What is SECR?

Spatially explicit capture–recapture (SECR) deals with observations at an array of detectors (traps) that may be summarised as spatial encounter histories something like this:

             Occasion
ID    1    2    3    4    5
1   A9 . .   . .
2   A12 A12    .   .    .
3   . .   C6 B5    .
4   . .   G3    .   F3
...

Each entry (e.g. A9) records the detector at which a known animal (ID) was observed on the given occasion (sample time). '.' indicates no detection. Each detector has known x-y coordinates.

For 'proximity detectors' multiple entries are possible on each occasion.

The primary aim of SECR is to estimate the population density of free-ranging animals (Efford 2004; Efford, Dawson & Robbins 2004; Borchers & Efford 2008; Efford, Borchers & Byrom 2009, Efford, Dawson & Borchers 2009). Animals are assumed to be distributed independently in space and to occupy home ranges.

As in conventional capture–recapture (e.g. Williams et al. 2002), the approach is to fit a model that includes both population parameters and parameters for the detection process. In SECR, the basic population parameter is density D, rather than population size N. The detection process is represented by a mathematical function that describes an animal's declining probability of being detected as its home range centre gets further from a detector. A simple detection function has the parameters g₀ (intercept) and σ (spatial scale). We fit the model to obtain estimates of D, g₀ and σ. Population size N may be estimated as a derived parameter.

Williams BK, Nichols JD, Conroy MJ 2002. Analysis and management of animal populations. Academic Press, San Diego.