Estimating the detection probability of the geometric tortoise

The geometric tortoise Psammobates geometricus, found in the Western Cape, is one of the world's rarest tortoises. The cryptic colouration and sedenta ry behaviour of the geometric tortoise causes problems when conducting coun ts using standard capture-recapture methods. The primary purpose of this st udy was to determine the detection probability of a geometric tortoise in t he field on a purely experimental basis. Plaster of Paris tortoise models w ere placed in predetermined transects in the natural habitat. Observers sur veyed these transects for the tortoise models. Three group and two individu al experiments were conducted. These experiments suggest that a large perce ntage of geometric tortoises are not found during a census. In individual e xperiments no observer detected more than 50% of the models, but in some ca ses this percentage was slightly higher in group experiments. The effect of the size of the tortoise, density of the habitat, level of experience and searching ability of the observers are discussed. Estimates of the various detection probabilities are derived and suggestions made for conducting tor toise counts.