Distance sampling with camera traps 671
FIG. 2 Kernel density estimates of daily activity pattern of the bharal and the musk deer in summer and winter in the Upper Bhagirathi basin.
detection (Howe et al., 2017). If the population surveyed is not available for detection during the data collection pe- riod selected for analysis, temporal sampling effort is over- estimated, and as a result, density could be underestimated (Cappelle et al., 2019). To avoid this bias, either the sam- pling period should be defined as the time during which the entire population was available for detection (peak ac- tivity period) or the proportion of time when animals were available for detection should be included as a parameter in the model (Howe et al., 2017). In our study, the bharal was active during 6.00–18.00,without amarked peak in activity (Fig. 2). The Himalayan musk deer was active at night (18.00–6.00) in summer and during the day (6.00–20.00) in winter (Fig. 2).We used the active period of each species as the sampling period for the analysis. We corrected for the bias caused by animals being unavailable for detection by calculating the mean proportion of animals that were active during the period selected for analysis and incor- porating this proportion in the density estimates. For example, for the bharal we first plotted the number of independent captures (i.e. at least a 30-minute interval between subsequent captures) to visualize the activity pat- tern of the species (Fig. 2).We assumed that if all animals were active throughout the day, then the curve would be a flat line between 6.00 and 18.00. On the other hand, if all the animals were active around 12.00 (at the highest point of the curve), then this flat line will coincide with the curve at 12.00.We calculated both the areas under the imaginary flat line and under the actual activity curve shown in Fig. 2. We then calculated the mean proportion of animals that are active between 6.00 and 18.00 by dividing the propor- tion of the area under the actual activity curve by the area under the imaginary flat line, where animal activity reaches a peak. The estimatedmean proportion of animals that are active during the period selected for analysis was 0.75 in summer and 0.8 in winter for the bharal. For the Himalayan musk deer it was 0.65 in summer and 0.7 in winter. We used the proportion of time animals are active
to correct the naïve density estimate by dividing it by propor- tion of time active, using Distance 7.0 (Thomas et al., 2010).
Density estimation Distance sampling with camera traps requires calculating the distance between the animal and the camera at snapshot momentsto ensure that animalmovement does not bias the distribution of detection distances (Howe et al., 2017).We thus defined a finite set of snapshotmoments (2 s apart) within the sampling period (as suggested in Howe et al., 2017). For each snapshot moment when the species was captured, we estimated the radial distance between each animal and the camera trap, using a regression equation developed fromthe field calibration. In this equation, the de- pendent variable was the ratio of the actual height of an indi- vidual to its height in the photograph, and the explanatory variable was the distance at which the individual was photo- captured (see Supplementary Material 1 for details). We ob- tained information on actual heights for different age and sex classes of the bharal by comparing the camera-trap photos of the species with the heightof the calibration poleheight.We identified eight, 14,two and 10 comparable photographs of adult males, adult females, subadults and fawns, respectively. We calculated the mean height as 76.3 ± SE 2.4 cm (adult male), 70.0 ± SE 1.1 cm (adult female) and 64.0 ± SE 1.0 cm (subadult) and 47.3 ± SE 1.9 cm (fawn). For adult Himalayan musk deer we used a mean height of 50 cm (Sathyakumar et al., 2013b) to estimate their distance from the camera.
Density was estimated following the equation for camera-trap point transects (Howe et al., 2017):
K
D = ˆ k=1
pw2 K
k=1
where nk is the number of observations of animals at a point k (camera-trap location), ek is the temporal effort, and ˆ
Oryx, 2021, 55(5), 668–676 © The Author(s), 2021. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S003060532000071X Pk is ek ˆ Pk nk ×
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