Roadkill rates change through time – Mitigation needs to be adapted to the species.
Written By Anthony Rendall
Deakin University
14th March 2022
Conservation areas are critical in safe-guarding biodiversity, in doing so, they also attract tourists wanting to interact with wildlife. This combination of higher wildlife abundance and high traffic places these areas at particular risk of wildlife-vehicle collisions.
Most studies of roadkill rates assess a relatively short period of time (a few years of sampling); or reflect low frequencies of occurrence for species making it hard to understand how roadkill rates may change through time.
We used two datasets collected at the same time of year (February to June) 15 years apart (1998, 1999 and 2014), using the same methods and protocols, along the road network on Phillip Island in south-east Australia. A total of 714 roadkill (1.59 mortalities per km per month) were recorded in the two years of 1998 and 1999, and 688 (2.30 mortalities per km per month) were recorded in 2014 alone, and these data were used to assess how roadkill rates may have changed. We also considered whether the spatial and temporal influences on roadkill changed to inform long-term viable mitigation measures
Figure 1. Aerial photo of Phillip Island in south-eastern Australia with a varied land use of conservation reserves, agriculture, and urban settlements. Images represent the diversity of road contexts considered throughout the study.
Changing roadkill rates through time
We found dramatic variation in roadkill rates between 1998-99 and 2014 with total roadkill rates for all species combined increasing by ~25%. Swamp wallabies, possums, and bird species showed the most notable increase (Figure 1). The decline in short-tailed shearwater deaths was attributed to extremely poor breeding success of the species in 2014 leaving few individuals on roads.
The dramatic changes are likely to be the result of several changes that have occurred on Phillip Island between these two time periods:
An increase in road traffic volume (visitors and permanent residents)
An increase in wildlife species abundance, attributable to effective fox control and habitat conservation efforts.
Wallabies and possums are both known to respond positively to fox control activities elsewhere, suggesting these increases are likely associated with increased abundances of these two species and hence interactions with the road network. When increases in wildlife numbers is combined with an increase in the traffic intensity, the likelihood of a vehicle collision further increases.
Figure 2. The number of roadkill per kilometre per month between February and June 1998-99 (white bars) and in 2014 (grey bars). Birds(all*) excludes short-tailed shearwaters.
How did factors influencing roadkill rates change?
We found similar trends in the factors influencing roadkill rates during the two time periods. Speed limit was the dominant factor influencing roadkill rates during both time periods with the highest mortality rates observed at moderate speed limits (60-80 km/h), although this was species-specific. Our data shows that weekends experienced higher roadkill rates. In 1998-99 rural areas had higher roadkill compared to urban areas, however in 2014 differences between urban and rural areas were not supported – although this is likely influenced by competing species-specific trends. We found that spatial factors influencing roadkill were broadly similar between the two time periods, albeit with some species-specific differences. These results suggest mitigation measures implemented now will remain beneficial, even with future changes in species wildlife composition or abundance.
Species-specific roadkill rates showed contrasting trends with respect to spatial influences on roadkill. Swamp wallaby roadkill for example was greater on rural roads, while rabbit and possum roadkill were greater in urban areas. Similarly, roadside vegetation increased possum roadkill while decreasing roadkill of rabbits and birds – a trend likely attributable to foraging modality of these species.
Species-specific mitigations?
Roadkill rates in our study were species-specific adding to the complexity of implementing mitigation measures (Figure 2). The prevalence of roadkill across Phillip Island facilitated our consideration of factors influencing different species, to appropriately inform location-specific mitigation. Across all species, speed was a dominant feature, but not in the same way for all species. Swamp wallabies were more regularly hit at higher speeds (80km/h) while possums and rabbits were more regularly impacted at lower speeds (60km/h).
Figure 3. Roadkill hotspots for different taxonomic groups across Phillip Island roads between February and June 2014. The size of dot represents the number of roadkill (square root transformed).
Reducing speed limits is likely to reduce the prevalence of roadkill for most species, and these species activity around roads varies throughout the day. Managers could therefore consider different speed limit regulations applied during specific times of the day that seeks to reduce vehicle speeds during those periods where wildlife are most active around roads (i.e., dawn and dusk in our study system).
Additionally, we recommend a combination of crossing structures with fencing along critical road segments where valuable wildlife habitats are found on both sides of the road – previous studies have reported > 95% reduction in roadkill rates for some species with these interventions. Where this is not possible, rope-bridges can help possums safely cross the road.
Reducing roadside vegetation has been recommended to increase the visibility of wildlife to drivers, however, our results suggest that this measure may be effective only for some species (i.e., possums and woodland birds) but could increase roadkill rates for others (i.e., ground foraging birds). The value of wildlife habitat may therefore be more important than roadkill mitigation through vegetation management. We recommend speed limit reductions as the most viable consideration for mitigation in these circumstances.
Our research has extensively informed the “Phillip Island Safer Speeds” initiative which seeks to reduce the impact of road speed limits on wildlife across Phillip Island. Further research could investigate the efficacy of these policy implementations and how they translate into positive outcomes for wildlife.
Author information:
Anthony Rendall - a.rendall@deakin.edu.au
Source:
Rendall, AR., Webb, V., Sutherland, DR., White, JG., Renwick, L., Cooke, R. (2021). Where wildlife and traffic collide: Roadkill rates change through time in a wildlife-tourism hotspot. Global Ecology and Conservation e01530. https://doi.org/10.1016/j.gecco.2021.e01530
Editor:
Carme Rosell and Rodney van der Ree
Cite this summary:
Rendall, AR. (2022). Roadkill rates change through time – Mitigation needs to be adapted to the species. Edited by Rosell, C. and van der Ree, R. TransportEcology.info, Accessed at: https://transportecology.info/research/roadkill-rates-through-time-phillipisland [Date accessed].
