Remnant Asian elephant population 475
seconds of video per trigger, although malfunctions caused eight cameras to take exclusively either photographs or video, and two cameras took a 3 second video only. Individuals were identified, aged and sexed using dis- tinguishing traits, and ages of non-adults were estimated by comparing animal heights relative to an adult female where they co-occurred in the same photograph (de Silva et al., 2011; Vidya et al., 2014). Individuals were grouped into four age classes: calf (#6 months), infant (7 months– 2 years), juvenile (3–7 years) and adult ($8 years). The adult age class incorporated subadults, as distinguishing between adults and subadults based on relative height measurements is unreliable. All juveniles and adults were assigned a sex, but calves and infants were left unsexed as these age classes lack discriminating sexually dimorphic features (Varma et al., 2012). Population size was estimated using Chao’s moment es-
timator (Mth model) in CAPTURE (Hines, 1987), which ac- counted for the effects of time (t) and individual differences (h) (Seltmann et al., 2018). A closure test was applied to ensure the population met the assumption of a closed population. Capture probabilities across the survey area, and ele-
phant density, were assessed using a spatially explicit capture–recapture model with the package secr (Efford, 2018) implemented in R 3.4.1 (R Core Team, 2017). The spatial scale of capture location data (σ) was estimated to be 1.8 km, as a proxy of elephant home range size in the reserve, calculated as the mean maximum distance moved. The initial secr buffer width was taken as 2σ (3.6 km), which was adequate for density estimates to stabilize across the camera grid. Social structure was determined by assigning individuals
to the same group if they were captured within 15 minutes of each other (Head et al., 2013). Any residual individuals were considered part of a group if they were captured with one or more of its members. Individual elephants captured more than 30 minutes apart on the same camera were considered to be independent capture events.
Results
Camera trapping in Nangunhe National Nature Reserve yielded a total of 154 images and 43 videos of elephants on six of the 36 camera traps, of which 89 images (58%) and 37 videos (86%) were suitable for elephants to be individu- ally identified, sexed and assigned an age class. Sixteen elephants were individually identified: eight adult females, one adult male, three juvenile males, and two infants and two calves of indeterminate sex (Supplementary Table 1). Using Chao’s Mth model, the total population size in the reserve was estimated to be 20 individuals (95%CI 17–33). The spatially-explicit likelihood capture model estimated
the detection probability (g0)to be 0.31 (95%CI 0.26–0.37) over the trapping grid, with an elephant density of 0.39 indi- viduals per km2 (95%CI 0.14–0.67 individuals per km2). Of the 16 elephants identified, 11 formed one herd
(Supplementary Table 2), although not all members were captured together on every occasion (Supplementary Table 3). Females F04 and F06, juveniles J01 and J03, and calves C04 and C06 were recorded together in four capture events on camera traps 2 and 3. Female F05, juvenile J02 and calf C05 were absent from one capture event. Adult female F07 was captured only once in the presence of a recognized herd member (juvenile male J02), although there was a 22-minute separation, and more than an hour after the rest of the herd was captured on the same camera trap. An adult female (F02) was captured once with her calf (C02) on camera trap 5 (Fig. 1). Three solitary adult fe- males (F01,F03 and F08) were detected, with F03 and F08 captured once on camera traps 1 and 2, respectively, and F01 captured in seven separate events on camera traps 3, 4, 5 and 6 (Fig. 1). Only one adult male (M01) was encoun- tered, detected on his own three times on camera trap 2, and once with the herd on camera trap 3, although one capture on camera 2,on 29 May, was only 24 minutes after the other herd members.
Discussion
We estimate that the elephant population of Nangunhe National Nature Reserve is 20 individuals, with an estimated density of 0.39 elephants per km2. This density is relative- ly low compared to densities of 3.3 elephants per km2 in Nalkeri Reserve Forest, India (Karanth & Sunquist, 1992), and 5.0 elephants per km2 in Bandipur National Park, India (Johnsingh, 1983), although densities can be ,0.1 per km2 (Sukumar, 1989). The area of suitable habitat for elephants in Nangunhe covers only 29 km2 of the Reserve (Liu et al., 2016), which is less than the estimated minimum species’ home range size of 100 km2 (Jathanna et al., 2015; Liu et al., 2016), potentially limiting the carrying capacity of the Reserve (Zhang et al., 2015). The elephant population in Nangunhe has not increased
for more than 4 decades, which equates to approximately two generations (Choudhury et al., 2008). The size of the population has reportedly fluctuated around 20 individuals since 1976, with the exception of a decline to 12 individ- uals in 1983 (Zhang et al., 2015). Although apparently stable over this period, the population remains vulnerable to accelerated inbreeding and loss of genetic diversity leading to inbreeding depression and a compromised ability to re- spond to changing environmental conditions (Frankham, 2003, 2005). This is compounded by demographic and environmental stochasticity and local catastrophes that together lead to an increased risk of population extinction
Oryx, 2021, 55(3), 473–478 © The Author(s), 2020. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605319000504
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