Assessment of large-vertebrate species richness and relative abundance in Neotropical forest using line-transect censuses: what is the minimal effort required?

Line-transect sampling is a strategy commonly used to assess richness and abundance of large diurnal vertebrates in tropical forests, but the relationships between the sampling effort (measured as transect length in km) and the accuracy of the estimates based on the field data have rarely been investigated. Using data from 17 distinct surveys in French Guiana, we demonstrated that 85 km of transect are sufficient to extrapolate species richness whatever the forest type and the disturbance level of the habitat. Concerning species abundances, reliable estimations were obtained after 40-90 km of transects for large birds, howlers, tamarins, and agoutis. In contrast, relative abundances of capucins, sakis, and ungulates, were still not stabilized after 100 km but can still be reliably assessed with this effort. These species have larger home range than the former, and the accuracy of abundance assessment may be related to use of space. Since species with small area requirements regularly use their entire home range, abundance prediction with a moderate sampling effort may be facilitated. On the contrary, species with large home ranges may exhibit strong seasonal habitat partitioning, therefore decreasing the accuracy of abundance estimation on a low-effort survey. This analysis provides the first evidence of the minimal efforts required to assess large vertebrate richness and relative abundance of some species in a neotropical rainforest. We encourage similar works on other sites, to collect additional information on the influence of forest productivity and species assemblage composition on the minimal required sampling effort. This would permit confident extrapolations of species richness and abundance in other Neotropical forests and may provide efficient guidelines to integrate the predictive analytical tool developed in this work in future biodiversity management plans.