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A camera trap is a camera arranged to record animals at a location without a person continuously operating it there. The trigger and sampling procedure determine what becomes a recorded encounter. Images can reveal occurrence, activity or recognizable individuals, but a photograph is not automatically an independent animal count.

In a 1995 tiger study, cameras along travel routes produced 31 photographic captures of ten identified animals over nine sampling occasions. Researchers analyzed the resulting encounter histories with mark and recapture models, including variation in detection probability among individuals.^[ref]

An original toy example has three images of animal A, two of B and one of C. There are six photographs, three detected animals and an unknown number of undetected animals. Counting images answers the first question. Recognizing individual markings answers the second. Estimating total abundance additionally requires assumptions about the observation process.

Sampling effort matters too. Two cameras operated for three nights provide six camera-nights of nominal effort, but that equality alone does not make them equivalent to six cameras operated for one night: location and temporal coverage differ. State how encounters are defined and how repeated images are handled before interpreting a photographic detection rate as a biological difference.

K. Ullas Karanth developed a photographic capture-recapture model approach for estimating tiger populations. A 1995 study used repeated camera trap sampling in Nagarahole, recognizing individual tigers across photographs instead of equating the photograph count with the number of animals.^[ref]

The study reported $31$ photographic captures of $10$ individuals across nine sampling occasions. A model allowing animals to differ in capture probability was appropriate for those data.^[ref]

The count of ten distinct photographed tigers and an estimate of the underlying population answer different questions. Repeated sightings identify recaptures, while unseen individuals require inference. Variation in how readily animals pass a camera trap matters even when their identities can be recognized accurately.

The tiger, Panthera tigris, is a species of Felidae in the taxonomic genus Panthera. A striped individual encountered in the field is both a member of that species and a particular animal with its own movement and detection history.^[ref]

This distinction made a useful measurement technique possible. A 1995 study in Nagarahole used camera traps along travel routes and identified individual tigers from photographs. Thirty-one photographic captures across nine sampling occasions represented ten different animals, not 31 animals. Their capture histories were then analyzed using mark and recapture methods that allowed different individuals to have different detection probabilities.^[ref]

For an original toy capture history, write 101 for an animal photographed on the first and third occasions, and 010 for another photographed only on the second. These are three detections but only two known individuals. Animals never photographed leave no such history, which is why an abundance estimate needs a detection model in addition to a tally.

The method connects a natural visible marking with statistical identification. It also exposes a limitation: placing cameras on travel routes samples encounters with those cameras, rather than guaranteeing that every tiger in the study region has been seen.