Age-stratified interview campaigns suggest ongoing decline of a threatened tortoise species in the West African Sahel

Face-to-face interviews with local populations are often used to determine the distribution and population trends of elusive threatened species. Although interviewee responses may suffer from some bias, historical trends in the status of a species can be investigated from age-structured questionnaires. In this paper, we tested this idea by analysing separately answers given by older (> 60 years age) and younger respondents (25-44 years old) on the status of the African spurred tortoise, (Centrochelys sulcata), a charismatic large reptile listed as Vulnerable in the IUCN Red List. We interviewed 619 persons (hunters/farmers/cattle farmers) of different ages in three of the species’ habitat countries (Burkina Faso, Niger and Nigeria). Interviewees were asked whether in their experience the tortoise was common, rare or absent. By using Generalized Linear Models we showed that the probability to answer “common” increased with age in Nigeria and Burkina Faso, whereas the probability of responding “absent” declined with age in Nigeria and Niger. There were no significant effects of age for the answer 'rare' in any country and no differences were found between villages in any of the studied countries. From our data we conclude that spurred tortoises have been extirpated in 16.7% of study sites. We argue that if statistical differences emerge between answers given by respondents of various age classes on the population status of a target species, it is possible to conclude that the species’ situation may have significantly changed during the last 30-40 years.