Spatial–temporal patterns in Mediterranean carnivore road casualties: Consequences for mitigation

Abstract

Many carnivores have been seriously impacted by the expansion of transportation systems and networks; however we know little about carnivore response to the extent and magnitude of road mortality, or which age classes may be disproportionately impacted. Recent research has demonstrated that wildlife–vehicle-collisions (WVC) involving carnivores are modulated by temporal and spatial factors. Thus, we investigated road mortality on a guild of small and medium-sized carnivores in southern Portugal using road-kill data obtained from a systematic 36 months monitoring period along highways (260 km) and national roads (314 km) by addressing the following questions: (a) which species and age class are most vulnerable to WVC? (b) are there temporal and/or spatial patterns in road-kill? and (c) which life-history and/or spatial factors influence the likelihood of collisions? We recorded a total of 806 carnivore casualties, which represented an average of 47 ind./100 km/year. Red fox and stone marten had the highest mortality rates. Our findings highlight three key messages: (1) the majority of road-killed individuals were adults of common species; (2) all carnivores, except genets, were more vulnerable during specific life-history phenological periods: higher casualties were observed when red fox and stone marten were provisioning young, Eurasian badger casualties occurred more frequently during dispersal, and higher Egyptian mongoose mortality occurred during the breeding period; and (3) modeling demonstrated that favorable habitat, curves in the road, and low human disturbance were major contributors to the deadliest road segments. Red fox carcasses were more likely to be found on road sections with passages distant from urban areas. Conversely, stone marten mortalities were found more often on national roads with high of cork oak woodland cover; Egyptian mongoose and genet road-kills were found more often on road segments close to curves. Based on our results, two key mitigation measures should help to reduce WVC in Portugal. The first involves the improvement of existing crossings with buried and small mesh size fence to guide the individuals towards to the passages, in road segments with high traffic volume (>1200 vehicles/night) and located in preferred carnivore habitats. The second mitigation involves cutting or removal of dense vegetation in verges of road segments with curves to aid motorists in seeing animals about to cross.

Introduction

As roads are built and upgraded to accommodate greater traffic densities in rural areas, the rate of wildlife–vehicle collisions (WVC) has increased significantly (Forman et al., 2003). Although WVC involving ungulates and other larger animals have been well documented in North America (Hansen, 1983, Conover et al., 1995, Romin and Bissonette, 1996, Hubard et al., 2000, Joyce and Mahoney, 2001, Biggs et al., 2004) and in Europe (Bruinderink and Hazenbroek, 1996, Seiler, 2005), much less information is available for the suspected high number of small fauna–vehicle collisions (e.g., Huijser and Bergers, 2000, Ramp et al., 2006).

WVC involving small fauna can as well cause human death and injury (Williams and Wells, 2005) and are increasingly relevant in the context of biodiversity conservation. It is well known that some carnivores have been severely affected by the expansion of transportation structures (Ferreras et al., 1992, Van der Zee et al., 1992, Caro et al., 2000, Taylor et al., 2002, Forman et al., 2003, Iuell et al., 2003), however we know little about carnivore response to the extent and magnitude of road mortality, or which age classes may be disproportionally impacted (Noss et al., 1996, Ferreras et al., 2001, Blanco et al., 2005). Forman and Alexander (1998) suggested that population effects of road mortality on wildlife may be restricted primarily to a few rare species, but clearly collision records for all wildlife species are underestimated; further, data on the impact of roads on population stability is scarce. In general, carnivores are particularly vulnerable to WVC and barrier effects because of their life-history characteristics; e.g., low population density, often low fecundity, and relatively large home ranges (Spellerberg, 1998, Trombulak and Frissel, 2000, Sunquist and Sunquist, 2001, Forman et al., 2003). In fact, for some carnivores, road mortality has been identified as a serious source of mortality; e.g., the Florida panther (Taylor et al., 2002), wolves in Canada (Paquet, 1993); and Iberian lynx in Spain (Ferreras et al., 1992). In Britain, traffic is believed to kill more than 40% of the estimated adult Eurasian badger population annually (Clarke et al., 1998) and mortality on roads accounts for ∼10% of the endangered Iberian wolf population south of the Douro river in Portugal (Petrucci-Fonseca, 1990, Grilo et al., 2004). Although road mortality is the most visible impact, less is known about the barrier effect caused by road avoidance (McLellan and Shackleton, 1988, Lovallo and Anderson, 1996, Whittington et al., 2004, Alexander et al., 2005, Blanco et al., 2005, Jaeger et al., 2005). In a few mammal species (e.g., desert bighorn sheep (Ovis canadensis nelsoni), bobcat (Lynx rufus) and coyote (Canis latrans)), there is evidence that highways have blocked movement of individuals between road sides affecting reproductive ability and accelerating the loss of genetic diversity (Epps et al., 2005, Riley et al., 2006, Strasburg, 2006). These impacts impose serious concerns about stability and sustainability of wildlife populations in roaded landscapes (Lodé, 2000, Ramp et al., 2005).

Despite the large number of road mortality studies (e.g., Mech et al., 1988, Brody and Pelton, 1989, Dodd et al., 1989, Bruinderink and Hazenbroek, 1996, Hubard et al., 2000, Carr and Fahrig, 2001, Ramsden, 2003, Kolowski and Nielsen, 2008), only a few have focused on a group of species (Clevenger et al., 2003, Malo et al., 2004, Ramp et al., 2006, Eigenbrod et al., 2008) and no study has ever focused on the small and medium-sized carnivore guild of a country. To our knowledge, only a few papers have attempted to summarize and interpret the patterns of some carnivores in relation to roads (Clarke et al., 1998, Philcox et al., 1999, Cain et al., 2003, Seiler, 2003, Saeki and Macdonald, 2004, Baker et al., 2007). Recent research has demonstrated that WVC involving carnivores are modulated by temporal (Philcox et al., 1999, Guter et al., 2005, Orlowski and Nowak, 2006) and spatial factors (Clarke et al., 1998, Caro et al., 2000, Cain et al., 2003, Clevenger et al., 2003, Malo et al., 2004, Ramp et al., 2005). Temporal variations in road-kill are related to differences in species behavior and activity, e.g., foraging and resting, mating, and dispersal of juveniles (Saeki and Macdonald, 2004). Additionally, WVC show significant spatial clustering and appear to depend on population density, species biology, habitat and landscape structure, as well as road and traffic characteristics (Clevenger et al., 2003, Malo et al., 2004). Vehicle speed (Jaarsma et al., 2006), adjacent vegetative cover (Ramp et al., 2005), roadside topography (Clevenger et al., 2003), and type of nearby passages (Clevenger et al., 2003, Malo et al., 2004) are road-related features that have been shown to have varying effects on road-crossing by wildlife. Moreover, the influence of traffic volume both on road mortality and on the barrier effect has long been recognized (e.g., Clarke et al., 1998, Alexander et al., 2005). Up to a threshold volume of traffic, WVC increase with increasing traffic; above that threshold, some species are discouraged from crossing roads (Forman et al., 2003, Seiler, 2003, Alexander et al., 2005). Thus, road managers need reliable data to identify when and where particular species are susceptible to high road-kill rates in order to implement mitigation measures during the road design planning and/or exploration stage. Given the complexity of factors contributing to fatalities and the specific life-history characteristics of individual species, a fruitful approach involves identifying explanatory variables that influence the probability of collisions.

In this paper we provide an assessment of road impacts on the guild of small and medium-sized carnivores in Portugal by addressing the following questions: (a) which species and age classes are most vulnerable to WVC? (b) are there consistent temporal and/or spatial patterns in road-kill? and (c) are there life-history and/or spatial factors that influence the likelihood of collisions? In this context, we: (i) quantified the number of fatalities of each species, (ii) calculated the proportion of juveniles vs. adults fatalities, (iii) compared the mean number of fatalities among each known biological period for distinct species, and (iv) developed predictive models for each species to identify explanatory variables that may influence the probability of road mortality. We further discuss potential preventive measures.