Policy analysisPreventing wildlife roadkill can offset mitigation investments in short-medium term
Introduction
Roads are the most common human-made features on the globe, spreading across nearly all of its surface (Ibisch et al., 2016). An additional 25 million km of new roads are expected to be built by 2050, 90% of which in developing countries (Dulac, 2013), particularly in tropical regions, which have support exceptional biodiversity and vital ecosystem services (Laurance et al., 2001, Laurance et al., 2015; Alamgir et al., 2017). These infrastructures allow human population development by facilitating the movement of people and goods, as well their accessibility to different services, e.g. schools and markets (Ali et al., 2015). However, particularly outside urban areas, roads can also become areas of conflict with biodiversity, notably due to the high number of wildlife-vehicle collisions (WVC). For example, over two million mammals are estimated to be road-killed every year solely in Brazil (González-Suárez et al., 2018).
Collisions with wild animals, mainly large ones, can also be a serious threat to human safety, causing significant injuries to drivers and passengers, in addition to considerable economic impact (Bissonette et al., 2008; Huijser et al., 2009, Huijser et al., 2013; Abra et al., 2019). In USA, it is estimated that 1–2 million WVC occur involving large mammals, of which ca. 5% cause human injury with costs surpassing US$ 8bn (Huijser et al., 2008). In California, in particular, it was estimated a material damage cost of US$ 76 million due to large wildlife-vehicle collision, for 2018 alone (Shilling et al., 2019). The annual cost of WVC involving ungulates in Sweden was estimated at US$ 406 million (Gren and Jägerbrand, 2019), and in the São Paulo State, Brazil, the total annual cost to society was estimated at US$ 25 million (Abra et al., 2019). Yet, there is a great lack of knowledge about the material costs that WVC entail in most regions, even though this information is particularly relevant for making decisions about the implementation of mitigation measures on roads.
Roadkill data is usually acquired through systematic monitoring of roads. However, roadkill surveys generally fail to fully record all casualties due to imperfect detection issues, and yet accurate estimation of the total mortality is important for precise estimates of the material costs associated with WVC. Imperfect detection may be due to one or more detection biases, including: i) researchers failing to find carcasses along the road or verges while surveying the road; ii) traffic volume, scavengers or management staff removing carcasses before searches are conducted; and iii) carcasses ending up outside the searched area (Santos et al., 2011, Santos et al., 2016; Teixeira et al., 2013). To account for such biases, researchers typically conduct field trials to estimate the effects of the first two components (Barrientos et al., 2018). For the third component, while generally less considered in mortality estimates, one can perform extra surveys outside the road corridor to count the number of carcasses therein or search for signs of injured animals.
Once a detailed knowledge of roadkill patterns is obtained, different mitigation measures can be considered to reduce WVC. Fencing is one of most common mitigation measures, and one with higher effectiveness at reducing road mortality, particularly when connecting to existing road passages (Seiler et al., 2016; Rytwinski et al., 2016; Spanowicz et al., 2020). Other measures, such as raising the road on pillars or placing the road underground are potentially more effective than fencing but are also cost prohibitive and unrealistic for most regions. Conversely, less expensive measures than fencing, such as wildlife warning signs and reflectors, are apparently ineffective (Rytwinski et al., 2016; Brieger et al., 2017; Benten et al., 2018). Hence, while also requiring regular maintenance, wildlife fencing seems to be an important cost-effective strategy for road mortality mitigation of most species (Seiler et al., 2016; Rytwinski et al., 2016; Spanowicz et al., 2020).
Due to financial constraints, it is rarely realistic to fence an entire road network simultaneously, and thus mitigating particular road sections may be more cost-effective than fencing all roads (Ascensão et al., 2013; Spanowicz et al., 2020). One common approach is to focus on road sections with significant aggregation of collisions (Spanowicz et al., 2020). Given that collisions with large body mass species represent a higher threat for humans, with corresponding higher material costs, fencing may be more beneficial in sections where these species tend to be more road-killed, as where roads cross landscape connectivity corridors (Grilo et al., 2011; Santos et al., 2017), in opposition to fencing where higher overall roadkill occurs.
Knowledge of the patterns of mortality, namely how it varies along time, its spatial aggregation, and their costs allow for better management of existing roads as well as better planning for future road networks. This information is even more necessary in developing regions where road networks are expected to have greater expansion and impact in near future (Meijer et al., 2018; Ascensão et al., 2018; Ascensão, 2020). In this study, our main goal was to estimate the material costs due to WVC in a region of fast development as is the state of Mato Grosso do Sul, Brazil. The reduction of roadkill is essential therein due to the large number of WVC that has been evidenced in several studies, but also due to the threat to human safety that collisions with species of large body mass represent, including the lowland tapir (Tapirus terrestris), giant anteater (Myrmecophaga trydactyla) or capybara (Hydrochoerus hydrochaeris) (Ascensão et al., 2017, Ascensão et al., 2019).
We used the information from a 3-year systematic roadkill monitoring program over of 1158 km, for which we estimated the real number of casualties accounting for bias in roadkill counting. We obtained information on the material costs on vehicles (cars and trucks) due to WVC from car mechanics and online surveys and estimated the total material costs resulting from collisions with larger animals considering the traffic volume characteristics (proportion of cars and trucks). We then performed a cost-benefit analysis comparing the costs of material damage with the necessary investment to implement fences (e.g. Huijser et al., 2009), either along the full road corridor or only in sections with significant roadkill aggregation (i.e. hotspots of mortality). With this information we estimated how long it would take to obtain a return on the investment in fencing that prevent collisions, considering the avoided costs due to WVCs.
Section snippets
Study area
The study was conducted in the state of Mato Grosso do Sul (MS), located in center-west region of Brazil, throughout six different transects on four roads (Fig. 1), namely the federal roads BR-262 (R1A, R1B and R4) and BR-267 (R3B), and the state roads MS-040 (R2) and MS-338 (R3A), totaling 1158 km (Fig. 1, Table 1). All roads have two-lanes and are paved.
The land use bordering these roads was dominated by grassland/pasture and agriculture, followed by remnant native vegetation and riparian
Roadkill systematic surveys
We recorded 10,942 carcasses of road-killed animals of which 9431 had a body mass >1 kg, including 4208 larger animals likely to cause material damage with monetary cost to vehicles (see Table S2.1). The roadkill rate was generally higher in the transect R4 for species pertaining to body mass classes <5 and >30; and had similar values across transects for the intermediate class <30 (Fig. 2). A considerable proportion of animals pertaining to weight classes <30 and >30, mainly reptile species
Discussion
We performed over three years of roadkill monitoring in which we recorded >10,000 WVC along the 1158 km. About 40% of collisions involved large animals that cause significant material damage to vehicles, some of which represent a real threat to human lives. Moreover, some of these larger species are of conservation concern and had considerable roadkill rates, such as the lowland tapir (267 records) and giant anteater (608 records), probably threatening their population persistence in the long
CRediT authorship contribution statement
Fernando Ascensão: Conceptualization, Writing - original draft preparation, Reviewing and editing; Débora R. Yogui: Data collection and curation, Reviewing and editing; Mario H. Alves: Data collection and curation; Amanda Carolina Alves: Data collection and curation, Reviewing and editing; Fernanda D. Abra: Reviewing and editing; Arnaud L.J. Desbiez: Conceptualization, Reviewing and editing.
Declaration of competing interest
The authors declare not to have any conflict of interest.
Acknowledgements
We would like to thank the donors to the Anteaters & Highways Project especially the Foundation Segre as well as North American and European Zoos listed at (http://www.giantanteater.org/). We would also like to thank Gabriel Massocato for interviewing the mechanics in Campo Grande, the numerous project volunteers that collaborated with team in the highway monitoring, and Rafael Barrientos for commenting an early version of the manuscript.
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