POPAN

POPAN is an open-population capture–recapture model used in wildlife biology to estimate population size and dynamics across multiple sampling occasions. It represents a parameterization of open population models, such as the Jolly-Seber framework, by incorporating a super-population concept that includes all individuals who could potentially appear during the study.

The model focuses on five sets of parameters. N^ denotes the super-population size, the total number of individuals that could enter the population over the study period. pent_t is the probability that an individual from the super-population enters the population before sampling occasion t. phi_t is the apparent survival probability from occasion t to t+1. p_t is the probability of capturing an individual at sampling occasion t. N_t represents the size of the population just before sampling at time t and can be derived from N^, pent_t, and survival.

Data for POPAN consist of capture histories collected across a series of sampling occasions. These data are used to jointly estimate the parameters typically via likelihood-based or Bayesian methods. The approach yields estimates of the super-population size, time-specific abundance, entry (recruitment) probabilities, survival, and capture probabilities, providing insight into population dynamics and trends.

Advantages of the POPAN formulation include a clear interpretation of abundance and entry into the population, and compatibility with standard capture–recapture data. Limitations involve assumptions common to open-population models, such as correct mark retention, accurate capture records, and, in some cases, potential identifiability issues in sparse data or when heterogeneity is not modeled. POPAN is implemented in software packages such as MARK and RMark and is widely applied to birds, mammals, and other taxa to inform management and conservation decisions.