ORIGINAL ARTICLE Pablo Ruiz-Capillas • Cristina Mata • Juan E. Malo How many rodents die on the road? Biological and methodological implications from a small mammals’ roadkill assessment on a Spanish motorway Received: 6 May 2014 / Accepted: 19 December 2014 Ó The Ecological Society of Japan 2014 Abstract Some small mammal species use road verges as a habitat for sheltering. As a result, their populations suffer mortality from roadkill in an unknown extent. We analysed the road mortality of small mammals and the population impact of roadkill on wood mouse along 1 year on a 4-km motorway stretch. To account for potential bias in carcass sampling, we conducted walk- ing surveys with the traffic interrupted and we performed three carcass persistence experiments to estimate total number of roadkills. The estimations were contrasted with previous studies and the population abundance on the road verges. The spatial pattern and the effects of seasonality and traffic volume on roadkill were analysed. We recorded 84 rodent carcasses with a mean carcass persistence of 6.65 days (SE = 0.41). The estimations of roadkill ranged between 190.2 and 238.3 individuals km 1 year 1 , with a mean value close to a monthly 6 % mortality for the most common specie. Roadkill was more frequent during the hottest and driest months, and showed a unimodal relation with traffic peaking at 8100 vehicles day 1 . A spatial aggregation of roadkill was detected with 80–190 m clustering. Our results suggest that, although rodents suffer greater roadkill mortality than previously reported, it affects a small percentage of the wood mouse population in our study area. This source of mortality is unlikely to affect population via- bility in the short term. Our results show that roadkill surveys should be specifically designed for small mam- mals to have a more accurate analysis of the effects of roadkill on their populations. Keywords Carcass persistence Æ Detectability Æ Road verges Æ Walking survey Æ Apodemus Introduction Transport infrastructures cover some 0.5 % of the glo- bal land surface (Hooke et al. 2012) and they are one important cause of biodiversity loss (Vitousek et al. 1997; Alkemade et al. 2009). The infrastructure disrupts the physical environment and alters animal behavior (Forman and Alexander 1998; Trombulak and Frissell 2000; Coffin 2007) leading to changes in abundance patterns of wildlife there. A number of studies have al- ready analysed for several taxa the direct and indirect effects of roads on their abundance patterns (Fahrig and Rytwinski 2009; Benı´tez-Lo´ pez et al. 2010) showing that effects are usually negative (e.g. 60 % of 133 species analysed by Fahrig and Rytwinski 2009). Such effects are associated with negative impacts on populations resulting from their fragmentation, habitat loss or destruction, as well as from roadkill (Forman and Alexander 1998; Trombulak and Frissell 2000; Coffin 2007). Nevertheless, several species like some raptors, small mammals or generalist predators, regard proxim- ity to transport infrastructures favourably and positively select the road verges (Fahrig and Rytwinski 2009; Benı´tez-Lo´pez et al. 2010). Such species find a new habitat in roadside verges, which they may use for for- aging (Bennett 1991), for their movements or as refuges from predation (Muhly et al. 2011). However, little is known about the negative repercussions on their popu- Electronic supplementary material The online version of this article (doi:10.1007/s11284-014-1235-1) contains supplementary material, which is available to authorized users. P. Ruiz-Capillas (&) Ecological Restoration Section, R&D Service, Obrasco´n Huarte Lain, S.A., Paseo de la Castellana, 259D, Torre Espacio, 28046 Madrid, Spain E-mail: pablorui@ohl.es Tel.: +34 91 497 80 11 C. Mata Æ J. E. Malo Terrestrial Ecology Group, Departamento de Ecologı´a, Universidad Auto´noma de Madrid, 28049 Madrid, Spain E-mail: cristina.mata@uam.es J. E. Malo E-mail: je.malo@uam.es Ecol Res DOI 10.1007/s11284-014-1235-1