Estimating species tolerance to human perturbation: expert judgment versus quantitative approaches

Species tolerance guilds are frequently used as direct indicators of ecological quality and frequently they are among those with the strongest responses to disturbances. Usually the tolerances are based largely on expert judgment, with little support from ecological or physiological data. This is particularly true for fish of Mediterranean-type rivers, in which there are many basin-endemic taxa with little information on basic life history traits. The main objectives of this study were to compare different quantitative methods for computing species tolerance with qualitative expert judgment classifications. Thirteen alternative approaches were used to compute fish tolerance values for the Mediterranean basins of SW Europe. Three main kinds of approaches were used: (1) based on the concept of niche breath along an environmental gradient; (2) based on deviations from expected values at disturbed sites, as predicted by statistical models; and (3) based on the relative independent contributions of pressure variables to the data variation explained by statistical models. The approaches were evaluated according to their consistency with the other methods and their agreement with the expert judgment-based classifications. Results show that there is a high degree of uncertainty in tolerance estimates that should be accounted for when applying them in ecological assessments.