The surveys described in this chapter do not require individual identification of wild animals. However there are similarities between the models developed here and various models and approaches elsewhere in the book, for instance on estimating the size of a closed population described in Chapter 3. Of interest is estimating the probability of species occurrence at a set of sites. Thus we are interested in the presence or absence of species, rather than individuals. The fundamental approach was proposed independently by Hoeting et al. (2000), Young (2002), MacKenzie et al. (2002) and Tyre et al. (2003), and has given rise to an appreciable amount of new research and applications since then. In this chapter we shall outline the basic ideas and also illustrate more complex applications, involving data collection along a transect. The experimental paradigm is one where several visits are made to S sites, and on each visit a record is taken of whether a particular animal or plant species is detected at each site. There is a close relationship with the robust design, to be encountered in Section 8.4. Occupancy data are more easily obtained than capture-recapture data. We note here the importance of an appropriate definition of what constitutes a site, and we return to this issue in Section 6.9.

Example 6.1 Tiger footprints Sumatran tigers, Panthera tigris sumatrae, are threatened by both habitat

loss and poaching of the tigers and their prey, such as red muntjac, Muntiacus muntjac and sambar, Cervus unicolor. A study has taken place in the Kerinci Seblat National Park, the largest in Sumatra, in which detection and nondetection of indirect tiger and prey signs, such as footprints, were recorded in 80 2 km2 grid cells in a 963 km2 forest mosaic. Occupancy models have been used to estimate detection probabilities and tiger and prey occupancy parameters; see Linkie et al. (2008).