Introduction

Roads that cross protected areas are built to simplify movement of goods and services, but they have been shown to negatively affect wildlife resources in various protected areas worldwide due to roadkill incidents [1–3]. These roads are expected to grow in size, number, and traffic in years ten to come [1,3] and some might be upgraded from unpaved into paved roads shortly including the NCA.

Worldwide, there are about 36 million km of road networks [4]. It is estimated that an additional 25 million km of roads by 2050 [4,5]. Expansion of road network is also expected in developing countries containing exceptional biological diversity and majority of hotspot areas [6]. In India, roadkill incidences were reported to highly threaten lives of many native wild animals and birds [7]. It is estimated that 340 million birds killed on the roads annually in the USA and 13.8 million birds annually in Canada [3]. In a 13 years monitoring that was done in Changbai Mountains of northeast China, a total of 13, 309 vertebrate roadkill belonging to 91 species with a roadkill rate of 6.35 ± 2.41 per 100 km per day was recorded [8]. In sub-Saharan Africa, roads in protected areas were found to highly affect animals foraging site, habitat, and acting as barriers to access corridors for their movement [9]. Due to forecasted impacts on biodiversity due to expansion of road networks worldwide, evaluating their current and future impacts is critical for improving current management practices and biodiversity conservation [1,10].

The NCA which is a UNESCO World Heritage Site and part of the Serengeti-Ngorongoro Man and Biosphere Reserve [11] is not an exceptional to this scenario. Since, in the course of supporting tourism and administration activities, anti-poaching operations, and community development, the Ngorongoro Conservation Area Authority (NCAA) has established over 1300 km long road networks classified based on various categories of road use [11]. The established road networks connect different tourist attractions, and villages as well as link the Northern Tanzania circuit Arusha, Manyara, Kilimanjaro, and Tanga with the lake zone regions (Mara, Shinyanga, Simiyu, and Mwanza) [12]. The main road from Lodoare to Golini (82 km) bisects the conservation area, allowing passage of vehicles carrying services such as goods, food, and people [11].

Moreover, between 1997 and 2013, vehicles entering the NCA increased from 24,164 to 129,968 [13,14]. With the expected increase in tourist demands in line with vehicle traffic, NCAA has planned to construct and upgrade different roads to intensify the accessibility of natural attractions as a strategy to diversify tourism activities. Yet the increased road network and upgrading of roads is a major concern in wildlife conservation and priority research in road ecology due to its potential impacts on wildlife and the ecosystem health at large [10,14–16]. In addition, previous studies have found that linear infrastructures such as roads have negative impacts including habitat loss and fragmentation, disturbance due to edge effects, and wildlife-vehicle collisions leading to road mortality [15–18]. Most roadkill studies are conducted in developed countries, but there are limited information on the impacts of roads on wildlife in the African countries [19]. Only partial information exists on wildlife roadkill’s within the road networks of NCA [1].

Therefore, this study aimed at determining factors influencing wildlife roadkill in the area. The study firstly hypothesized that small bodied animals would be more hit by vehicles compared to medium to large animals along the Lodoare- Golini Road in NCA. Secondly, the frequency of wildlife roadkill would occur differently between habitat types within the NCA. The information accrued from this study would help in minimizing the impacts of wildlife roadkills and increase the safety of people and their properties within NCA. Further, it will create awareness and issues to consider for different stakeholders in designing and expanding road networks, especially within protected areas.

Materials and methods

Results

Diversity and composition of wildlife roadkill

The study results revealed that 85 individual animals (28 and 57 in the dry and wet seasons, respectively) belonging to 21 families, and 5 mammalian, 3 reptilian, and 10 bird orders were recorded killed during 10 field trips in the area. These animals comprised 26 species, which included 10 mammalian, 5 reptilian, and 11 bird species at an overall rate of 0.065 roadkill km per month. Moreover, more birds (n = 59; 69.4%) than mammals (n = 16; 18.8%) and reptiles (n = 10, 11.8%) were found killed in the area. Hence, birds are more affected group due to roadkill in the area. Amongst the bird species, the night jars (Caprimulgus europaeus; n = 26, 30.6%) were most frequently recorded killed followed by black rat (Rattus rattus; n = 6, 7.1%) on the mammals’ side and chameleon (Chamaeleo chamaeleon; n = 3, 3.5%) on the reptiles’ side (Table 1). An average of 164 kilometers was surveyed per day, which resulted a total effort of 131,200km of driven transects per year. In addition, the number of wildlife roadkill did not differ significantly in the area (X² = 14.408, df = 26, p-value = 0.9672). Moreover, regarding conservation status, the wildlife roadkill differed significantly based on their IUCN status (X² = 3.771, df = 125, p < 0.001), as one of them was found only endangered (Secretary bird, 1.2%) and the majority were least concerned (90.2%). While the remaining belonged to near-threatened (8.6%) IUCN status. Least concern species are plentiful in the wild, their chances of being affected by roadkills incidences is higher.

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Table 1. Wild animal Species found killed along the Lodoare-Golini Road transect.

https://doi.org/10.1371/journal.pone.0323994.t001

Discussions

Worldwide, wildlife species have to maneuver through roads considered as modified landscapes, and face the risk of a collision with a vehicle on the road [28]. The study findings show that different taxa were involved in roadkill cases within NCA as had been previously reported to occur elsewhere in the world [1,2,25,29]. Therefore, they confer to previous findings, that wildlife taxa in most protected areas around the world, and more specifically in NCA are impacted by roadkill accidents. This study found that more road kill incidences occurred during wet season compared to dry season. This might be the result of increased animal activity during wet season [30,31], poor visibility of drivers [32,33], waterlogged habitats forcing animals to use avoided areas such as roads during wet season might attract certain species especially in arid and semi-arid areas where moisture is scarce [34]. Again, the study found that more bird species were impacted by roadkill cases compared to other taxa within NCA, similar to those reported previously [1,2,35]. A study that was done in Latin America found similar observation that birds are highly affected taxa in road kill incidences [36]. Similar findings were observed by [28] in South Africa. In a study that was done in Northern Tanzania by [25] found that birds were the most frequently killed taxon, followed by mammals, reptiles and amphibians. This could be because birds are known to search for food on roads more likely than other taxa and, hence, become more prone to roadkill [35]. Bird roadkill may occur while they are either perching on or near the road and are scared away by the oncoming vehicle, or they may clash when flying across the road at a low elevation [25,29,35]. In addition, heat stored on the road surface is released into the atmosphere at night, creating heat islands around roads, birds and snakes preferentially aggregate on or near warm roads, increasing their risk of being hit by cars [37]. Moreover, many bird species are probably put in danger in traffic because of over speeding of drivers as birds lack the cognitive ability to swiftly digest all the information about distance, direction, and speed from vehicles [35].

Amongst the birds in our study, the nocturnal Night jars (C. europaeus) were most frequently killed. This might be contributed by their habit of settling on roads in the open space and warmth of the road surface attracting them, particularly in the cooler evening or early morning hour [38]. Further, headlights of cars stun nightjars, hence they are considered as traffic casualties species [39]. Due to its altitude, topography and temperature inversion, NCA is often shrouded in fog, particularly in the early mornings or during the rainy season. Majority of drivers turn on headlights of their cars for increased visibility, detect road hazards and vehicle visible to others reducing the likelihood of accidents [40]. A study that was done by [2], found higher road mortality likely to occur in birds with smaller bodies, higher population densities, longer lifespans, omnivorous diets or diets predominately made up of plants and seeds. Explanation killing birds could be that the late drivers who want to get out of the NCA gate before 06:00 pm or late hours are overspeeding their vehicles while at the same time, the vehicle’s lights are on to an extent that they blind the birds’ eyes and also do not give a chance for the species to take off and run away [1,41]. The study also recorded the mortality of Secretary Birds (S. serpentarius), a species listed by the International Union for Conservation of Nature (IUCN) as endangered [42]. Previous studies found that Secretary Birds prefer road surfaces as hunting grounds for capturing different prey species or for taking advantage of the presence of carcasses on road, which consequently exposes them to vehicle collisions [1]. When roadkill impact secretary birds, it significantly result into their population decline, reducing breeding success and decrease their genetic diversity [43].

Furthermore, this study found that few numbers of mammals (n = 16) were recorded killed by vehicles likely because they spent very short time around the roads. After all, food and water are limited near roads [1]. Moreover, probably they might have been scavenged or removed by other animals/people in the area [44]. On the other hand, the Black rat (Rattus rattus) was the one species frequently hit by vehicles in the area. This might have been caused by their strong preference for feeding along vegetated road verges or sometimes due to their high population density [1,45]. Other contributing factors could be the fact that they are small in size which makes them more difficult for drivers to detect their presence and avoid those [46].

Despite that reptiles are most likely to be victims of roadkill’s due to their distinguishing long body morphology and their behaviour of frequently using roads as thermoregulation sites [47] but again, few numbers of them were recorded killed in NCA, and most representing the chameleons (C. chamaeleon). In a study that was done across Europe, reptile casualties was found more pronounced compared to other taxa which is in agreement with current study [48]. This could be probably because this species is a slow-moving reptile, and therefore commonly hit by vehicles [47].

Interestingly, the study recorded only two killed carnivore species including bat-eared fox (Otocyon megalotis), and wildcat (Felis silvestris). These animals were probably killed while either foraging or crossing the road or due to their inability to escape from overspeeding vehicles [1]. When water is scarce, carnivores normally expand their home ranges and often cross roads through their expanded territory [28]. This might lead them into roadkill cases. Additionally, the wildlife roadkills with small body sizes (<30 Kg) were most frequently recorded killed compared to medium body-sized (31–50 Kg) and large-body sized (>50 Kg) probably because they are small in size which makes them more difficult for drivers to see and avoid [47].

The study results again found that wildlife roadkills occurred mostly in woodland habitats in the area. This finding strongly agrees with that found in southern Portugal, where there was a strong relationship between vulnerability to traffic deaths and foraging behavior and habitat type, with the most vulnerable species being the small woodland species that prefer to feed in shrubs and trees in the area [2,49]. Furthermore, ecological and behavioral characteristics increases the vulnerability of some species [2]. In addition, in woodland habitats probably there are more forage resources, shelter, and cover for most of the wildlife found in the area, and thus the increased movements encountered during searching for those resources [50] could have increased their chances for collision with vehicles. Additionally, the type of habitat close to the roads can attract different wildlife species especially birds to the roads and increase their risk of becoming roadkill victims [47]. Furthermore, [51] reported that high species richness areas were having more roadkill incidences compared to low species richness areas.

Conclusion and recommendations

The present study recorded a substantial number of reptiles, mammals, and birds as roadkill victims on a stretch of the 82 km main road located in the NCA. The loss of individual wildlife species by roadkill has negatively impacted the structure of the terrestrial wildlife community and caused a severe loss of biodiversity in the area. Therefore, study recommends enforcing punishments and penalties for overspeeding drivers, installation of cameras to monitor speed limit especially within protected areas, providing regular awareness on wildlife roadkill to different road users, constructing underground tunnels in suitable areas and placement of road signs for different road users. This needs to be included in strategies and plans to mitigate the loss of biodiversity in the area. Furthermore, it is critical to proceed with roadkill monitoring in the area to avoid further biodiversity loss.

Acknowledgments

We are thankful to Ngorongoro Conservation Area Authority for logistics support to accomplish this work. Also, we thank the College of African Wildlife Management (MWEKA) and Tanzania Wildlife Research Institute (TAWIRI) for their technical guidance in designing the research data collection, analysis, and manuscript writing. We are very much grateful to Flora Assey, Peter Nyanswi, Eugene Cyrillo, Francis Makari, James Rite, Emmanuel Kija, Houssein Kimaro, Emmanuel Lyimo, Alex Kisingo and Pius Mollel for their support in accomplishing this work. Furthermore, we are also grateful to the research assistants for their commitment during data collection. Lastly the drivers; Nsajigwa Mwaisemba, Dissmas Harya, Mzee Juma Bakari, Omari Msakamali, and Asantael Uronu for their full commitment to participating during the whole period of the study.