DENSITY ESTIMATES OF CANE TOADS FROM NATIVE POPULATIONS BASED ON MARK-RECAPTURE DATA

The cane toad (Bufo marinus) has received considerable attention becau se of its rapid spread in Australia and the potential threat it may re present to native species. Although the introduction of pathogens from native populations is now being considered to control this species, p opulation estimates based on comparable methods that demonstrate that native populations are in fact less dense than the introduced ones are not available. Accurate population estimates are necessary to evaluat e potential techniques for the control of the cane toad. We estimated population densities of cane toads over a wide range of habitat types and climate conditions by means of mark-recapture data. The capture hi story and location of toads each night were analysed to explore the va lidity of some of the assumptions of mark-recapture models. Because mi grations, deaths and recruitment over three nights appear to be unimpo rtant, populations may be legitimately regarded as closed for that per iod. However, cane toads seem highly sensitive to disturbance effects due to trapping and/or handling. Consequently, density estimates based on removal methods seem the most reliable because they are not sensit ive to handling and trapping effects. Similarly, analyses of residuals of regressions between I-night counts and density estimates suggested that toad nightly activity is affected by the air temperature during sampling. However, only 60% of the variation in estimated densities ca n be predicted by I-night counts and air temperature. Estimates of pop ulation density over a wide range of habitats in South America were on e order of magnitude lower than estimates in Australia. We speculate o n the possible factors that may account for the lower densities in pop ulations in the native range of the species.