Road Development in the Tropical Forests of Southeast Asia and the Pacific: A Systematic Review of Forest Loss, Ecological and Health Effects, and Mitigation

Background Road networks in Southeast Asian and Pacific tropical forests are expanding through highways, logging and plantation roads and unmapped access roads. This review synthesized primary evidence on road effects on forest loss, ecology and human health, and evaluated mitigation. Methods Scopus, PubMed and OpenAlex were searched on 7 October 2026 for English-language journal articles published from 2015 (PRISMA 2020). Reviewer 1 and Reviewer 2 independently screened 496 records (κ = 0.94) and assessed 80 full texts (κ = 0.80); the Adjudicator resolved disagreements. Studies were appraised with MMAT 2018, synthesized following SWiM and rated with GRADE-CERQual. Results Sixty-five studies were included, most from Malaysia, Indonesia and Thailand; 19 reported forest-loss, 47 ecological and six mitigation outcomes. Forest cover loss was greater nearer roads, at higher road density or length and after construction or completion in nine studies, and no study showed less loss (moderate confidence). Loss extended to 17.7 km with secondary roads in New Guinea (moderate). Structural edge effects reached 34 m at a logging road and 350 m at a highway, whereas soil effects stayed largely on the road surface (moderate). Reptiles or amphibians led roadkill in all seven surveys recording every vertebrate class, and mortality concentrated on short sections near water (moderate). Paved roads reduced crossing by elephants and gibbons and were linked to genetic differentiation in some taxa (moderate). Crossing structures were used by mammals and a large snake (moderate); measured effects rested on single spatial comparisons (very low). No study reported a human health outcome, and fire, landslides and water quality were unmeasured. Conclusions Roads act on these forests at two scales: kilometers for access-driven clearing, and tens to hundreds of meters for edge, mortality and barrier effects. Assessment boundaries and mitigation should match each mechanism’s scale, and mitigation needs before-after evaluation.