Public Engagement Officer, Sivi Sivanesan, looks at a 2001 study which investigated the effect of road kills on amphibian populations.
The diurnal movement of newts, frogs and toads move was studied by a team in Denmark. They investigated two questions:
- What was the probability of an individual amphibian getting killed when crossing the road?
- For a known population, what fraction of the amphibian’s population were killed by traffic?
They found that the probability for amphibians being killed on a road with a traffic load of 3,207 motorised vehicles /day, ranged from 0.34-0.61, increasing to 0.89-0.98 when crossing a motorway (0 = no probability of death to 1 = 100% probability of death).
They state that their figures should be considered as the minimum probability given that their calculations were on based on perpendicular crossing patterns, while angular crossing patterns resulted in an increased risk of being hit, and the subsequent risk of injury and death.
The study results indicated that the percentage reproductively active adults lost annually was up to 25% of Pelobates fuscus and up to 21% of Rana temporaria/R.arvalis.
There were a number of points made in the paper:
- Activity patterns for the site in Denmark showed movement variation at the species level. The frog species were active soon after sunset (sunset was between 8-9pm over the course of the study); most common toads were active between 10pm and 11pm and the newt species tended to peak later still, from around midnight to 2am.
- They noted that road mortality may be more frequent in juveniles in the populations as they are slower movers.
- Their calculations were based on motorised vehicles and the chance of being hit by the wheel (even a partial hit). I, and other patrollers, have witnessed toads killed in the same manor by non-motorised vehicles such as bicycles on roads and bridleways.
There are some interesting implications in this study.
- If amphibian species at different British sites were to show a marked preference for the time of day/night they travelled, as did the species at the study site, then some species at patrol sites may be missed if patrols are operating at a different time.
- It highlights the need for pre-emptive road designs that maintain habitat connectivity in areas that already have evidence of a lot of established migration routes.
- Pre-emptive designs also need to be considered when sustainable urban drainage systems, such as balancing ponds, are planned close to roads. On some sites, this newly created habitat has been shown to attract amphibians to cross over existing/new roads, thus creating new areas of amphibian mortality.
- Long term solutions may include installing wildlife tunnels as standard on all new roads or road redevelopment schemes. However this solution would only work if they were based on the latest scientific research into wildlife tunnel design and frequency.
- Where tunnels are installed or proposed, the management agencies must also incorporate proper maintenance levels to keep tunnels clear and active for amphibians.
- In addition to the above, better designs of curb side drainage needs to be implemented on sites with amphibian migrations across roads, to stop animals falling in and dying in the drains.
These are problems that need to be addressed now as our road usage by all vehicles (motorised + peddle) is likely to continue to increase as both urban and rural areas are under increased pressure to absorb more development to accommodate increases in local populations and for economic stimulus.
What you can do:
- Volunteer with your nearest toad patrol – use the new interactive map to find your nearest patrol.
- The Traffic counts website tells you the traffic load per day/year for some of the roads in the UK – all major roads and some minor roads. Interesting links for those interested in this study and the traffic loads near patrol sites near (only available for some roads, but may give you an indication of the traffic loads near you.
Hels, T. And Buchwald, E. 2001. The effect of road kills on amphibian populations. (In) Proceedings of the 2001 International Conference on Ecology and Transportation, Eds. Irwin, C.L., Garrett, P. And McDermott, M.P. Centre for Transport and the Environment. North Carolina State University, Raleigh, NC: pp 25-42. [Accessed online, December 2013].