Abstract
Simple SummaryThe Australian state of Tasmania has a high rate of roadkill, so any method that reduces roadkill in this state deserves attention. A commercial roadkill mitigation device, which combines an auditory warning signal with flashing blue and amber lights in linked units to form a so-called virtual fence, is said to reduce roadkill by up to 90%. For the current trial, a virtual fence was installed on a 4.5-km segment of Tasmanian highway south of Hobart and roadkill was monitored on a daily basis for a period of 126 days. Sections of the virtual fence were switched on or off, according to a predetermined experimental design. Bennett’s wallabies, Tasmanian pademelons, and common brush-tail possums accounted for most of the total roadkill of 174 animals over the study period. For these three species, four complementary methods of analysis failed to reveal any significant effect of the virtual fence in reducing roadkill. This study does not confirm previously reported estimates of reduction in roadkill rates of 50%–90%.When wildlife and motor vehicles collide, the result for the animals is often death (roadkill). A commercial roadkill mitigation device that forms a so-called virtual fence (VF), is said to reduce roadkill by up to 90%. A field trial to test its effectiveness was undertaken along a 4.5-km segment of a Tasmanian highway subdivided into 6 equal sections. A total of 126 days of monitoring of roadkill by species was conducted, with alternate sections being switched on or off, according to a variation of Crossover and Multiple Before-After-Control-Impact experimental designs that divided monitoring into five periods. From the six sections over the five periods, the 30 aggregated values of daily counts of roadkill for each species were modelled. Bennett’s wallabies (BW) (Notamacropus rufogriseus), Tasmanian pademelons (TP) (Thylogale billardierii) and common brush-tail possums (BP) (Trichosurus vulpecula) accounted for most of the total roadkill of 174 animals. Although initially there appeared to be an effect, linear model fits to standardised roadkill rates were not statistically significant for each of BW, TP, and BP using each of the Crossover, Multiple Before-After-Control-Impact, and simple On versus Off comparisons. Adjustment for spatial and temporal trends using a Generalised Additive Model with Poisson error also failed to detect a significant VF effect. A simulation study used to estimate the power to detect a statistically significant reduction in roadkill rate gave, for median estimates of reduction of 21%, 48%, and 57%, estimates of power of 0.24, 0.78, and 0.91, respectively. Therefore, this study failed to confirm previously reported estimates of reduction in roadkill rates claimed for this VF of 50%–90%, despite having adequate power to do so. However, point estimates obtained for these three species of reductions ranging from 13% to 32% leave open the question of there being a real but modest effect that was below statistical detection limits.
Highlights
Road infrastructure is expanding rapidly on a global scale as industrialisation and urbanisation increase [1,2]
One consequence is a global rise in animals being killed or injured in wildlife vehicle collisions (WVC) in Europe [3], the Americas [4] and Australia [5], and a rising number of injured and orphaned animals being rescued by welfare organisations [6,7,8,9]
We present four different analyses because they each exploit different aspects of the experimental design and, in doing so, they either use different subsets of the data or, in the case of the omnibus LM
Summary
Road infrastructure is expanding rapidly on a global scale as industrialisation and urbanisation increase [1,2]. One consequence is a global rise in animals being killed or injured (roadkill) in wildlife vehicle collisions (WVC) in Europe [3], the Americas [4] and Australia [5], and a rising number of injured and orphaned animals being rescued by welfare organisations [6,7,8,9]. The ‘One Welfare’ concept [15,16], where animal health, human well-being, socio-economic development and environmental sustainability are inexorably linked, might suggest that any reduction in WVC could have strategic value, such as fewer car insurance claims and less roadkill for tourists to encounter. Three main approaches to mitigating the problem of WVC, which can be undertaken individually or in combination, are infrastructure management, changing human behaviour and changing animal behaviour. Infrastructure management includes building roadside fences, culverts and land-bridges, but these are costly, ranging from AUD 50,000 for a culvert to AUD 2 million for a land-bridge [17,18]
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