Roadkill Surveys Research Articles

How many rodents die on the road? Biological and methodological implications from a small mammals’ roadkill assessment on a Spanish motorway

AbstractSome 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 walking 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 viability in the short term. Our results show that roadkill surveys should be specifically designed for small mammals to have a more accurate analysis of the effects of roadkill on their populations.

Implications of upgrading a minor forest road on traffic and road-kill in southeast Queensland

The planned upgrade of a minor road in Brisbane connecting two suburbs through a forested area raised concerns about possible increases in road-kill in an area of high wildlife abundance and diversity. Extensive consultation with community representatives resulted in the incorporation of traffic calming devices in the design of the forested section of the road. To assess the effectiveness of these, road-kill surveys were carried out before and after the upgrade and vehicle numbers and speeds were obtained from traffic census data. Vehicle numbers almost doubled following the upgrade. The number of road-kills increased by a factor of three after the upgrade but there were marked differences for different sections of the road, with higher rates in suburban areas and lower rates in the forest section. The study supports previous work showing the importance of low speeds for reducing road-kill and showing that low speeds are best achieved by the inclusion of traffic calming devices in the road design.

Culverts alone do not reduce road mortality in anurans

Roads are linked to population declines in amphibians, primarily due to road mortality. Culvert-type ecopassages, along with fencing to direct animals to the passage, have been used to mitigate these impacts. However, the effectiveness of the ecopassage itself, independent of the associated mitigation fencing, is largely untested. In regions with heavy snowfall, long-term maintenance of amphibian-proof fencing is extremely costly. Therefore, it is important to know whether ecopassages alone (without fencing) mitigate amphibian mortality. We used a Before-After-Control-Impact design to experimentally test the hypothesis that pre-existing drainage culverts of the type typically used to mitigate road effects on herptiles mitigate anuran road mortality. Grates were installed at both ends of 6 culverts to exclude anurans from the culverts. At an additional 4 sites fencing was installed on either side of culverts on both sides of the road, to keep anurans off the road. Ten control culverts were left un-manipulated. Roadkill surveys were conducted 1 y before treatments were installed and in each of 2 y after. We predicted that, if culverts alone mitigate mortality, road kill should increase following installation of the grates. If fencing is effective for mitigation, road kill should decrease following installation of fences. We found no evidence for the first prediction: culverts alone did not mitigate road kill effects. In contrast, there was a large decrease in mortality at fenced sites, relative to control sites, indicating that fencing is effective at mitigating road mortality. These results suggest that culverts alone do not reduce anuran mortality; to reduce mortality, animals must be excluded from the road surface.

English

The evaluation of road-kill spatial patterns is an important tool to identify priority locations for mitigation measures aiming to reduce wildlife mortality on roads. Single target or multi-species approaches are usually adopted to implement such measures, although their success must be evaluated. We aim to test if road-kill hotspots are coincident among different vertebrate groups. If this is true, data on accidents from one group could be used to plan measures applicable to other groups. We identified hotspots using five different grouping criteria: vertebrate classes (reptiles, mammals or birds), body size (large or small), species commonness (common or rare), type of locomotion (flying or non-flying), and time of activity (preferably nocturnal/crepuscular or diurnal). We analyzed data from road-kill surveys on four roads in southern Brazil, each one with at least one year of monitoring. Using SIRIEMA software, we performed a modified Ripley's K statistic to recognize scales of road-kill aggregation and we carried out Hotspot analyses to identify the location of road-kill aggregations for each group described above in each road. To test for similarity in hotspot location among different groups we performed an association test using correlation as resemblance measure. Hotspot analyses and association tests were done using different scales to evaluate scale effect on similarities. Correlation results between groups presented low values in small scales although had a tendency to increase with increasing scales. Our results show that road-kill hotspots are different among groups, especially when analyzed in small scales. We suggest that, for a successful biodiversity approach in mitigation, one should first select general hotspots in large scales and then identify specific hotspots in small scales to implement specific measures. These findings are relevant in a context of road networks already implemented, where mitigation measures are being planned to reduce their impact on wildlife.

Seasonal source–sink dynamics at the edge of a species' range

The roles of dispersal and population dynamics in determining species' range boundaries recently have received theoretical attention but little empirical work. Here we provide data on survival, reproduction, and movement for a Virginia opossum (Didelphis virginiana) population at a local distributional edge in central Massachusetts (USA). Most juvenile females that apparently exploited anthropogenic resources survived their first winter, whereas those using adjacent natural resources died of starvation. In spring, adult females recolonized natural areas. A life-table model suggests that a population exploiting anthropogenic resources may grow, acting as source to a geographically interlaced sink of opossums using only natural resources, and also providing emigrants for further range expansion to new human-dominated landscapes. In a geographical model, this source-sink dynamic is consistent with the local distribution identified through road-kill surveys. The Virginia opossum's exploitation of human resources likely ameliorates energetically restrictive winters and may explain both their local distribution and their northward expansion in unsuitable natural climatic regimes. Landscape heterogeneity, such as created by urbanization, may result in source-sink dynamics at highly localized scales. Differential fitness and individual dispersal movements within local populations are key to generating regional distributions, and thus species ranges, that exceed expectations.

Use of fauna road-crossing structures in north-eastern New South Wales

The vast network of roads around the world has had a significant effect on wildlife and ecosystems through habitat fragmentation, reduced dispersal and mortality by collision with vehicles. Road agencies worldwide now frequently install dedicated structures to facilitate the safe crossing of roads by wildlife. We conducted surveys to determine the use of dedicated wildlife overpasses and nearby underpasses at two locations on the Pacific Highway in north-eastern New South Wales. Road-kill surveys were conducted to provide some understanding of the species commonly killed and whether the rate of road-kill was lower at one location where crossing-structures were located. Use of the crossing-structures by wildlife was monitored with sand-transects. The most frequent users were macropods, bandicoots and rodents. Macropods made greater use of overpasses (n = 104 tracks) than underpasses (n = 36), whereas underpasses were used more by bandicoots (n = 87) and rodents (n = 82) than were overpasses (n = 28, n = 15, respectively). We identified 78 road-kills of 21 species on two sections of the Pacific Highway over a 7-week period. Bandicoots (n = 16) and macropods (n = 9) were the most frequently observed victims. The mortality of wildlife was lower along the highway section with the crossing-structures (0.04 road-kills km–1) than it was along the highway section without structures (0.15 road-kills km–1). The lack of replication precludes any firm conclusion that the crossing-structures reduced road mortality but the high level of use of the crossing-structures by species that were common victims of road-kill suggests an influence.

Diet and body size of North American mammal road mortalities

Previous studies suggest roadkill may not be evenly distributed among species or within species due to life history characteristics. We examine published data on mammalian roadkill surveys conducted in North America for a relationship between diet type, body size and roadkill frequency. Eight studies with comparable methods are used in the analysis for a combined survey mileage of 224,354 km that includes 3.101 individuals killed from 38 species. Carnivores were less likely to be found in roadkill surveys than omnivores or herbivores. After controlling for diet type, a peaked relationship was found between body size and roadkill frequency with the model predicting the highest roadkill for body sizes of about 1.06 kg. These results may be explained by both biological factors and sampling methods.