Population Dynamics and Survival of an Endangered Wallaby: A Comparison of Four Methods

Abstract

The bridled nailtail wallaby Onychogalea fraenata is a critically endangered marsupial, now restricted to a single locality in central Queensland, eastern Australia. Its small size and its nocturnal, solitary, and cryptic behavior present problems for monitoring of population dynamics. The population apparently declined during a major drought between 1991 and 1995, when juvenile survival was low. We carried out a detailed demographic analysis from 1994 to 1997 in order to estimate survival and abundance in two different habitats, using mark–recapture, mark–resight, radio-tagging, and line-transect methods. The finite rate of increase (λ) and its sensitivity to changes in survival and reproduction of each age class were also calculated using a matrix population projection model. We then assessed biases and the value of each method for management. For data other than those based on radio-tagging data, population size and survival estimates were negatively biased and had large confidence limits. Line-transect estimates were likely to be the least biased. The reasons were that probabilities of capture and sighting varied among sexes, sites, and times; habitat use varied according to vegetation conditions; and the populations were small. Despite this, all methods indicated an increasing population size. The projected value of λ based on radio-tagging data was most sensitive to adult survival. Vegetation conditions improved throughout the study after an initial drought, and the mean projected intrinsic rate of increase was 28% per year. We conclude that management should aim to increase adult survival via predator control and that line-transect estimation is the most appropriate long-term monitoring method.