- The Nilgiri pipit is a songbird that is endemic to the montane shola-grasslands of the Western Ghats. Scientists surveyed almost the entire expanse of the bird’s distributional range to see what factors govern both the presence and numbers of the species in an area.
- Surveys across 170 locations and statistical models revealed that elevation matters greatly for both pipit presence and numbers. The team also found that fewer pipits are likely to be present if grassland patches contained invasive plants such as Eucalyptus.
- Their results imply that several management actions could be crucial for the persistence of the bird in its habitat: from restoring highly fragmented and disturbed grasslands to tackling the problem of invasives such as Eucalyptus that are taking over the remaining grassland patches.
If it’s bright-hued birds you prefer watching, you’d barely look at a Nilgiri pipit twice. Moreover, the bird can be difficult to spot: the drab-coloured tones on its small, streaked body will quickly melt into the caramel of the high altitude grasslands in the Western Ghats, where it dwells.
But even within this specialised habitat, the bird occurs far more frequently at the highest elevations, finds a new study. This, coupled with the finding that invasive plants signify poor-quality habitats for the pipit, points to the urgent need to conserve the remaining patches of high elevation shola-grasslands, caution scientists.
Introduced invasive plants are indeed a worry in the higher reaches of the Western Ghats. And more so because these areas – above elevations of roughly 1,400 metres Above Sea Level (ASL) – are home to a unique but gradually-disappearing ecosystem called the shola-grassland mosaic, where montane grasslands occur beside small patches of stunted evergreen forests called sholas.
Between 1973 and 2014, for instance, the Palani Hills in Tamil Nadu alone lost almost 250 square kilometres of shola-grasslands and shola-forests; 88 percent of this loss occurred in shola-grasslands and the main culprit was the invasive, exotic black wattle tree, found a study by scientists including V. V. Robin, an assistant professor at the Indian Institute of Science Education and Research Tirupati (IISER Tirupati).
Decreasing shola-grasslands mean lesser space for grassland dwellers such as the Nilgiri pipit Anthus nilghiriensis, a songbird endemic to the Western Ghats.
So what factors drive the Nilgiri pipit’s distribution and numbers across its range?
To find out, scientists including Abhimanyu Lele (IISER Tirupati) and Robin surveyed 85 percent of shola-grasslands above 1,600 metres ASL from where Nilgiri pipits have been reported: the Nilgiris (Western Ghat hill ranges that lie north of the Palghat Gap) and the Anamalai-Palani hills (south of the Palghat Gap).
Based on available ecological information (including Nilgiri pipit home ranges), the team divided these grasslands into grids of between four to 25 hectares. They surveyed 170 such grids, visiting each grid multiple times across seven months in 2017-18 to look out for pipits.
At each site, the team also noted the presence of exotics such as black wattle Acacia mearnsii and Eucalyptus, signs of burns (since apart from accidental fires, some grasslands are set on fire in a controlled manner as a means of management), grass height, and presence of water sources. They derived other information on the grasslands – such as patch size and habitat isolation – from remote sensing techniques as well.
Higher is better
Nilgiri pipits were not uncommon in these high elevation shola-grasslands: the team spotted the birds in 109 sites (out of 170). Their statistical models – that analysed the importance of pipit habitat characteristics on bird presence and numbers – revealed that elevation matters to pipits a great deal. Grasslands at higher altitudes were more likely to have pipits occupying them.
“The species occurs far more frequently in montane grasslands above 2,000 m than in grasslands at 1,600 m, which are ostensibly the same habitat type,” wrote Lele, in an email to Mongabay-India.
Elevation also had a similar, ‘positive’ effect on pipit numbers: pipit densities (the number of pipits in each grid) were likely to be higher in higher altitude grasslands.
According to the authors, this significance of elevation – for both pipit presence and numbers – could make the bird vulnerable to climate change. Worldwide, scientists have noticed that species at higher elevations often feel the impacts of climate change first. For instance, the range sizes of high elevation birds in the Peruvian Andes have shrunk; some species have declined in abundance, while some common species have even disappeared.
“While there is no current evidence of range-shifts in the Western Ghats driven by climate change, it is quite possible such shifts have already occurred and have not been observed, because of the absence of rigorous historical distribution data,” commented Robin.
The importance of elevation to pipits also makes preserving the existing grasslands more critical, added Lele. “Because total grassland area declines rapidly with altitude, and so the extent of grasslands above 2,000 m is much smaller than the total area of montane grasslands,” he wrote.
Invasive aliens and local extinctions
The team found that pipits were also more likely to occur in larger grassland patches, and less likely in patches that were isolated from other grassland tracts. The presence of water – such as the small hill streams that trickle through grassland patches – meant higher numbers of pipits could occur in the area. Other features that had such a positive, but smaller, impact on bird numbers included the presence of burnt grassland patches and the native rhododendron tree.
However, the presence of the alien, invasive tree Eucalyptus affected pipits drastically: if it occurred in an area, it meant that fewer pipits likely used that tract. It is probable that low-density populations in areas affected by invasive species are non-viable, write the authors in their study published recently in Ecology and Evolution. Their surveys also confirm the local extinction of the pipit in some areas.
Read more: Did you ever think that tree plantation drives could threaten an ecosystem?
“Our study documented the complete absence of the Nilgiri pipit in the Kotagiri region (indeed, in the entire eastern portion of the Nilgiris), and historical data suggest that the species was previously present there, which would imply a local extinction,” said Robin.
Several management actions, ranging from tackling invasives to restoring connectivity between fragmented grasslands (many of which are outside protected areas) are urgently required, they add in their study.
Indeed, the findings of this study have significant implications for the conservation of the Nilgiri pipit, wrote scientist Dr. Kelvin Peh of the University of Southampton, who has studied tropical montane ecosystems in Malaysia and was not involved in this study. While the study is “well-designed” it would have been interesting to also explore the “habitat amount hypothesis”, or whether the amount of habitat around a grassland patch is an important predictor of Nilgiri pipit density, he added.
A factor that could influence both pipit presence and numbers more than elevation is habitat structure, opined scientist Dr. Uma Vinod, who studied Nilgiri pipits for her doctoral research in 2007. The heterogenous mix of many species of grasses and herbs in such tracts, and the microhabitats they create could be a crucial factor, commented Vinod, currently Faculty and project Facilitator at the Integrated Rural Technology Centre in Palakkad, Kerala. The current study did, however, incorporate some factors that encompass this such as grass height and the presence of rhododendron trees. These results suggest that the birds could be dependent on habitat heterogeneity too, though more detailed studies on this aspect are needed.
Vinod agrees that losing grasslands to exotic plantations and agriculture is worrisome for pipits. Along with restoring severely fragmented grasslands outside protected areas as the study recommends, restoring grass and herb species heterogeneity could be crucial to save the Nilgiri pipit too, she added.
Robin et al 2014. Reassessment of the distribution and threat status of the Western Ghats endemic bird, Nilgiri Pipit Anthus nilghiriensis. Current Science 107(4), 622–630
BirdLife International. 2016. Anthus nilghiriensis. The IUCN Red List of Threatened Species 2016: e.T22718582A94586848. https://dx.doi.org/10.2305/IUCN.UK.2016-3.RLTS.T22718582A94586848.en. Downloaded on 17 August 2020.
Arasumani et al 2018. Not seeing the grass for the trees: Timber plantations and agriculture shrink tropical montane grassland by two-thirds over four decades in the Palani Hills, a Western Ghats Sky Island. PloS One doi: https://doi.org/10.1371/journal.pone.0190003.
Arasumani et al 2019. Invasion compounds an ecosystem-wide loss to afforestation in the tropical grasslands of the Shola Sky Islands. Biological Conservation, 230, 141–150.
Freeman et al 2018. Climate change causes upslope shifts and mountaintop extirpations in a tropical bird community. PNAS 115 (47): 11982-11987.
Banner image: The Nilgiri pipit is endemic to the higher reaches of the Western Ghats. Elevation matters to the pipit a great deal finds the new study. Photo by Prasenjeet Yadav.