% in Malaysian Borneo and 0.50% in Brunei; it held 59% of the island's mangrove and 77% of its gross loss. Matched comparisons show long-standing strict protected areas reduced loss in Malaysian Borneo by about 2 percentage points but had no detectable effect in Kalimantan. Protection nonetheless explains little of the gap: 84% of it arises outside protected areas, and Sabah's 2015 upgrades of low-loss production reserves to full protection added nothing measurable. Classifying what replaced mangrove lost from 2000 to 2024 shows that conversion to aquaculture ponds, concentrated in the early 2000s, accounts for all of Kalimantan's 15.9-point excess loss, while loss away from ponds was similar in total on both sides of the border (clearing in Kalimantan and Sarawak, mostly edge change in Sabah) and oil palm replaced little mangrove overall. Annual loss outside conversion was lowest in El Niño years and highest in high-sea-level years, consistent with humid mangroves buffered against El Niño sea-level fall. Reducing Borneo's mangrove loss therefore depends on governing pond expansion outside protected areas more than on adding protected areas.