Assessing Growth Rates of European Rabbit Populations Using Spotlight Transect Counts

Abstract

Reliable estimates of population growth rates and densities are fundamental for effective wildlife management programs. We assessed the precision of spotlight-counting as a technique to monitor changes in rabbit abundance by fitting simple population growth models to observed trends in spotlight-count data during an 18-month period of increase in European rabbit (Oryctolagus cuniculus) abundance at sites in North Canterbury, New Zealand. A model-selection approach identified a simple exponential population growth model, allowing the observed rate of increase to differ between spring-summer (Aug-Feb) and autumn-winter (Mar-Jul) to be an adequate description of the observed data. The exponential rate of increase (r) over the study period differed markedly between spring-summer and autumn-winter periods, and averaged 2.3 yr -1 and 2.5 yr -1 for the 2 sites monitored. We used the fitted exponential model to estimate the observation error per spotlight transect. The estimated coefficient of variation for 1 night of counting of 10-km transects was 16%. With such data and model assumptions, only a modest number of surveys would be required to detect, with a high degree of certainty, moderate changes in spotlight-count indices of rabbit abundance either by analysis of variance or log-linear regression.