Nesting behaviour of Indian Giant Squirrel (Ratufa indica) in the exotic timber plantations of the Palani Hills, Western Ghats, India

doi:10.21203/rs.3.rs-2447130/v1

Exotic timber plantations have expanded in the montane habitats of the Western Ghats in the last century, often replacing the natural grassland-rainforest mosaic. This has inadvertently increased tree connectivity across the landscape, affecting arboreal mammals in varied ways. Here we report that the endemic Indian giant squirrel nests in exotic timber plantations in the Palani Hills, a behaviour not previously described. An extensive survey of timber plantations revealed a total of 70 nests, with 73% of the nests found on exotic Eucalyptus trees. An investigation of the landscape and environmental factors that were associated with the choice of nesting locations revealed that nest sites were significantly closer to rainforest patches. Squirrels also nested on east-facing slopes, away from human disturbances like agriculture. This study is the first to report giant squirrel nesting behaviour in exotic timber plantations across a landscape mosaic. We suggest that policymakers and managers proceed with caution while clearing plantations in an attempt to restore native vegetation.

Exotic timber plantations

Eucalyptus

Ratufa indica

Palani Hills

Nesting sites

Forests worldwide are declining, and planted forests or plantations, which are evenly-aged single-species stands, are expanding rapidly (Paquette and Messier 2010). Globally, plantations constitute over 7% (294 million ha) of the total forest area, and the area of planted forests is increasing by about a percent annually (FAO 2020). A large body of research shows that plantations support lower biodiversity than native forests across forest biomes and taxa groups (Barlow et al. 2007). The widespread expansion of oil palm plantations in Southeast Asia failed to support mammal diversity (Yue et al. 2015). Monocultures of Eucalyptus, bamboo and Japanese cedar in China resulted in reduced diversity of both birds and bees compared to the native forests (Hua et al. 2016). Small mammal densities in teak plantations in the Anamalai Hills, Western Ghats, were far lower than in natural forests (Chandrasekar-Rao and Sunquist 1996). Additionally, planted forests, especially of exotic tree species, are often hotspots of alien plant invasions (Csecserits et al. 2016). Over the last two decades, however, there have been contrasting results from studies that examine the impact of plantations on native biodiversity. Tree plantations were found to retain and provide valuable habitat for a wide variety of species, including native plants, animals and fungi (Humphrey et al. 2000; Pawson et al. 2008; Rodríguez et al. 2021). In exotic Eucalyptus plantations of Mexico, densities of black howler monkeys (Alouatta pigra) were higher than in adjoining rainforests (Bonilla-Sánchez et al. 2012). A 17-year-long multi-season temporal study in southeastern Australia showed that Eucalyptus plantations with remnant vegetation had unexpected support for diverse ground-dwelling and arboreal mammal communities (Law et al. 2017).

Mammals, particularly small to medium-sized groups, play a significant role in forests as seed predators, dispersers and prey, but they are largely unstudied in converted plantation systems. The Indian giant squirrel (Ratufa indica; hereafter IGS) is a large-bodied, arboreal, diurnal squirrel endemic to India. It occurs in southwestern, central and eastern peninsular India, where its distribution is severely fragmented (Borges 2015). IGS inhabits mainly moist deciduous, riverine and evergreen forests. In the Kalakad–Mundanthurai Tiger Reserve in Tamil Nadu, IGS occupy large contiguous stretches of moist and wet evergreen forests (Srinivas et al. 2008). Kanoje (2008) recorded 245 IGS nests along riparian areas of Sitanadi Wildlife Sanctuary, Chhattisgarh. Multiple studies have shown that the nests are built in tall, profusely branched trees in the highest parts of canopies (Datta and Goyal, 1996; Borges 2007; Pradhan et al. 2017), including in teak stands (Ramachandran 1992). IGS is also recorded in coffee plantations with largely natural rainforest overstory shade trees (Bali et al. 2007).

An important challenge in biodiversity conservation is to quantify the impacts of timber plantations, especially in regions undergoing large shifts in land-use change. The Western Ghats mountains along the west coast of the Indian peninsula are amongst the most biodiverse regions in the world. High-altitude mosaics of undulating grasslands interspersed with stunted evergreen forests, locally known as ‘shola’, occur on the tops of the mountain chain, and are a unique habitat (Robin and Nandini 2012). Over the last half century, large expanses of these rainforests and grasslands were replaced by homogeneous plantations of tea, Eucalyptus and Acacia due to their higher profitability (Arasumani et al. 2018). In the upper Palani Hills of the Western Ghats, exotic timber plantations were first established by European settlers, starting in the 1830s (Matthew 1962). Over the last five decades, the Palani Hills has undergone an expansion of recognised invasive species such as Acacia mearnsii, Eucalyptus spp., Pinus spp. (Arasumani et al. 2018). Though timber was primarily planted to supply wood, many such plantations were eventually not harvested due to changing forestry policies, and thus have stayed undisturbed for the last 20 years. This provides a unique opportunity to study the use of timber plantations by species that occur in the adjoining rainforest patches. The landscape is home to several arboreal mammals, including the canopy-dwelling IGS. We conducted a survey to identify nests of IGS in timber plantations and identified landscape variables which are most important in discriminating between nest sites and non-nest sites.

Fieldwork was carried out in the high-altitude landscape of the Palani Hills, Tamil Nadu, southern India. This eastern offshoot of the Western Ghats is located between 10 ̊6’ to 10 ̊20’N and 77 ̊16’ to 77 ̊24’E. The elevation of the study area ranges from 1400m to 2300m above mean sea level. The Palani Hills mountains were once widely covered by continuous grassland with montane rainforests (‘shola’ forests) in the valleys. Currently, the landscape is a mosaic of settlements, farmlands, exotic timber plantations of Acacia mearnsii, Eucalyptus spp., Pinus spp. and a few remnant patches of natural forest and grassland (Fig Ⅰ). Plantations account for 42% of the landscape (Arasumani et al. 2018). The IGS is the only giant squirrel that occurs in the higher altitudes of the Palani Hills, while the other large squirrel found in southern India, the grizzled giant squirrel (Ratufa macroura), is restricted to lower altitudes. The region also harbours two species of palm squirrels-Funambulus palmarum, F. sublineatus, and one nocturnal flying squirrel- Petaurista philippensis.

Targeted searches for nests were conducted in the exotic timber plantations between January and March 2019 and in September 2021. IGS builds large globular nests of leaves and twigs, with a discreet opening either on the side or the top. The nests are conspicuous and easily located during searches. The mapped timber plantation land cover classification (Arasumani et al. 2018) was gridded into 250 x 250m quadrants and 234 of these were randomly selected for nest searches. We walked around each quadrant to search for nests in the canopy. For each nest sighted, we recorded the tree species used for nesting, tree height, height of the nest, nest condition (fresh or dry) and nest shape (cup-shaped, globular, flat or messy) visually. Nest materials used were determined from photographs taken with a Canon 60D DSLR and a 300mm lens. Tree abundance was measured in a circular plot of radius 5m centered around the nest tree. Plants with GBH > 30 cm were counted as trees. Plantation trees were identified to genus and/or species and were grouped into Eucalyptus spp., Pinus spp. and Acacia mearnsii for further analysis. All native trees were grouped as shola trees. Random quadrants were selected from the same timber plantation land cover map, across the study area. Identical vegetation characterisation was done around the central tree in the random quadrants. Distance to the nearest shola and agriculture were extracted from a land use-land cover layer (Arasumani et al. 2018) using ArcMap 10.5. Mean temperature (2017 to 2019) and mean precipitation (2016 to 2018) were extracted from CHELSA version 1. Topographic variables (aspect and altitude) were extracted from the Terra Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (USGS). The aspect and slope values were extracted in degrees and converted to linear variables using a sine transformation (sin(π degree/180)). Results for statistical tests reporting means are indicated as mean ± SE. We used Chi-square analyses and univariate Mann-Whitney U tests to test for the significance of observed differences between the site characteristics in nest and random quadrants. To examine the factors influencing the presence of IGS nests, we ran generalized linear mixed models (GLMM) with a binomial family. Akaike’s information criterion (AIC) was used to rank the models. All statistical analyses were performed using R Statistical Software (version 4.2.1).

A total of 70 nests of IGS were recorded in 35 distinct quadrants in plantations during the study period (Fig Ⅰ). IGS was found to use five tree species in exotic tree plantations for nest construction. Nests were found on Eucalyptus spp. (73%), Alnus nepalensis (17%), Prunus ceresoides (6%), Pinus patula (3%) and Eleaocarpus tuberculatus (1%) (Χ² = 127.57, df = 4, p < 0.0001). The nests were cup-shaped (31%), globular (25%), messy (25%) or flat (17%) (Χ² = 2.914, df = 3, p < 0.05) and were made up of twigs and leaves. Eucalyptus leaves was the most used nest material (47% of nests), followed by twigs (20%) and shola tree leaves (5%) (Χ² = 58.508, df = 4, p < 0.0001). Over 31% of the nests faced northeast (between 22.5° and 67.5°), followed by east (17%) between 67.5° and 112.5° (Χ² = 18.257, df = 7, p-value < 0.01).

IGS built nests on tall trees (34.8m ± 1.8, n = 70), and nests were typically located high up on the trees (26.9m ± 1.4, n = 70). We found variation in the heights of the trees used (range 10m to 60m), as well as the nest heights (range 8m to 50m). Overall, however, the tree height influenced the nest height across all tree species (R² = 0.79, F(1, 68) = 258.9, p < 0.0001) (Fig ⅠⅠ).

Acacia mearnsii (not used by IGS for nesting) was more abundant in nest quadrants (6.94 ± 1.55) than random quadrants (4.19 ± 0.50) (W = 2488.5, p < 0.01). The mean number of Eucalyptus trees around nest trees (5 ± 0.57) was not significantly higher than around random trees (4.82 ± 0.35) (W = 3387.5, p = 0.7956). Pinus patula was more abundant in random quadrants (0.94 ± 0.25) than in nest quadrants (0.91 ± 0.56) (W = 3335, p-value < 0.05). Nest quadrants had a density of shola trees (4 ± 0.92) that was not significantly different from the random quadrants (3.54 ± 0.44) (W = 2869.5, p-value = 0.7325).

At the landscape level, we found that nest trees were predominantly located near shola forest patches (509m ± 103, W = 4695, p < 0.001), and away from agricultural fields (1601m ± 208, W = 1605, p < 0.0001). Lower mean temperature was recorded in nest quadrants compared to random quadrants (W = 5815, p < 0.0001). Nest quadrants had higher precipitation than random quadrants (W = 2312.5, p < 0.001). Nest quadrants had lower aspect values (east-facing slopes) than random quadrants (W = 5076.5, p < 0.0001) (Fig ⅠⅠⅠ). Distance to land cover classes (shola and agriculture), topography (slope and aspect) and climate (mean temperature and mean precipitation) were included in the full model (AIC = 149.02 ). The model receiving the best support included distance to shola, mean temperature and aspect (AIC = 145.77) (Table I).

There has been no prior study on the nesting behaviour of IGS in the Palani Hills, and this is the first report of them using exotic timber plantations at a landscape level for nesting. Our survey also quantified habitat characteristics of nest sites of IGS. Interpreting these results requires caution since the design of this study was not intended to test habitat selection. We did not study other habitats used by IGS, such as shola forests. Our results showed that the studied exotic timber plantations were a novel nesting habitat for IGS. Large Eucalyptus trees were a key structural element, which can allow squirrels to climb higher in the canopy, staying out of the reach of terrestrial predators. The relatively strong Eucalyptus trees could also provide good support for building a large nest. This is in agreement with studies that have found that IGS primarily prefer mature trees (Datta and Goyal 1996; Pradhan et al. 2017), which provide nest-building resources (Pradhan et al. 2017). It was evident that IGS did not construct nests only in the topmost canopy. This might be to provide some nest concealment from avian predators (Pradhan et al. 2017). Twigs and leaves of Eucalyptus were significant contributors to the IGS nests. A report of a single IGS nest on Eucalyptus in Satpura Tiger Reserve found that 231 E. globulus twigs were used in the nest wall (Kumbhar et al. 2012). Although we did not observe IGS using any part of Eucalyptus for feeding, on multiple occasions, they were found eating the seeds and flowers of Acacia mearnsii, which is the dominant tree species in the nest quadrants. Most of the nests were east-facing, probably to provide the best thermal environment as the sunlight can reach nests earlier in the morning. Previous studies on IGS nests in Kuldiha Wildlife Sanctuary, Odisha and Satpura Tiger Reserve, Madhya Pradesh reported similar results (Kumbhar et al. 2012; Nayak and Patra 2015). Although studies on habitat use by IGS in exotic plantations are scarce, there are reports of IGS nesting (Kanoje 2008) and feeding in teak stands (Baskaran et al. 2011). These suggest that IGS might be able to occupy monoculture plantations, though it is likely that the limited food choices in these plantations are only overcome by foraging in adjoining forests. IGS can easily move to the surrounding forest matrix for food (Borges 2015), although details of their movement and feeding behaviour need to be studied in detail. The nest trees in this study were located predominantly near shola forest patches with high structural complexity, and it is likely that these forest patches offer forage diversity, relatively fewer predation risks and more favourable microclimatic conditions than the more open, exposed monoculture plantations. Most of the located nests were found away from the agricultural fields (always associated with settlements in Palani Hills), suggesting they are actively avoiding human disturbances. An occupancy survey at Kalakad–Mundanthurai Tiger Reserve similarly reported that IGS actively avoid human disturbances (Srinivas et al. 2008).

Our findings suggest that IGS is able to survive to some extent in largely monoculture timber plantations, but only when these are contiguous with intact rainforests and away from human disturbance. It is necessary to carry out intensive ecological studies on occurrence and behaviour of, and habitat use by IGS in timber plantations and the surrounding forest matrix to determine the adaptability of the species to modified habitats. Till we have a clearer understanding of how endemics like the IGS use modified landscapes, we suggest that managers proceed gradually when replacing exotic timber plantations with native species. It is important to guarantee the availability of mature forest patches or trees for the secure nesting of such arboreal mammals. It might be possible to somewhat mitigate the negative impacts of monoculture stands by increasing their habitat quality with measures such as planting diverse native understory vegetation. Such measures will increase the structural heterogeneity of these exotic timber plantations, which will benefit not just arboreal mammals, but other native taxa as well.

Statements and Declarations

The work was supported by a Science and Engineering Research Board (SERB) Early Career Research (ECR) grant (grant number: ECR/2016/002041). Author Nandini Rajamani received research support.

The authors have no relevant financial or non-financial interests to disclose.

All named authors have contributed to conducting the research and Aravind P S and Nandini Rajamani were involved in drafting the manuscript.

All authors contributed to the study conception and design. Data collection was performed by Aravind P S and Sanjay Prasad Ganguli. Material preparation and analysis were done by Aravind P S, Senan D’Souza and Nandini Rajamani. The first draft of the manuscript was written by Aravind P S and all authors commented on all versions of the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The work was funded by a Science and Engineering Research Board (SERB) Early Career Research (ECR) grant (grant number: ECR/2016/002041) to Nandini Rajamani. We are grateful to Tamil Nadu Forest Department for permission to carry out the study. We thank the Kodaikanal International School and IISER Tirupati for their support. Eswaran and Kamaraj assisted with the field work. We also thank Robin VV and his field team for sharing field logistics. We thank all the Sciurid Lab members for their comments on the manuscript.

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Table Ⅰ: Results of GLMMs assessing the effect of (A) all variables and (B) variables with significant differences on IGS nest presence.

	Estimate	Std. error	Z value	P value	Sign.	AIC
Quadrant ~ distance to shola + distance to agriculture + mean temperature + mean precipitation + slope + aspect						149.02
Distance to shola	-0.8568	0.3951	-2.168	0.0301	*
Distance to agriculture	-0.2284	0.2480	-0.921	0.3571
Mean temperature	-1.8607	0.4340	-4.287	<0.0001	***
Mean precipitation	-0.3273	0.4385	-0.746	0.4554
Slope	-0.1920	0.2330	-0.824	0.4100
Aspect	0.6335	0.2324	2.726	0.0064	**
Intercept	-2.8546	0.3799	-7.513	<0.0001	***
Quadrant ~ distance to shola + mean temperature + aspect						145.77
Distance to shola	-0.7362	0.3676	-2.003	0.0452	*
Mean temperature	-1.4612	0.3050	-4.791	<0.0001	***
Aspect	0.6016	0.2273	2.647	0.0081	**
Intercept	-2.7340	0.3558	-7.685	<0.0001	***

^{significant categories were set as ‘*’ p <0.01, ‘**’ p <0.001, ‘***’ p <0.0001}

Nesting behaviour of Indian Giant Squirrel (Ratufa indica) in the exotic timber plantations of the Palani Hills, Western Ghats, India

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Abstract

Figures

Introduction

Methods

Results And Discussion

Declarations

References

Tables

Status:

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