Capture-recapture (or mark-recapture) methods have been around since the eighteenth century when Laplace used them to estimate the population size of France. For many decades they have been one of the most widely-used methods for assessing and monitoring wildlife populations. They always involve some sampling in space (because animals are captured at specific points in space) but until 2004 there was no statistically rigorous method of dealing with the spatial aspect of capture-recpture methods. As a consequence, while these methods could be used to estimate the number of animals susceptible to capture, they could not be used to reliably estimate the area that these animals occupied, or the density of animals.

Spatial capture-recapture methods take specific account of the capture locations and use these to estimate the probability of capturing animals as a function of where the animals activity centres are. They are able to reliably estimate both the density and the abundance of animals, and are fast replacing non-spatial capture-recapture methods for assessing wildlife populations. They can also be used without captures, but with detections instead. They have been used with visual surveys, with acoustic surveys, with mist nets, with camera traps, with pitfall traps, to detect animals themselves, or their sounds, the genetic material they leave in hair snares or in dung, and their footprints, and for a very wide range of species.

leopardarea

The Figure shows some leopard camera trap data and the contours of the resulting estimated probability of detecting an animal with activity centre anywhere in the survey region (blue lines). The red crosses are camera traps and the coloured dots are individual leopard captures, with captures of the same individual joined by lines.


Recapturing individuals

How do you know you captured the same individual again that you captured before? For this you need to identify the individual animals. There are some animals that you can identify to the individual using their natural markings relatively easily. Have a closer look at the jaguar in the mural, for example, and pay attention to the pattern of black spots. If you compare this pattern with that of any other jaguar you will find that they differ. In fact, each individual jaguar can be identified by its spot pattern (just like a human being by their finger print).

There are a few other animals in the mural that we can identify to the individual by using their markings. These are the humpback whale (pattern on the underside of the fluke), Blainville's beaked whale (markings on their back and dorsal fin), loggerhead turtle (pattern on their back), grey seal (pattern of spots) and giraffe (pattern of dark patches).

jaguar

Capture-Recapture

Capture-recapture methods have been around since the eighteenth century when Laplace used them to estimate the population size of France. The method relies on the ability to recapture the same animals on more than one occasion. For many decades they have been one of the most widely-used methods for assessing and monitoring wildlife populations. They are rapidly being replaced by spatial capture-recapture methods.


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