Time-for-space substitution in N-mixture modeling and population monitoring

Population size is a fundamental state variable in ecology, and the analysis of temporal variation in abundance (i.e., detection of trends) is a prime objective in wildlife monitoring. However, population abundance cannot be directly observed because part of the population remains undetected and methods that account for imperfect detection are often not used. Capture-mark-recapture approaches give reliable estimates of abundance but are time- and effort-consuming. In the last decade, the application of hierarchical, or N-mixture, models that use repeated counts of unmarked animals seem to give great advantages in the estimation of population size. Hierarchical models require repeated surveys at multiple sites, but sometimes only data obtained for a single site in successive years are available. We applied the time-for-space substitution implemented within the N-mixture modeling framework to estimate population size and evaluate the dynamics of the endangered European leaf-toed gecko (Euleptes europaea) surveyed >20 years. We compared these results with capture-mark-recapture estimates obtained from the same population and over the same time period. Estimates and trends were comparable and both methods indicated similar population declines; moreover, N-mixture modeling indicated temperature affected detection. Therefore, the application of the time-for-space substitution in hierarchical modeling seems valuable and may be useful in species monitoring and conservation. (c) 2019 The Wildlife Society.