Density estimates of cane toads from native populations based on mark-recapture data

In: Wildlife research, Band 23, Heft 3, S. 305

Abstract

The cane toad (Bufo marinus) has received considerable attention because of its rapid spread in Australia and
the potential threat it may represent to native species. Although the introduction of pathogens from native
populations is now being considered to control this species, population 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 evaluate 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 history and location of toads each night were
analysed to explore the validity of some of the assumptions of mark-recapture models. Because migrations,
deaths and recruitment over three nights appear to be unimportant, populations may be legitimately regarded
as closed for that period. However, cane toads seem highly sensitive to disturbance effects due to trapping
andlor handling. Consequently, density estimates based on removal methods seem the most reliable because
they are not sensitive to handling and trapping effects. Similarly, analyses of residuals of regressions between
1-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 can be predicted by 1-night
counts and air temperature. Estimates of population density over a wide range of habitats in South America
were one order of magnitude lower than estimates in Australia. We speculate on the possible factors that may
account for the lower densities in populations in the native range of the species.