Estimating the detection probability of the geometric tortoise

Gardner S. ; Baard E.H.W. ; Le Roux N.J. (1999)


The geometric tortoise Psammobates geometricus, found in the Western Cape, is one of the world's rarest tortoises. The cryptic colouration and sedentary behaviour of the geometric tortoise causes problems when conducting counts using standard capture-recapture methods. The primary purpose of this study was to determine the detection probability of a geometric tortoise in the field on a purely experimental basis. Plaster of Paris tortoise models were placed in predetermined transects in the natural habitat. Observes surveyed these transects for the tortoise models. Three group and two individual experiments were conducted. These experiments suggest that a large percentage of geometric tortoises are not found during a census. In individual experiments no observer detected more than 50% of the models, but in some cases 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 tortoise counts.

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