A rapid survey method for estimating population density of European rabbits living in native vegetation

Abstract

European rabbits are severe environmental pests in Australia but reporting of density-damage relationships has been hindered by a lack of simple methods to estimate the density of rabbit populations in native vegetation. A methodology for quantifying rabbit densities suitable for use in sparse populations of rabbits in conjunction with surveys of the condition of native vegetation is proposed. Dung pellets were counted in 11 629 regularly-spaced quadrats of 0.1 m2 in semiarid, coastal and cool-temperate areas of southern Australia. Mean pellet counts in latrines and the relationship between dung counts outside of latrines and the proportion of quadrats falling on latrines were quantified. This allowed density of dung pellets to be estimated by using a correction factor for latrines instead of attempting to count all pellets within quadrats that included parts of latrines. Rabbit density was calculated from pellet density based on mean pellet size, pellet breakdown rate and estimates of rabbits’ dry matter intake and digestive efficiency. Results were validated against estimates of rabbit density from long-term studies using a combination of spotlight transect counts and burrow entrance counts. The proposed methodology allows estimates of rabbit density in native vegetation to be obtained from just a few hours work and can be used in conjunction with surveys of the condition of native vegetation to quantify rabbit impacts. This methodology is seen as particularly useful in providing a tool to allow rabbit densities to be estimated and then compared with the thresholds, determined separately, at which damage occurs for given ecosystems.