Abstract
Camera traps have proven very useful in ecological, conservation and behavioral research. Camera traps non-invasively record presence and behavior of animals in their natural environment. Since the introduction of digital cameras, large amounts of data can be stored. Unfortunately, processing protocols did not evolve as fast as the technical capabilities of the cameras. We used camera traps to record videos of Eurasian beavers (Castor fiber). However, a large number of recordings did not contain the target species, but instead empty recordings or other species (together non-target recordings), making the removal of these recordings unacceptably time consuming. In this paper we propose a method to partially eliminate non-target recordings without having to watch the recordings, in order to reduce workload. Discrimination between recordings of target species and non-target recordings was based on detecting variation (changes in pixel values from frame to frame) in the recordings. Because of the size of the target species, we supposed that recordings with the target species contain on average much more movements than non-target recordings. Two different filter methods were tested and compared. We show that a partial discrimination can be made between target and non-target recordings based on variation in pixel values and that environmental conditions and filter methods influence the amount of non-target recordings that can be identified and discarded. By allowing a loss of 5% to 20% of recordings containing the target species, in ideal circumstances, 53% to 76% of non-target recordings can be identified and discarded. We conclude that adding an extra processing step in the camera trap protocol can result in large time savings. Since we are convinced that the use of camera traps will become increasingly important in the future, this filter method can benefit many researchers, using it in different contexts across the globe, on both videos and photographs.
Highlights
Camera traps, triggered by motion and/or heat of a passing subject, are a non-invasive way to study animals and their behavior
Both filter methods discriminated between non-target and potential target recordings, but Filter 2 performed slightly better than Filter 1, especially when lower false positive rate (FP-rate) were tolerated (5% False positives (FP)-rate marked by a vertical dashed line)
Irrespective of the FP-rate, the true positive rate (TP-rate) in Dry circumstances was always higher compared to Wet circumstances
Summary
Camera traps, triggered by motion and/or heat of a passing subject, are a non-invasive way to study animals and their behavior. Camera traps have been used to study niche separation [2], competitive exclusion [3], population structure [4,5], density estimation with [6,7] and without individual recognition [8,9], abundance estimation [10], foraging behavior [11], biodiversity [12], activity patterns [13] and habitat use [14,15]. Camera traps can replace other study methods or add to direct observations, track inventories, knowledge of local inhabitants or genetic surveys [16,17,18,19,20]. Species such as small arboreal primates [23,24], ectothermic Komodo dragons [25] and birds [26,27] have been subjects of camera trap study, showing wide ranging applicability
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