Assessing the conservation status of species is essential for implementing appropriate conservation measures. A lack of evidence of threats, rather than showing an absence of impacts, could reflect a lack of studies on how human activities could result in species population declines. The range of Prince Bernhard's titi monkey Plecturocebus bernhardi is restricted to the Arc of Deforestation, a deforestation hotspot in south-eastern Amazonia. Despite this, it is categorized as Least Concern on the IUCN Red List. To reassess the conservation status of P. bernhardi, we carried out surveys during 2015–2017 to delimit the geographical distribution of the species and estimate its population density and abundance. We then used spatial predictive modelling to examine future habitat and population loss within its range. Plecturocebus bernhardi occurs over an area of 131,295 km2. Its mean group size was 2.8 individuals/group and its density 10.8 individuals/km2 and 3.8 groups/km2. Habitat loss was estimated to be 58,365 km2 (44.5% of its current range) over the next 24 years (three P. bernhardi generations) under a conservative governance model of deforestation and 105,289 km2 (80.2%) under a business-as-usual model. These numbers indicate that P. bernhardi is threatened and should be categorized as Vulnerable, at least, using the IUCN Red List criteria. We recommend the reassessment of other Least Concern primate species from the Arc of Deforestation using a similar approach.

Tropical forest hotspots have a high diversity of species but have lost > 70% of their original vegetation cover and are characterized by a multitude of small and isolated fragments. Paradoxically, conservation actions in these areas are still mainly focused on protection of large tracts of forests, a strategy now infeasible because of the small area of forest remnants. Here we use the Vulnerable black-handed titi monkey Callicebus melanochir as a model to study the effects of habitat loss, fragmentation and degradation on arboreal mammals and to provide insights for science-driven conservation in fragmented landscapes in tropical forest hotspots. We surveyed 38 Atlantic Forest fragments in Bahia State, Brazil and assessed the effects of patch area, quality and visibility, and landscape connectivity on the occurrence of our model species. Patch area was the single best model explaining species occurrence. Nonetheless, patch quality and visibility, and landscape connectivity, positively affect occurrence. In addition to patch area, patch quality, patch visibility and landscape connectivity are useful for predicting the occurrence of arboreal mammals in the fragments of tropical forest hotspots. We encourage the assessment of habitat quality (based on remotely sensed vegetation indices) and habitat visibility (based on digital elevation models) to improve discoverability of arboreal mammal populations and selection of fragments for conservation purposes across fragmented landscapes of tropical forest hotspots. Large remnants of tropical forest hotspots are scarce and therefore we require baseline data to support conservation actions and management in small forest fragments.