Combining Open and Closed Models
A range of models for closed populations are described that allow for such things as heterogeneity and variable catchability that are difficult to deal with in open populations. Pollock, in 1982, introduced the simple but elegant idea of combining models for both closed and open populations (the so-called “robust” design) by dividing the study period into primary and secondary periods where the primary periods are well spaced in time, but within each primary period there is a series of short secondary periods. The primary period is regarded as being of short enough duration for the population to be regarded as closed then. This approach has many advantages, for example, there is a variety of closed population models that can be used. Also, recruitment can be separated into immigration and birth with at least two age classes, and various types of migration can be incorporated. A number of versions of the robust design are described in detail, and recaptures, resightings, and dead recoveries can be introduced in various ways into the model. Transients can be modeled also. The chapter ends with a comparatively new topic called spatial capture–recapture models, which is introduced here as they can be incorporated into the closed parts of a robust model. This topic will no doubt eventually warrant its own chapter as extensions to open populations are considered.