Estimation of Animal Abundance When Capture Probabilities Are Low and Heterogeneous

Abstract

Obtaining reliable estimates of abundance or relative abundance under conditions of low numbers of captures and recaptures is crucial to properly assess population status of species that are of management concern; however, these characteristics make estimation difficult. We applied the commonly used jackknife (Burnham and Overton 1978, 1979) and moment (Chao 1988) estimators of abundance to capture-recapture data from northern flying squirrel (Glaucomys sabrinus) populations that had low (p ≅ 0.10), heterogeneous, capture probabilities and low densities (approx 2 squirrels/ha). The jackknife estimator selection procedure, higher-order jackknife estimators, and moment estimator were sensitive to the number of trapping occasions. These estimators tended to have low precision. Comparisons of estimators suggested specific, lower-order jackknife estimators performed well. Monte Carlo simulations corroborated results from field data. The moment estimator tended to have low bias, but the high root mean square error made the estimator less reliable than lower-order jackknife estimators. First- and second-order jackknife estimators tended to be the most reliable (low bias and precise) estimators when the number of trapping occasions (t) was ≥ 12. However, confidence interval coverage (% replications in which the constructed confidence interval included true N) was low with the first-order jackknife estimator, reflecting the negative bias of the variance estimator. We improved confidence interval coverage by an ad hoc adjustment to the variance estimator; coverage with the adjusted estimator approached the nominal 90% level at t ≥ 12. Reliable estimates of abundance can be achieved under conditions often encountered in field studies (small N and low, heterogeneous, capture probabilities) with lower-order jackknife estimators, a modification of the variance estimator for the first-order jackknife estimator, and ≥ 12 trapping occasions