Gazelle–livestock interactions 209
FIG. 2 Estimated densities of (a) dorcas gazelles Gazella dorcas and (b) livestock (TLU, tropical livestock units) in the study area during 2011–2019. The vertical lines represent the 95% CI. Note the order of magnitude difference in y-axis scale and unit body weights (c. 16 kg/dorcas individual vs 250 kg/tropical livestock unit).
between maximum and minimum numbers correspond with the movement patterns of the species in this area, with some seasonality in reproduction, although calves can be found in all months. Average dorcas density during the wet season was marginally lower than during the dry season (Table 1). Bayesian analysis showed an 81% proba- bility of decline over 2011–2019 (Fig. 3a), influenced by high densities in the smaller scale 2011 surveys. Analysis across the latter six standardized surveys (2015–2019) showed a positive trend, with 95% probability of increasing numbers (Fig. 3b).
Livestock numbers Mean estimates for livestock numbers ranged between c. 2,000 and c. 68,000 tropical livestock units, with low precision (CV 30–74%; Fig. 2b, Table 1). The low precision was associated with large variation in en- counter rate and group size between transects, and prevents analysis of trend in livestock numbers. The average live- stock density during the wet season was c. 16 tropical live- stock units/km2, more than twice that of the dry season (Table 1). Livestock made up .90% of ungulate biomass (range 93–99%) present in the study zone in every survey except the first, which was a dry season survey when un- usually low livestock numbers were recorded.
Dorcas gazelle distribution in relation to livestock, artificial water supply, season and fire The multiple regression model that included livestock encounter rate and season as explanatory variables had the lowest AIC value and showed a significant negative association (P,0.05) be- tween dorcas and livestock encounter rates, with little vari- ation between seasons (Figs 4 & 5). Although there was no
significant relationship between dorcas gazelle density and fire frequency during either 1–60 days or 61–180 days prior to the surveys at the 10 × 10 km grid scale, there was a positive relationship (P,0.05) between dorcas gazelle density and fire frequency over the full interval of 1–180 days prior to each survey at the scale of the whole study area.
Livestock distribution in relation to artificial water supply, season and fire Livestock encounter rate was positively associated with the distribution of seasonal hafiri waterholes (P,,0.05), particularly in the wet season when hafiris were full (Fig. 6). Livestock distribution was not significantly associated with fire detections at the 10 × 10 km grid scale during the 60-days or 61–180 days prior to the survey. Across the entire study area, livestock density did not have a significant relationship (P = 0.09) with fire detection frequency.
Dama gazelle distribution in relation to livestock and fire The maximum opportunistic single-day count for dama ga- zelles across 2011–2019 was 32. Only two sightings of dama gazelles outside the study area have been reported in this period. Distribution of dama gazelles during the wet season of 2016 compared to the dry season of 2017 suggests they moved to avoid both livestock and burnt areas (Fig. 7).
Sample effort and precision The mean encounter rate across the eight surveys indicated a survey effort of c. 470 km is needed to achieve a CV of 10% for dorcas gazelles. The estimated effort to achieve a CV of 20% for livestock groups is 483 km.
FIG. 3 Bayesian posterior probability distributions of trend in numbers of dorcas gazelles using the line transect sample zones across (a) all eight surveys (2011–2019) and (b) the latter six surveys (2013–2019), when the transect layout was standardized.
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
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