Gazelle–livestock interactions 207 Methods
Surveys The aim of the eight surveys conducted during 2011–2019 was to establish baseline estimates of densities and relative distribution of wild and domestic ungulates in the central Reserve. The survey area is a small part of an open system across which dorcas and dama gazelles and livestock move freely. The term ‘population’ thus refers here only to the animals using the study area at the time of a survey. Surveys used standard line transect methods (Buckland
et al., 2001; Young et al., 2010; Baamrane, 2013; Le Moullec et al., 2017). The survey zone covered c. 3,500 km2 (3%of the Reserve) and was selected as an area of relatively low human activity, with no local development of water wells, and a rep- resentativemix of vegetated fixed dune and floodplain habi- tats in the core of the Reserve. This included the only area where the Critically Endangered dama gazelle was reported to be regularly observed. Transect orientation and the cen- tral survey area were consistent throughout the study, but transect layout varied slightly during the initial exploratory phases in 2011–2013, before the design was fixed once longer-term funding was assured (Table 1). During 2015– 2019, surveys used a standard core framework of seven fixed transects, spaced 10 km apart (Fig. 1). A continuous line transect design improved efficiency. Transects were or- ientated to traverse low dune habitats running east to west across the survey zone and extended into floodplain habitats either side. In the latter four surveys, an east–west transect was added to the south within the floodplain of Ouadi Haddat (Fig. 1). In the 2017 wet and dry seasons, we extended the system-
atic assessment of wild and domestic ungulate distribution by surveying an additional 40 × 50 km contiguous recon- naissance survey block to the north of the line transect study zone (Fig. 1). In the reconnaissance block, data were collected as counts only, by two teams in separate vehicles working on parallel transects 10kmapart. Species encounter rates (per 10 × 10 km grid cell) of the reconnaissance survey block and line transect zone were combined to generate dis- tribution maps of wild ungulates and livestock. In the final line transect survey (2019), an additional set of transects was inserted between the established transects to create 5-km transect spacing, driven by two teams working in parallel, to increase sampling effort to 15 transects and improve precision of counts and density estimates (Bårdsen & Fox, 2006; Strindberg et al., 2010). Because dama gazelles were encountered only a total of 13
times during all surveys, we also analysed the distribution of 78 additional, opportunistic observations of this species col- lected during 2–3 month periods of continuous field work in the 2016 wet season and the following dry season in 2017. In these periods we recorded all observed wildlife and
Oryx, 2023, 57(2), 205–215 © The Author(s), 2022. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605321001629
TABLE 1 Summary statistics and results from eight line transect surveys of the central zone of the Ouadi Rimé–Ouadi Achim Reserve, Chad, 2011–2019.
Survey month Season
Feb. 2011 Dry Sep. 2011 Wet Sep. 2013 Wet Sep. 2015 Wet Feb. 2017 Dry Sep. 2017 Wet Mar. 2018 Dry Jan. 2019 Dry
Number of transects
8
11 6 7 8 8 8
15
Total transect length (km)
222 295 240 272 345 350 345 575
Survey area (km2)
Number of dorcas gazelle groups (truncated at 400 m)
1,109 199 2,213 269 2,400 150 2,715 160 3,500 236 3,500 124 3,450 222 3,500 510
Number of dorcas gazelles in survey block (95% CI), CV1 (%)
7,777 (5,929–18,751), 12.8 14,645 (11,437–18,751), 12.0 8,379 (4,775–14,704), 23.5 9,002 (4,570–17,732), 29.3 16,765 (12,733–22,072), 12.9 5,736 (3,109–10,684), 27.3 12,890 (8,469–19,618), 18.7 18,250 (14,517–22,945), 11.0
1CV, coefficient of variation. 2Tropical livestock unit, equivalent of mature individual weighing 250 kg (see text for details).
Number of dama gazelle groups
1 0 0 6 0 1 3 2
Number of individual dama gazelles
1 0 0 9 0 1 7 7
Number of livestock groups (truncated at 490 m)
13 31
109 108 89 72 67 90
Number of livestock units2 in survey block (95% CI), CV1 (%)
1,791 (426–7,533), 74.6
14,020 (5,683–34,586), 46.7 63,548 (18,996–213,000), 51.4 68,485 (21,428–219,000), 51.3 50,187 (26,579–94,765), 31.2 23,119 (7,089–75,394), 55.3 27,326 (11,535–64,732), 42.1 15,221 (7,885–19,380), 33.5
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