LINKED PAPER
Demographic history of the fragmented Yellow-throated Bulbul (Pycnonotus xantholaemus) population in the Deccan Peninsula, India. Jha, A. & Vasudevan, K. 2020 Endangered Species Research. doi: 10.3354/esr01062 VIEW
Our protagonist Yellow-throated Bulbul (Pycnonotus xantholaemus) (hereafter ‘YTB’) has it really tough. It is endemic, rare and vulnerable to extinction due to habitat loss. Its prime habitat holds huge reserves of granite, a key raw material for all urban construction projects. Currently it lacks any conservation attention. This sparrow-sized fruigivore prefers rugged terrains along the hill slopes of the Eastern Ghats Mountain range and inland hillocks (inselbergs) in Southern India. Exposed granitic rocks and open scrub forests characterize its preferred habitat. Even in its preferred habitat it is rare, found mostly in pairs or small flocks of 4-5 birds. It shares its habitat with the congener White-browed Bulbul (P. luteolus), another endemic but super abundant big bully and Red-vented Bulbul (P. cafer), a generalist species notoriously famous for being one of the 100 ‘World’s Worst Invasive Species’. YTB is found in patches of suitable habitat dispersed across its vast extent of occurrence (Jha 2022). Most information on its natural history comes from opportunistic sightings and works of ornithologists who were active during the mid-19th to mid-20th century. Evolutionarily, YTB does not have any sister species and shared a common ancestor with other Pycnonotus species ca. 6–8 Ma. It is the descendent of one of the oldest bulbul (Pycnonotidae spp.) lineages in South Asia (Jha et al. 2021).
Figure 1 Typical Yellow-throated Bulbul habitat: hillock with exposed granitic boulders and scrub vegetation © Ashish Jha.
Figure 2 Location of sampling sites across four states in Southern India. Names of the states are in bold; the number shown near the sampling location denotes the total sample obtained from that particular region. Inset shows the location of mist netting sites and the area of occurrence of Yellow-throated Bulbul.
Research questions
Are the disjunct YTB populations genetically connected?
Are these ‘refugia populations’ restricted to what’s left of once widespread habitat? Or are these populations the result of gradual range expansion as populations hopped from one habitat to the nearest suitable habitat?
Data collection and analysis
We collected tail-feather samples from 63 individuals after a cumulative 245 hours of mistnetting at 30 locations across the species’ distribution range in Southern India. We amplified 1050 base pairs using a mitochondrial Control region and looked at genetic diversity, historical demography and population structure. Results revealed high genetic diversity (hd > 0.5) and low nucleotide diversity (π < 0.005). The negative Tajima’s D values (−0.155, P < 0.05), Fu’s Fs (−0. 60, P < 0.05) and a small positive value for Ramos-Onsins and Rozas’ R2 (0.10, P < 0.05) indicated population expansion. The Bayesian skyline plot estimated the timing of the onset of population expansion of YTB at approximately 0.1 Ma. The median-joining haplotype network consisted of 39 haplotypes and 12 median vectors; the topology of the network suggested lack of structuring in the populations. Almost all of the variation was observed within populations rather than among populations, which suggests that there was no critical historical disruption of gene flow.
Figure 3 Median-joining haplotype network for the displacement loop (control region) in populations of yellow-throated bulbul. Coloured circles represent the 39 haplotypes from 10 different populations. Lines on bars that separate haplotypes represent the mutational steps between the haplotypes. Empty white circles represent haplotypes that were not sampled, also known as median vectors.
Figure 4 Prime threats to Yellow-throated Bulbul habitat. (a) Mining of soil for brick making (b) Ficus tree, a crucial food source for Yellow-throated Bulbul, loped for use as goat fodder (c) Illegal quarrying of granite boulders for obtaining construction aggregate.
Conclusion
Southern India has undergone gradual aridification since the mid-Miocene (ca. 12–15 Ma). It is possible that the gradual aridification opened new habitat for the species and it expanded its range, occupying suitable habitats as it became available. Even in its optimal habitat it is a rare species and is found in small numbers which increases the chance of stochastic local extinctions. Dispersal via sub-optimal habitat corridors could be leveraging against such odds. We hypothesize that dispersal events from natal areas are essential for long-term persistence of the isolated populations of YTB. Ongoing mining activities to meet the growing demand for construction aggregate are quintessential threats to YTB habitat and dispersal corridors (Jha and Vasudevan 2020). Because YTB habitats are not well represented in the protected area network in the region and dispersal events have shaped the YTB population, conservation efforts should focus on the landscape level. To leverage support from local communities and managers, we propose that YTB be considered a non-traditional flagship species for isolated inland hillocks and their associated scrub forests.
References
Jha, A. 2022. Yellow-throated Bulbul (Pycnonotus xantholaemus), version 2.0. In Birds of the World (Rodewald, P.G. Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. VIEW
Jha, A. & Karak, S. 2021. Counting bulbuls to conserve the Deccan hillocks and scrub forests [Commentary], Mongabay, 11 January 2021. VIEW
Jha, A. & Vasudevan, K. 2020. Environmental niche modeling of globally threatened Yellow-throated Bulbul for conservation prospects in the Deccan Peninsula, India Current Science 119: 1815-1823. VIEW
Jha, A. 2019. Endemic Bulbul of an unconventional forest. The Cerd Magazine, volume 1, page 28. VIEW
Image credit
Top right:The Yellow-throated Bulbul (Pycnonotus xantholaemus) in its natural habitat © Ashish Jha.
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