A simple mixed model for trend analysis in wildlife populations

This article is concerned with a test for time trend in ecological surveys conducted at multiple sites across several years. The test is useful for studying long-term trends in wildlife populations. We propose a modification of a mixed model—recently suggested by Van Leeuwen, Murray, and Urquhart—which leads to invariance to translation in the origin of the time covariate. The modified model is easily fitted using standard software. Using simulation, the performance of the procedure for un balanced data is shown to be satisfactory. Data on the population density of Thomson’s gazelles (Gazella thomsoni Günther) are used as an example.