574 N. van Doormaal et al.


and provides a controlled environment with few external factors that could potentially affect the study. Because resources are limited and rangers have other re- sponsibilities, searches for snares in the buffer zone were only possible when sufficient time and resources were avail- able. In general, rangers patrolled the buffer zone twice per week. The Reserve’s management used a grid system for planning their operations; each grid cell is 1.02 × 1.1 km (0.01 × 0.01 °) and teams were normally assigned to one particular grid cell for their search. A search team typical- ly consisted of 2–3 rangers. Ranger teams were equipped with a GPS that automatically recorded their positions at c. 10-s intervals. The searches were done in the early morn- ing to avoid the heat of the day and took c. 2 h, but may have lasted longer if signs of illegal activity were found. All obser- vations, including detected imitation snares, were reported through the Reserve’s patrol monitoring system. The ran- gers received a small financial reward for every snare they reported, in addition to their base salary. This financial in- centive was already in place before this study was conducted.


Methods


Experimental design A thorough study of snare detection probabilities for differ- ent patrol strategies requires a field experiment in which the true spatial distribution of snares is known. We achieved this by setting imitation snares at 166 random locations throughout the study area, with a minimum spacing of 25 m and a density of 40 snares/km2. The numbers were based on a small pilot carried out in September 2018, which involved 10 imitation snares per 0.25 km2. Imitation snares were set at the nearest suitable location from the ran- domly generated point, to realistically mimic the use of real snares. For example, imitation snares were set as wire circles along game trails, often tied to a nearby tree or set between bushes, and a member of the Reserve Manager’s team was present when the snares were set to ensure that they ap- peared realistic. The spatial distribution of actual poacher snares is unlikely to be random, but patrol teams still need to detect one snare to find a cluster of snares.Weused a ran- dom distribution rather than locations where snares were detected in the past, to avoid potential bias in recorded poaching data. Randomized controlled trials are often con- sidered the gold standard for testing the efficacy of treat- ments (Baylis et al., 2016; Pynegar et al., 2019), and our experimental design aimed to approximate this, without disrupting ongoing law enforcement operations. We used snares previously removed by the Reserve’s pa-


trol teams as imitation snares. They were set without the usual trigger mechanism, which ensured that the imita- tion snares looked realistic but would not catch or injure


animals. We recorded the time and location of every imita- tion snare placement using a GPS. Each imitation snare was marked with a small piece of black tape so that it could be distinguished from actual poacher snares. This approach was tested in a pilot study and the results suggested that the black tape did not increase detection probability. Dis- tinguishing imitation snares from actual poaching snares was also necessary to minimize the likelihood that reported locations of imitation snares influenced future patrol de- ployment decisions. The operations manager of the Re- serve was informed about the study design, but did not know the exact locations of all imitation snares. All imita- tion snares were set within 1 week, with the last imitation snares set on 4 April 2019. The imitation snares were left in the field until detection or the end of the study, in the last week of October 2019. We were later able to retrieve all undetected snares from where we had set them, suggest- ing that rangers and poachers did not use the imitation snares for their own profit.


Baseline detection


The first phase focused on estimating the baseline detection probability of snares under normal law enforcement opera- tions, in which rangers are assigned to search a 1 km2 grid cell. This system was already in place before the study, and we did not influence how the grid cells were chosen. Rangers rely on their training and experience to decide where within their assigned grid cell to search for snares. The rangers were not informed about this research, to min- imize any impact on operations. Because of logistical and technical issues, no data on patrols were recorded in April 2019. Therefore, we used all snare searches and observations recorded during 1 May–12 August 2019 to estimate the base- line detection rate. The buffer zone was not always accessible during this period because of hunting activities by the local landowners. Snare searches were planned around the hunt- ing schedule, but were infrequent, especially during May– July. The baseline detection rate served as a reference point for comparing the different patrol strategies.


Spatially focused patrols


We compared the detection probability of spatially focused patrols to the baseline detection probability. The remaining, undetected imitation snares still in the field were used to identify locations with a density of at least 10 imitation snares per 0.25 km2. This ensured that the remaining areas that were included in this study did not comprise only iso- lated snares that would have been difficult to locate. These locations were passed on to the operations manager, who then decided which of these locations should be patrolled. On the day of the search, the manager informed the ranger


Oryx, 2022, 56(4), 572–580 © The Author(s), 2021. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605320001301


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