Last week, Kate and I headed northwest from the Neuras lodge, past the airstrip, to the Tsauchab riverbed. We wanted to select a suitable camera trap site for the survey, which means identifying a place where our cameras are most likely to take photographs of our focal species. It sounds simple. It’s not!
The biggest issue with camera traps is that you’re limited by space. Once you set a camera, it does not move, and it can only observe an area of approximately 100 square meters. When dealing with a study area of hundreds of square kilometers, it seems like a few dozen stationary cameras cannot possibly provide an accurate count of animals as mobile as leopards and hyenas. Surprisingly, not only is it possible, but a growing body of research suggests that camera-trapping is among the most effective methods for counting carnivores.
In the 1990s, K. Ullas Karanth and James Nichols pioneered the use of camera traps to study carnivores when they used a method called capture-recapture to estimate the abundance and density of tigers in four national parks in India. Their work resulted in a seminal paper that has influenced virtually every camera-trap study of carnivores since they published their research in 1998. Karanth and Nichols used the distinctive stripe patterns on tigers to identify individuals they photographed with their camera traps (capture). They then observed how often and where they photographed the same individuals (recapture). Using an open-access statistics program designed for analyzing capture-recapture data, called CAPTURE, they built and tested a series of models that predicted the abundance of tigers in their study areas.
The abundance of a wildlife population (especially of endangered species) is very useful information, but it’s also ecologically helpful to know the density of a population. Karanth and Nichols used a strict methodology for positioning their camera traps to ensure that they covered every inch of the national parks, but they knew that their results could also include tigers that lived primarily in areas adjacent to the parks. Such an event is called an edge effect and to calculate an accurate tiger density, the researchers needed to account for it. Therefore, to estimate their effective sampling area, they applied a well-studied statistical method called Half Mean Maximum Distance Moved. For each tiger that they identified, they found the greatest distance between any two camera-traps that photographed that particular tiger, or the Maximum Distance Moved between cameras. Then they calculated the Mean Maximum Distance Moved for all the tigers that they recaptured.
Tigers, like leopards and other big cats, regularly patrol their entire home range, the area of habitat that they consistently use. The Mean Maximum Distance Moved value provided an estimate of how far the average tiger traveled, or the width of a typical tiger’s home range. In other words, we know that tigers and other big cats regularly travel the entire extent of their home range and rarely travel outside their home range. Therefore, if we know how far they regularly travel (as measured by the distance between camera traps) then we should know how big their home ranges are. To account for tigers living on the edge of their study area, they buffered their sampling area by half of the mean maximum distance moved for the tigers in their study. Adding this buffer to the study area accounted for tigers whose home ranges partially overlapped with the edges of the study area.
Now let’s talk about Neuras. We lack the time and resources available to Karanth and Nichols. Nor do we have decades-worth of experience and knowledge about my study population. Instead of pursuing ground-breaking research, the primary goal is to maximize the actionable data on wildlife for N/a’an ku sệ Foundation. That means identifying as many large carnivores as possible so that the Foundation’s researchers can monitor these individuals. Therefore, my study is relying on a few decisions (and a bit of bias) about how to get as much data on these carnivores as possible in the time that I have here. With the available local knowledge, we’re currently streamlining all our efforts to provide the best scientific assessment we can, and we’re confident that it’ll be a reasonably good one, “robust” as we say in research circles. I’ll send more information soon…