EVALUATING POPULATION-SIZE ESTIMATORS - AN EMPIRICAL-APPROACH

We used nine closed populations of gray-tailed voles (Microtus canicau dus) in 0.2-ha enclosures to empirically select the best fit among 11 probabilistic estimators of population size. We also examined the infl uence of population size and number of trap occasions on performance o f estimators. Population size was known in all instances, providing a basis for comparison of performance of estimators. Three replicates of three population sizes (30, 60, and 90 voles/enclosure) were used in this experiment. The most accurate and precise estimators, selected on the basis of four consecutive trapping occasions, were the Pollock an d Otto's M(bh), Chao's M(h), and jackknife estimators. Examination of the hypothesis tests included in the Model Selection Procedure of the program CAPTURE identified individual heterogeneity as the prevailing source of variation in capture probabilities and suggested that the ap propriate estimator would be the jackknife. Reliability of the heterog eneity estimators (the jackknife and Chao's estimators for M(h) and M( th)) was positively related to population size, whereas reliability of almost all of the other estimators varied inversely with population s ize. The jackknife estimator was unique in the stability and quality o f its performance in the first few trap occasions. Using the jackknife estimator and three trap occasions offered the best tradeoff between reliability and trapping effort.