Occupancymodellen

Occupancymodellen, or occupancy modeling, is a statistical framework used in ecology to estimate the probability that a given site is occupied by a species while explicitly accounting for imperfect detection during surveys. The two central quantities are the occupancy probability psi (the probability that a site is occupied) and the detection probability p (the probability of detecting the species during a survey, given it is present). This approach separates true occupancy from non-detection caused by survey limitations.

Data for occupancy modeling consist of presence-absence observations collected in repeated visits to many sites, producing detection histories such as 1-0-0 or 0-1-1. By modeling both psi and p, researchers can obtain unbiased estimates of occupancy even when a species is not always detected. Models can be single-season, estimating occupancy within a defined period, or multi-season (dynamic occupancy), which allows occupancy to change over time through colonization and extinction processes.

Covariates can be incorporated to explain variation in psi (e.g., habitat type, elevation, management actions) and p (e.g., observer experience, weather, survey effort). Estimation is commonly performed via maximum likelihood or Bayesian methods, using software and packages in programs such as PRESENCE, program MARK, or R packages like unmarked.

Extensions of the basic model include accounting for false positives, exploring community occupancy with multiple species, and dynamic models that incorporate more complex temporal processes. Occupancy modeling is widely used in wildlife management and conservation planning to assess species distribution patterns, monitor changes over time, and evaluate habitat or policy effects while mitigating biases from imperfect detection.

Key assumptions include the closure of occupancy status within a survey period and independence of detections conditional on occupancy, with limitations arising from limited replication or highly variable detection probabilities.