Our study shows that although mammals are negatively impacted by the highway, the effects are mixed (Fahrig and Rytwinski 2009). We found that herbivores and small to medium sized mammals occurred more on the road and away from it, with particular species such as Rhesus macaque, Gray mongoose, Wild boar and Small Indian civet preferring roads. Sloth bears, Tigers, Leopards, Honey badgers, Four-horned antelopes, and Rusty spotted cats occurred mostly away from the road. In general, we found that carnivores avoided roads, whereas herbivores were more tolerant. The sole primate species detected, the Rhesus macaque, occurred mostly along the highway which can be directly linked to frequent food provisioning by the tourists that attracts them to roads and roadside habitats (Pragatheesh 2011; Underhill and Angold 2000). This is an instance of roads altering mammal behaviour inside forests. Studies on road avoidance behaviour are scarce, and studies indicate a reduction in the density of species in habitat near roads (Jaeger et al. 2005; Cullen et al. 2016). In addition to this, species may avoid crossing roads, leading to changes in home range, foraging patterns, and social interactions. Such behavioural shifts can negatively impact reproduction and survival (Bissonette and Adair, 2008). The PT route had no such food provisioning, and hence did not have any macaques.
Carnivores and larger mammals occurred largely away from the road (Fahrig and Rytwinksi 2009). For large carnivores like Tigers and Leopards which require undisturbed forests, National Highways carrying high vehicular traffic can pose a particular threat and can have a significant impact on their movement, habitat use and ecology (Jaeger et al. 2005; Zhou et al. 2022). Based on our study, we conclude that carnivores and large mammals in NSTR are particularly affected by vehicular traffic on the highway. For this reason, animals like Tigers, Dhole, and Honey badgers prefer areas away from the highway. For the same reason, they overall prefer the PT route much more than the highway. The PT route had a significantly higher richness of carnivore species, indicating that carnivores were most affected by vehicular traffic on NH. However, on the PT route carnivores had no difference in richness between the route and away from it, indicating that they are tolerant of the route itself, but not of vehicular traffic. Interestingly, herbivores showed no such trend, and occur at the same richness on the highway and the PT route. They also occurred at the same richness on RCT and ARCT, indicating that they are much more tolerant of vehicular traffic than carnivores in NSTR.
Roads and vehicular traffic movement through forests entail many negative impacts on wildlife (Barrientos et al. 2021). Although several studies on the impacts of roads on wildlife have been conducted, we still remain far from understanding how roads affect wildlife populations inhabiting roadside habitats in the long term, especially in biodiversity-rich tropical regions (Benitez-Lopez et al. 2010; Barrientos et al. 2021). Our study for the first time shows the impacts of a major highway, the NH 765 on spatial richness and occurrence patterns of mammals including large carnivores like the Tiger and Leopard, in India’s largest Tiger Reserve situated in the poorly known Eastern Ghats.
The NH 765 leading from Doranala to Srisailam passes through the core of NSTR, and experiences high vehicular traffic particularly in the mornings at the rate of 192 vehicles per hour (129 entering and 63 exiting between 6–7 am after the TR opens) comprising of 38 buses and trucks, 85 cars/jeeps, 8 three wheelers and 60 two wheelers. Vehicular traffic averaged 3.2 vehicles a minute in the mornings with a high proportion of buses, and nearly 50 vehicles await the opening of the Tiger Reserve every morning (observed by our team). By afternoon, traffic increased to 243 vehicles per hour (4.1 vehicles per minute). Despite this high movement of vehicles, our study detected a number of mammal species along NH 765 both on and away from the road. Our study shows that both small, medium and large mammals are using the roadside habitat along the NH (Fahrig and Rytwinski 2009; Adams et al. 1983). This habitat use could increase their proximity with humans, and increase the risk of roadkills, especially of those species with high densities in the roadside habitat (refer to Fig. 7, 8). In general, species with greater mobility, lower reproductive rates, and larger body sizes are known to be more vulnerable to the negative effects of roads and vehicular traffic (Rytwinski and Fahrig 2012).
The National Highways in NSTR comprise only 0.042km/km2 road density which is considerably lower than that recommended by Forman and Alexander (1998) which suggests a road density of 0.6 km/km2 as the maximum that would support a naturally functioning landscape containing sustained populations of large predators and other species. However, the road estimate in our study does not take into account the small trails. We suggest that the increasing spatial distribution of mammals along the 40 km section of the road away from the forest edge was partially driven by vehicular traffic and partially by avoidance of humans, but these variations in diversity and distribution patterns are mainly caused by ecological factors.
The impacts of traffic intensity on animal movements can also be difficult to disentangle from the influence of road characteristics, because of temporal variations in traffic volume and positive correlation with road width. There needs to be systematic monitoring of vehicular traffic movement and wildlife mortalities within protected areas, which is currently largely absent in India (Gubbi et al. 2012). Previous studies on mammal abundance from NSTR have found that the Reserve contains a relatively lower abundance of wild ungulates compared to other Tiger Reserves (Kumar et al. 2023; Srinivasulu 2001). Therefore, road expansion in NSTR, particularly expansion of the NH, could become a major threat to wildlife by impacting abundance and spatiotemporal behaviour patterns of mammals. We estimate that the potential widening of NH 765 will result in the loss of 42 hectares of forest that support important wildlife including IUCN red listed species such as Tiger, Leopard and Sambar, along the 40 km stretch. Our findings suggest that herbivores and carnivores are differentially affected by roads and vehicular traffic, with carnivores being much more negatively impacted. To ensure wildlife movement and population connectivity, mitigation measures such as improving forest cover and habitat connectivity along the roadside need to be undertaken, and dedicated studies are required on evaluating the effectiveness of such mitigation measures in minimizing the impacts of roads on wildlife populations (Barrientos et al. 2021; Rytwinski et al. 2015; van der Ree et al. 2015).