Projected development in Borneo and Sumatra will greatly reduce connectivity for an apex carnivore.

The islands of Borneo and Sumatra are strongholds for biodiversity and home for many endemic species. They also have experienced amongst the highest deforestation rates globally. Both islands are undergoing massive, rapid infrastructure development, leading to further deforestation and habitat fragmentation. Here, we identify priority areas for continued functional forest connectivity across Borneo and Sumatra, using spatial models of clouded leopard (Neofelis diardi, a forest indicator species) movement, and impacts thereto from existing and future infrastructure development. We specifically measure and map the anticipated impacts on forest functional connectivity of three major infrastructure development projects (Pan Borneo Highway, Trans-Sumatran Toll Road, and the new Indonesian capital city of Nusantara). We found that core clouded leopard habitats are already highly fragmented in Sumatra, constituting only ∼13 % of the island, with potential dispersal corridors still linking some habitat fragments. In Borneo, clouded leopard core habitats cover 34 % of the island, with one large central core area and several much smaller satellite cores, which are largely unprotected (15 % protected, compared to 42 % in Sumatra). The largest negative effect on habitat connectivity was predicted for Nusantara (66 % of the total connectivity loss predicted for all three infrastructure projects), reverberating across the entirety of Borneo with the strongest effects in East Kalimantan. The Pan Borneo Highway accounted for 28 % of the total connectivity loss, affecting every province in Borneo and Brunei, with 6 % of this decrease located within protected areas. The Trans-Sumatran Toll Road had the smallest negative effect on connectivity (6 %) but only when excluding the already built segments, which, when included, produce a total negative impact similar to that of the Pan Borneo Highway.