A multiple data source approach to improve abundance estimates of small populations: The brown bear in the Apennines, Italy

► We performed a theoretical and empirical evaluation of multiple data source sampling. ► We produced the first reliable population size estimate for the Apennine brown bear. ► Population size was of 40 bears and density of 32 bears/1000 km2. ► High capture probability is essential to mediate the negative effects of data correlation. ► Multiple data source sampling is particularly suited for small populations of elusive species.