Historical baselines reveal suitable conservation landscapes for Javan and Sumatran rhinoceros

Ecological baselines are challenging to establish for tiny surviving populations of threatened species, and historical archives can contain conservation-relevant information about past species distributions and environmental requirements that are unavailable from modern data.^1,2,3,4 The Javan rhinoceros (Rhinoceros sondaicus) and Sumatran rhinoceros (Dicerorhinus sumatrensis) have both experienced catastrophic range collapses due to hunting and habitat loss and are among the world's rarest mammals,⁵ with current conservation planning based upon inferences about ecological conditions in landscapes still supporting remnant populations.^6,7,8,9 We investigated a large-scale dataset of multi-century historical occurrence records of both species within a species distribution modeling framework to define rhinoceros environmental niches, evaluate habitat suitability within landscapes that contain surviving populations, and assess how realized rhinoceros niche space has changed over time. Historical records demonstrate that Southeast Asian rhinos are ecological generalists that formerly occurred across diverse habitats and a broad elevational range. Substantial suitable habitat still exists across Southeast Asia, including within many protected areas that currently lack rhinos, and both species are able to co-occur within many landscapes. Sites that retain remnant rhinoceros populations vary in suitability and may decrease in suitability under future climate change, and our results highlight the importance and ecological feasibility of re-establishing populations across areas of their former range. Older records demonstrate that both species occupied even broader environmental niches during the Holocene, which were reduced through protracted declines pre-dating the recent historical period. These findings demonstrate the need to integrate past and present evidence into conservation planning for the world's rarest species.