European Badger hit by a vehicle near Aznalcázar, Seville Province. Photo: Jacinto Román

After four years of intensive research, the technical guidelines document “Methodology for the Study and Analysis of Vertebrate Mortality on Transport Infrastructure" has been published. This report includes the findings of the SAFE – Stop Atropellos de Fauna en España (Stop Wildlife Colissions in Spain), led by the Doñana Biological Station – CSIC. Estimates suggest that up to 55 million vertebrates may be killed on Spanish roads each year, though the actual numbers could be even higher. The most affected groups include amphibians, lizards, and small birds

Wildlife-vehicle collisions are among the most visible impacts of transport infrastructure on ecosystems. However, in Spain, data on the scale of road-related wildlife mortality was previously limited to a 2003 study by the Society for the Conservation of Vertebrates (SCV) and other scattered local or regional research.

To address this gap, Spain’s Ministry for Ecological Transition and Demographic Challenge launched the SAFE project with the goal of conducting the first comprehensive study on vertebrate mortality on Spanish roads, using predefined transects and applying rigorous corrections to account for potential biases in data collection.

The Doñana Biological Station – CSIC has been responsible for the project's scientific leadership, overseeing everything from the selection of sampling methodologies to the analysis of volunteer-collected data, including bias estimation. Meanwhile, the Spanish Herpetological Association (AHE), SECEM, and SEO/BirdLife have been responsible for coordinating and managing volunteer participation.

Citizen science, key to the project

The project has embraced citizen science as a tool to engage society in research and expand knowledge of this phenomenon. “SAFE is the first project in the world to investigate wildlife collisions on a national scale through citizen science, implementing a scientific protocol,” says Marcello D'Amico, researcher at the Doñana Biological Station – CSIC. “The participation of volunteers has been crucial. Without them, we would not have been able to achieve the geographical coverage over time that we’ve accomplished.”

For at least one year, volunteers walked, cycled, or drove along fixed routes at least once a month. During each visit, they recorded the vertebrates they encountered using a mobile app and took photos on the walking and cycling routes. Thanks to their participation, data was collected at 304 different sites across 45 of Spain’s 50 provinces between October 2020 and March 2024. Without this collective effort, it would not have been possible to gather such a large amount of data.

Field tests conducted by the scientific team at the Doñana Biological Station revealed that the records collected by volunteers exhibited typical sampling biases, which needed to be corrected during data collection and processing. One of the mobile applications used for walking and cycling surveys helped improve species identification through expert-validated photos. Additionally, it was found that roadkill incidents were recorded 10 times fewer during car journeys, especially for small animals like amphibians and lizards.

“The data could not be used directly to estimate mortality, as it had to be corrected beforehand to minimize the effect of several biases,” explains Marcello D'Amico. “In this regard, we have investigated these biases over the past few years and published the results in various articles.”

For example, one of the factors to consider is that roadkill does not always remain at the scene of the collision, making it difficult or even impossible to record. The animal may be ejected from the impact, become trapped in the vehicle, or die elsewhere due to injuries sustained from the collision. Another factor is the persistence of the carcass. Scavengers, road maintenance, or the continuous passage of vehicles may cause the carcass to disappear from the search area. The final bias relates to search efficiency. Small-sized species or those with colors resembling asphalt are more difficult to detect.

Amphibians, lizards and small birds among the most affected groups

After bias corrections, the estimates revealed that, in total, between 18 and 55 million vertebrates could be dying on Spanish roads. Amphibians stand out as one of the most affected groups, underlining their vulnerability in a context where they already face multiple threats due to global change. At the same level, lizards also show high mortality, which could have important implications for their conservation, as they usually receive less attention in studies and monitoring programmes. Another of the groups most vulnerable to roadkill is small birds. Other groups studied include micromammals, rabbits and hares and carnivores, also with high mortality rates.

‘These figures reflect an alarming situation regarding wildlife mortality on Spanish roads,’ explains Carlos Rodríguez, research technician at the Doñana Biological Station - CSIC.  ‘While some species have large populations, others, especially those in decline or with reduced populations, face a higher risk due to additional mortality caused by traffic’

The findings underline the urgent need for conservation and mitigation measures to protect biodiversity and reduce wildlife road deaths. ‘Since 2018, we have been working with the Ministry on a number of work plans to reduce the impact of animal road kills. The combination of wildlife crossings and fencing is the most effective, as long as they are well designed and executed,’ explains Carlos Rodríguez.

Safeguarding these species is essential not only for their survival, but also to preserve the ecological balance and health of ecosystems.

Ministerio para la Transición Ecológica y el Reto Demográfico. 2024. Metodología para el estudio y análisis de la mortalidad de vertebrados en infraestructuras de transporte. Documentos para la reducción de la fragmentación de hábitats causada por infraestructuras de transporte, número 9. 75 pp. Madrid. https://www.miteco.gob.es/content/dam/miteco/es/biodiversidad/temas/ecosistemas-y-conectividad/fragmentacion/fragm-publicaciones/PT9_web.pdf