In Rowcliffe et al. (2008) REM was tested using data from a survey carried out over a 6 weeks period from 13 June to the 24 July 2005 at Whipsnade Wild Animal Park (WWAP), located in Bedfordshire, south England. The advantage of this data set is that it is from a zoo with a known number of animals. There was a total of n = 42 camera trap days. The park houses several free-ranging species but only four of these species were considered for the purposes of the analysis:
• red necked wallaby (Macropus rufogriseus) • Chinese water deer (Hydropotes inermis) • Revee’s muntjac (Muntiacus reevesi), and • mara (Dolichotis patagonum).
Note that for brevity, the common names: wallaby, water deer, mara and muntjac will be used in the rest of this thesis. The park was divided into four areas of contrasting habitat. The first two areas are open grasslands with scattered scrub:
1. Downs open grassland with scattered scrub and a steep scarp slope; 2. Institute P addock with gentle slopes;
3. Old F arm, which is an area of rough grassland and thicket on largely level but highly broken ground; and
4. Central P ark, which is an area of mixed lawns, roads, buildings and enclosures housing large animals with scattered trees.
2.7.1
Estimating Speed and Group Size
The calculation of density requires independent estimates of speeds, which could only be obtained during the day, and average group size. Rowcliffe et al. (2008) used day range to derive an approximate estimator of speed. To derive estimators of speed and average group size Rowcliffe et al. (2008) conducted 10 focal watches, for each species,
distributed evenly between 08:00 and 18:00 in order to control for any variation in move- ment patterns over the day. For the speed estimator, Rowcliffe et al. (2008) arbitrarily select individuals following each individual for 30 minutes during each focal watch and recording the total distance travelled during that time as the sum of all straight-line movements. Day range for each species was then calculated as the mean across all focal watches for that species. So the distribution of the speed data over a day was used as the distribution of speeds over the trapping period. It was observed that some animals were not moving during the period they were watched, hence, a zero speed was recor- ded. While the probability of observing a zero speed would be sensitive to observation timescale, tending to zero as timescale increases, the aim was to derive an estimate of average speed, which is insensitive to timescale, and not to obtain an unbiased estimate of the probability of observing a zero speed.
Average group sizes were estimated by systematically recording the numbers of indi- viduals in groups encountered along transects through the study area. In Table 2.7.1 we give a summary of the fixed parameters required to estimate the density. Note that the angle of detection, θ in Table 2.7.1 is quite narrow since the camera traps used was a model called DeerCam, which has this limitation. These results are taken from Rowcliffe et al. (2008).
We also provide the coefficient of variation (Cv) of the speed estimators for each species. The coefficient of variation is a measure of dispersion of the data relative to the mean. It is defined as the sample standard deviation, σ divided by the mean, that is Cv = σ/µ, where µ is the mean speed. Often the Cv is expressed as a percentage, which corresponds to the following formula
Cv% = 100(σ/µ). (2.7.1)
The Cv is useful for comparing the variability of two or more samples of data from different variables or from the same variables when the means are very different (Brown, 1998). In Table 2.7.1 the Cv% is quite large for the encounter rate and the speed estimators of the four species suggesting that the speed of movement variation among
animals and the variability in the encounters are substantial.
Table 2.7.1: Summary of fixed parameters required to estimate density (standard error in parentheses), which is taken from Rowcliffe et al. (2008).
mara muntjac wallaby water deer Mean encounter (ˆλ) 0.06 (0.02) 0.35 (0.07) 4.78 (0.47) 1.51 (0.19)
Cv% of encounter 389% 239% 117% 154%
Average day range (¯v, in km day−1) 2.56 (1.08) 8.27 (1.92) 0.71 (0.36) 1.17 (0.49)
Cv% of day range 133% 73% 160% 127%
Average group size (ˆg) 1.8 (0.63) 1.5 (0.53) 1 (0) 1 (0) Sample size of speed (m) 10 10 10 10 Detection arc (θ, radians) 0.175
Detection distance (r, km) 0.012
2.7.2
Animal Census
A census, counting the number of animals, was carried out at the end of the camera trapping period between 09:30 and 14:00 by a team of 12 counters. A systematic co- ordinated line approach (to avoid double counting), was adopted to cover areas in three of the four habitats (Downs, Institute Paddock, Old Farm) in a single sweep, while in the Central Park area small teams systematically counted central areas, co-ordinating movements to ensure complete coverage without double counting (see Rowcliffe et al., 2008). Since the census was taken during a specific time period, and animals moved among habitats there were observed zero census count in some habitats for some species, for example, the census of the mara species in Old Farm. However, during the camera trapping period there were records of the mara species in Old Farm (see Table 2.7.2). Table 2.7.2 gives the summary of the census data split by habitats for the four species at WWAP.
Table 2.7.2: Summary of the census data split by habitat, which is taken from Rowcliffe et al. (2008). Habitat
Downs Institute Paddock Old Farm Central Park Total Area (km2) 0.49 0.28 0.23 1.26 2.26 Camera hours (day time) 898 440 543 317 2198
Census count mara 15 2 0 136 153 muntjac 3 0 5 22 30 wallaby 544 213 185 120 1062 water deer 36 10 133 91 270 Density (animals km2) mara 30 7 0 108 68 muntjac 6 0 22 18 13 wallaby 1101 760 803 96 468 water deer 73 36 577 72 119 Day-time camera trap photos
mara 3 1 2 3 9
muntjac 10 1 4 23 38
wallaby 225 195 78 38 536 water deer 32 3 89 23 147