The potential of occupancy modelling as a tool for monitoring wild primate populations
Primates are a global conservation priority, with half of known species considered threatened with extinction. Monitoring trends in primate populations is important for identifying species in particular need of conservation action, and evaluating the effectiveness of interventions. Most existing primate survey methods aim to measure abundance. However, obtaining estimates of abundance with acceptable precision to detect changes in population is often expensive and time consuming. Evidence from other taxa suggests that estimating occupancy (the proportion of the area used by the species) may be less resource-intensive, yet still provide useful information for monitoring population trends. We investigate the potential of occupancy modelling for monitoring forest primates using a case study of three species of diurnal lemurs in the eastern rainforest of Madagascar. We estimated detectability and occupancy from a survey with three visits to 30 sites. Our estimates suggest that precision in occupancy estimates would be maximized by visiting a larger number of sites (therefore with limited repeat visits) for Indri indri, whereas the optimal monitoring design for Eulemur fulvus and Propithecus diadema, which showed very low detectability in our surveys, involves more frequent visits to fewer sites. Power analyses suggested that a meaningful reduction in occupancy could be detected with reasonable effort for easily detected species, but the method may prove impractical for more cryptic species. Primates pose a number of practical challenges for occupancy modelling, including choosing appropriate survey designs to satisfy closure assumptions. We suggest that if these issues can be overcome, occupancy modelling has the potential to become a valuable addition to the monitoring toolbox for the study of forest primates.