Animal reactivity to camera traps and its effects on abundance estimate using distance sampling in the Taï National Park, Côte d’Ivoire

Abstract

The use of camera traps (CTs) has become an increasingly popular method of studying wildlife, as CTs are able to detect rare, nocturnal, and elusive species in remote and difficult-to-access areas. It thus makes them suited to estimate animal density and abundance, identify activity patterns and new behaviours of animals. However, animals can react when they see the CTs and this can lead to bias in the animal population estimates. While CTs may provide many advantages, an improved understanding of their impacts on individual’s behaviour is necessary to avoid erroneous density estimates. Yet, the impact of CTs on detected individuals, such as human odour near the device and the environment, or the infrared illumination, has received relatively little attention. To date, there is no clear procedure to remove this potential bias. Here, we use camera trap distance sampling (CTDS) to (1) quantify the bias resulting from the different animal responses to the CTs when determining animal density and abundance, and (2) test if olfactory, visual and auditory signals have an influence on the animals’ reaction to CTs. Between March 2019 and March 2020, we deployed CTs at 267 locations distributed systematically over the entire Taï National Park. We obtained 58,947 videos from which we analysed four medium- to-large-bodied species (Maxwell’s duiker (Philantomba maxwellii), Jentink’s duiker (Cephalophus jentinki), pygmy hippopotamus (Choeropsis liberiensis) and Western chimpanzee (Pan troglodytes verus)) displaying different behaviours towards the CTs. We then established species-specific ethograms describing the behavioural responses to the CTs. Using these species-specific responses, we observed that the Maxwell’s duiker reacted weakly to CTs (about 0.11% of the distance data), contrary to Jentink’s duiker, pygmy hippopotamus and Western chimpanzee which reacted with relatively high frequencies, representing 32.82%, 52.96% and 16.14% of the distance data, respectively. Not taking into account the species-specific responses to the CTs can lead to an artificial doubling or tripling of the populations’ sizes. All species reacted more to the CTs at close distances. Besides, the Jentink’s duiker and the pygmy hippopotamus reacted significantly more to the CTs at night than during the day. Finally, as for olfactory signals, the probability of reaction to the CTs during the first days after CTs installation was weak in Maxwell’s duiker, but concerned 18% of the video captures in Western chimpanzees which decreasing with time, but they remained high in pygmy hippopotamus and Jentink’s duiker (65% and 70% of the video captures respectively). Careful consideration should be given to animal’s response to CTs during the analysis and in the field, by reducing human’s impact around the CTs installation.

Estimating effective survey duration in camera trap distance sampling surveys

“…In our data set, 19% of the roe deer and 34% of the red deer events included some form of behavioural reaction to the camera trap (Henrich et al., 2022), as well as 16% of wild boar events. Failure to account for behavioural responses to camera traps can strongly bias estimates of animal density, when they affect the staying time or position of animals in the FOV (Bessone et al., 2020; Houa et al., 2022). While behavioural responses can be corrected for when they can be classified as such (Delisle et al., 2023, submitted for publication), their effect on the retrigger delay $$$ v $$$ in data sets of PIR sensor‐triggered photographs cannot be directly observed.…”

Camera traps unveil demography, social structure, and home range of six unhabituated Western chimpanzee groups in the Moyen Bafing National Park, Guinea

“…If a rapid estimate of population density is needed within a large area, then transect lines based on nest counts or distance sampling survey with camera traps may be the most appropriate methods (Buckland et al, 2001; Cappelle et al, 2019; Kouakou et al, 2009). Alternatively, if documenting population trends over a 5‐year period is the objective, such large‐scale methods will be too imprecise to provide reliable answers for chimpanzees because of their slow reproductive rate (Cappelle et al, 2021; Houa et al, 2022). Smaller‐scale methods, such as the camera trap approach presented here, can provide the level of precision needed to answer such questions.…”

Reducing bias in density estimates for unmarked populations that exhibit reactive behaviour towards camera traps

“…For instance, Houa et al. (2022) documented a single type of reactive behaviour in <1% to 37% of detections; Henrich et al. (2020) documented a single type of reactive behaviour in 2%–43% of events; Henrich et al.…”

“…Different methods have been used to reduce suspected bias caused by reactive behaviour. For CTDS, these include left-truncating all detections close to the camera trap (Cappelle et al, 2019), removing detections of animals reacting to the camera (Bessone et al, 2020;Houa et al, 2022) and additionally omitting from consideration certain detection functions when estimating the detection probability (Delisle, McGovern, et al, 2023;Delisle, Miller, et al, 2023). For REM, independent observations, during which animals react to a camera trap, commonly are removed for the estimation of the movement speed parameter (Monteiro-Alves et al, 2019;Palencia et al, 2021Palencia et al, , 2022Palencia et al, , 2023Rowcliffe et al, 2016).…”

“…While cameras are collecting data, animals often react to these foreign objects, and in so doing alter their natural behaviour (Henrich et al., 2020; Ladd et al., 2022; Meek et al., 2016). Common reaction types towards cameras include attraction, freezing and fleeing (Houa et al., 2022). Attraction is when animals spot the camera trap and subsequently change their movement path towards the camera.…”