Jamie McKaughan describes his team’s latest research testing the use of distance sampling with camera traps as a way to establish meaningful density estimates of mesocarnivores in South Africa.
Frederick Forsyth’s novel, ‘The Day of the Jackal’, is a gripping political thriller that tells the story of a highly skilled assassin tasked with killing the French President. But have you ever wondered why the author chose the code name “Jackal” for the assassin? In fact, the author had the character choose his own name, reflecting, metaphorically, his own mindset and abilities – sly, adaptable and an elusive predator that is difficult to catch.
But what else do we really know about these animals?
Jackals are an example of a mesocarnivore – species that are ecologically defined as any mid-ranking carnivore in a food web, irrespective of their size or taxonomy. They are typically highly adaptable animals and play a critical role in maintaining a healthy ecosystem. Their adaptability often causes them to become unpopular with people though, making them an important focus for wildlife managers in many countries.
Despite their importance, we know remarkably little about them – particularly how many or few there are. We therefore aimed to use camera traps to estimate the population density of four mesocarnivores from southern Africa: black-backed jackal, caracal, brown hyena and African civet.
The experiment
Camera traps are a valuable tool to monitor wildlife remotely and unobtrusively, ideal for collecting data on elusive animals. However, managers and researchers need to account for a variety of factors when deploying this technology, and not all of them are widely discussed. A particular factor that we looked at was how estimates of animal abundance can be influenced by the recovery time of camera traps.
A useful technique for estimating the abundance of unmarked animal populations is known as distance sampling.
During distance sampling surveys, observers (humans or cameras) take snapshots of the animals present at different distances from the observer. The period in between snapshots is known as the snapshot interval.
In a sense, the snapshot interval determines how many times the area is surveyed; it is, as a consequence, crucial for relating the numbers of animals seen in surveys to the density of the population. The snapshot interval has a significant impact on the accuracy of density estimates, and selecting an appropriate interval is critical for achieving reliable results.
Many users of camera traps want to ensure that the cameras are collecting data as often as possible, whether using photos or videos, and so the camera reset time (the delay after recording one trigger event, before the camera can be triggered again) is normally set to the lowest possible time (often this is 1 second). Importantly, however, the performance of camera traps in the field can often be quite different from the performance indicated by the trap’s settings.
As an example, we found that our camera traps had an average downtime of 10.35 seconds between photo bursts during continuous triggering, even though we’d set the recovery time to be 1s! Assuming the settings were accurate would have greatly inflated our estimates of effort, substantially underestimating the density of animals in the environment. In fact, the estimation of camera downtime is made even more complicated by downtime resulting from false triggers, or triggers by animals not among the species of interest.
In our paper, we talk about different ways of estimating the total downtime and how they can affect estimates of animal abundance.
Our findings
Accounting for the details of camera performance will ensure quality data and more reliable information, leading to better informed wildlife management decisions. With camera performance accounted for, we found comparatively healthy population densities of three out of four of our mesocarnivores: brown hyena, civet and jackal appeared to prosper in the mixed farmland habitat, more than we expected given estimates from farmland elsewhere. Caracal densities were much lower and more in line with expectations from what we know of this animal’s behaviour, especially given the high jackal density.
That we had so few densities to compare against for these species shows just how much more work we have to do to gain a better understanding of healthy population abundances, both within and beyond protected areas.
Spoiler alert: the ‘Jackal’ ultimately fails to achieve his objective in “The Day of the Jackal”, with the authorities ultimately succeeding in tracking the elusive Jackal after an extensive pursuit. Camera traps have made it that little bit easier for us to catch a glimpse of these fantastic but elusive animals too; understanding the details of assumptions about survey parameters will help us to better document their abundances.
Read the full article: “Estimating mesocarnivore abundance on commercial farmland using distance sampling with camera traps” in Issue 4:2 of Ecological Solutions and Evidence.
