Northern white-cheeked gibbons in Lao 771


TABLE 2 Model ranking by Akaike information criterion (AIC) for gibbon counts at Nam Et-Phou Louey National Protected Area, Lao, during May 2014–May 2015, showing model parameters, AIC, difference of AIC from best-performing model (ΔAIC) and Akaike weight.


Model λ (deciduous) p(sun)


λ (deciduous+hunting) p(sun) λ (deciduous+slope) p(sun) λ (deciduous+distance) p(sun) λ (deciduous+elevation) p(sun) λ (hunting) p(sun) λ (global) p(sun) λ (slope) p(sun)


λ (distance) p(sun) λ (elevation) p(sun) λ (.) p(.)


No. of parameters 4


5 5 5 5 4 8 4 4 4 2


AIC


209.65 210.37 211.32 211.33 211.59 213.36 213.93 214.77 215.35 215.60 218.73


ΔAIC 0.00


0.72 1.67 1.69 1.95 3.71 4.28 5.12 5.70 5.96 9.09


Akaike weight 0.29


0.20 0.13 0.13 0.11 0.05 0.03 0.02 0.02 0.01 0.00


Gibbon group abundance varied across listening points,


ranging from 0.15 to 3.60 groups per km2. The mean abun- dance of gibbon groups per site (λ) estimated with the best model was 1.26 (95%CI 0.54–2.93), giving a density estimate of 0.4 groups/km2 (95%CI 0.17–0.93) with a detection prob- ability of 0.35 (95%CI 0.17–0.60) on sunny days. The density estimate was 0.74 groups/km2 (95%CI 0.32–1.74) for mixed deciduous forest, and 0.09 groups/km2 (95%CI 0.02–0.54) for evergreen forest. The estimated number of gibbon groups for the entire study area was 57 (95%CI 32–261). On rare occasions we located additional gibbon groups


outside the surveyed area, but no inference could be made about these groups from our dataset because the threat level was unknown, and could be relatively high (KS, pers. obs.). We also found solitary males both inside and outside the surveyed area; their presence could not be used for density estimate analyses.


Discussion


Our northern white-cheeked gibbon density estimates are low compared to other populations of the genus (Table 3). These low numbers may reflect a high level of human dis- turbance, at least over the last two decades, and are asso- ciated with the poor habitat quality across the range of the genus (Bleisch et al., 2008).Ahistory of logging, followed by relatively dry conditions in secondary forest, has been asso- ciated with low gibbon food availability and low group den- sity (Phoonjampa et al., 2011), because the animals need to maintain larger home ranges for securing sufficient food in dry habitats (Savini et al., 2008; Light, 2016). The way density is calculated, particularly the choice of


an effective listening area, is critical for accurate measure- ment. It is likely that the effective listening distance in the rugged terrain of Nam Et-Phou Louey National Protect- ed Area varied between listening points. For example, Chanthalaphone (2013) used two independent effective lis- tening distances in the same study site, yielding different density estimates. The degree to which this possibility has been accounted for in other published estimates from the region is inconsistent (Gilhooly et al., 2015). Despite this possible methodological bias, an undisturbed habitat should have a carrying capacity for most gibbon species of c. 4–5 groups/km2 (Brockelman et al., 2009), which is much higher than our estimates for the northern white- cheeked gibbon. According to the bestmodel, gibbon abundancewas posi-


FIG. 2 Relationship between predicted gibbon abundance (solid line) and area of mixed deciduous forest in 1-km radius around the survey point. Dashed lines represent the 95% confidence interval.


tively related to the proportion of deciduous forest area, with gibbon groups being unexpectedly less numerous in ever- green forest. As gibbons are exclusively arboreal and mostly frugivorous (Geissmann et al., 2000), they are primarily found in mature forests with dense canopy cover, which is mostly associated with evergreen forests (Hamard et al., 2010). Thus, we presume that gibbons do not prefer mixed deciduous forest over evergreen forest but that there has been more intense poaching in the latter, because species


Oryx, 2020, 54(6), 767–775 © 2019 Fauna & Flora International doi:10.1017/S0030605318001515


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