Some animals like tigers are identifiable as individuals in ‘camera trap’ shots. Others that aren’t could be estimated by the new method (Image: ZSL)

Even the shyest, most reclusive animals should now yield their secrets to a new census method that uses automatic cameras to snap photos of passing animals. Until now, such animals could only be counted through much more painstaking, expensive methods.

Conservation biologists already use such automatic cameras routinely in their field work. Triggered by infrared sensors, these “camera traps” can record the presence of stealthy animals such as tigers or deer, which tend to flee an approaching biologist long before they can be glimpsed.

For a few animals, such as tigers, which can be individually recognised by their pattern of stripes, these photos already give good estimates of population size. However, for the vast majority of species, in which individuals cannot be recognised, biologists cannot tell whether they are seeing a few individuals many times or many individuals a few times.


Gas molecules

Now Marcus Rowcliffe, a conservation biologist at the Institute of Zoology in London, UK, and his colleagues have figured a way around this problem. Rowcliffe began by assuming that animals move around randomly, just like molecules in a gas.

By modifying physicists’ equations for the motion of gas molecules, he found that he could estimate the population density of the animals from the frequency with which they are photographed, as long as he had some measure of the animals’ speed of movement.

“It’s surprisingly simple, given the amount of mathematics behind it,” says Rowcliffe.

The researchers tested their method by placing cameras in an animal park, Whipsnade Zoo in southern England, for a total of 3277 camera hours.

For three animal species – two kinds of deer and a wallaby – the population size calculated from the camera sightings and observers’ estimates of movement speeds matched the actual, known population in the park.

Speed checks

Other biologists are excited about Rowcliffe’s technique. “It really shows the potential to monitor the abundance of a whole community of animals with one technique,” says Roland Kays, curator of mammals at the New York State Museum in Albany, US.

In the wild, though, the technique won’t work until biologists have accurate information about how fast animals move around.

“That’s the big sticking point at the moment. We are not there yet,” Rowcliffe concedes. Radio-collaring animals is one way to get that information, but he and Kays are also developing a method for using video camera traps to record both presence and speed at the same time.

Journal reference: Journal of Applied Ecology (DOI: 10.1111/j.1365-2664.2008.01473.x)

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