Assessing changes in habitat quality due to land use changes in the spur-thighed tortoise Testudo graeca using hierarchical predictive habitat models

In this study we propose a model-building approach based on the hierarchical integration of the main environmental factors (climate, topography/lithology, and land uses) determining the distribution of the spur-thighed tortoise in south-east Spain. Data on the presence/absence of the species were primarily based on information derived from interviews to shepherds. The hierarchical modelling exercise consisted of three steps. First, we constructed a model for the entire region using climate variables, thus obtaining a potential climatical model. Second, we introduced variables referring to topography and lithology that fall within the climatic distribution range ( potential model). Third, by using this second model as a starting point, we included land use variables to obtain the actual distribution model.
We analysed the changes in the values of probability of the presence of this species for a given cell between the potential and the actual model, assessing areas where habitat quality has decreased, been maintained or increased. The spatial representation of these changes was highly coherent. A discriminant analysis linked areas where habitat quality has dropped with agriculture landscapes, whereas those areas where habitat quality has been maintained or increased were located mainly in shrublands. Twenty-five per cent (479 km²) of the potential distribution of the species became suboptimal when land use was included, which emphasizes the importance of land use changes in both the range dynamics and the conservation of the spur-thighed tortoise in south-east Spain.