Camera trap used for surveying leopards in the Boland region of South Africa.
Some species are very hard to monitor using conventional survey methods involving human observers. This may be, e.g., because they are difficult to detect in their natural environment, their habitat is hard to get to or they shy away from people.For some species, the solution in monitoring them was found in using automated detectors which are placed in the species' habitat. Examples of these are 'camera traps' which are motion-triggered cameras capturing images of the animals that come by (e.g. for jaguar), hydrophones that record underwater sounds such as vocalizations or clicks from cetaceans (e.g. CPODs used for monitoring the critically endangered vaquita porpoise, see: Jarmamillo-Legorreta et al. 2017) or microphones.
The species of frog depicted in the mural (cape peninsula moss frog) is so small that they are almost impossible to detect visually in their natural habitat. How can you find out how many there are? Their individual sounds can be matched across multiple detectors (microphones) and analysed using spatial capture-recapture methods.
The mural shows one frog producing calls which are detected by two of the four detectors (microphones). The red circles represent those microphones that detected the calls while the yellow ones did not. Based on this information (detected on A and C, not detected on B and D) probability contour lines are produced about the location of the frog.
In an actual survey, several detectors would be placed throughout the study area. Using the location information about all detected frogs we can estimate how many frogs were in the study area.
Borchers, D.L., Stevenson, B.C., Kidney, D., Thomas, L.J. and Marques, T.A. 2014. A unifying model for capture-recapture and distance sampling surveys. Journal of the American Statistical Association 10: 195-204.
Stevenson, B. C., Borchers, D. L., Altwegg, R., Swift, R. J., Gillespie, D. M., and Measey, G. J. (2015) A general framework for animal density estimation from acoustic detections across a fixed microphone array. Methods in Ecology and Evolution, 6(1), 38-48.
Jarmamillo-Legorreta, A., G. Cardenas-Hinojosa, E. Nieto-Garcia, L. Rojas-Bracho, J. Ver Hoef, J. Moore, N. Tregenza, J. Barlow, L. Thomas and B. Taylor. 2017. Passive acoustic monitoring of the decline of Mexico's critically endangered vaquita. Conservation Biology 31: 183-191.