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Publication Number: FHWA-HRT-08-034
Date: August 2008
Wildlife-Vehicle Collision Reduction Study: Report To Congress
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Chapter 4. Impacts to wildlife
Roads and traffic can negatively affect wildlife in various ways, including habitat loss, reduced habitat quality, reduced habitat connectivity (and associated potential demographic and genetic consequences), and direct road mortality. (See references 83, 84, 85, and 86.) This chapter focuses on the effects of direct road mortality on wildlife only, specifically for threatened and endangered species.
As previously stated, in most cases, an animal that has been hit by a vehicle dies immediately or shortly after the collision. For example, in Michigan, Allen and McCullough estimated that a minimum of 91.5 percent of all white-tailed deer that were hit by a vehicle died at the scene or shortly thereafter.(81) In Newfoundland, 88.5 percent of all moose collisions resulted in the death of the animal (4,800 moose fatalities out of 5,422 collisions). Many different wildlife species representing a wide variety of species groups have been observed as roadkill, sometimes in massive numbers. Seiler provided a review of estimates of the number of road-killed animals.(23) The combined number of road-killed amphibians, birds, ungulates, and other vertebrates runs in the multiple millions per year for most of the countries that were reviewed. In the United States the total number of road-killed vertebrates was estimated at 365 million per year.(87) The number of DVCs in the United States was estimated to exceed 500,000 per year, around 538,000 per year, and greater than 1,000,000 per year.(3,4,45)
The number of WVCs and animal carcasses is often underestimated (as previously discussed in chapter 2); researchers have calculated the underestimation by 10.3 percent, 25 percent, 50 percent, 77.5 percent, and 87.9 percent. (See references 4, 68, 72, and 88.) These estimates for underreporting apply especially to deer, as this species is involved in the vast majority of all reported AVCs or large WVCs in North America; for example, 80 percent in Saskatchewan, and 81.4 percent in Maine.(68,89) Underreporting may have various causes, including infrequent carcass checks, poor visibility of the carcass from the road, mutilation of the carcass by traffic to the point that the species can no longer be identified or that little to none of the carcass remains, decomposition, (illegal) removal by humans other than the data collectors, and scavengers.(90)
While deer are the species of primary interest from a safety perspective, their survival probability is typically not a concern. Species most affected in their population survival probability seem to be species that have relatively low population density, large home ranges, travel long distances, are long lived, and have a relatively low reproduction rate. (See references 85, 91, 92, and 93.)
Roads and traffic can reduce population densities for some species such as different frogs and toads, the western European hedgehog (Erinaceus europaeus), and the desert tortoise (Gopherus agassizii).(94, 95, 96) For some species, the survival probability of local or regional populations can be impacted too, especially if the species concerned also suffer from other human-related disturbances such as large-scale intensive agriculture and urban sprawl.(97,98) The effect of road mortality on the population viability of a species can not always be separated from other effects associated with roads and traffic, but road mortality is believed to have affected the population survival probability for multiple species representing different species groups: amphibians (moor frog (Rana arvalis)), (leopard frog (Rana pipiens); spotted salamander (Ambystoma maculatum)), reptiles (timber rattlesnakes (Crotalus horridus)), (land and large bodies pond turtles including the box turtle (Terrapene ornata)), mammals (western European hedgehog), (Eurasian badger (Meles meles)), (otter (Lutra lutra)), (ocelot (Leopardus pardalis)), (Florida panther (Felis concolor coryi)), (Iberian lynx (Lynx pardinus)), (Florida Key deer (Odocoileus virginianus clavium)). (See references 91, 92, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, and 110.)
This section reviews federally listed threatened and endangered animal species in the United States for which direct road mortality is among the major threats to the survival of the species or certain populations of that species. The threatened and endangered species were not reviewed with regard to other effects associated with roads and traffic such as habitat loss, reduced habitat quality, and the barrier effect of transportation infrastructure. Note that the list in this chapter (table 7) has no regulatory status and that it does not replace potential consultation with the appropriate agencies about the impact of road improvement projects on local endangered species. In addition, because the required data were often difficult to access, and since only limited time was available for this effort, the list in this chapter is not necessarily complete.
All threatened and endangered animal species (clams, snails, crustaceans, arachnids, insects, fishes, amphibians, reptiles, birds, and mammals) in each of the 50 states and Washington, DC, were combined into one list. If different populations of the same species were listed, they were treated separately.(111) Species (or populations) were identified for which direct road mortality is among the major threats to the survival probability of the species. Species that are aquatic were not reviewed with regard to vehicle collisions. Mortality as a result of collisions with trains and off-road vehicles was also excluded from the review. This review focused solely on the effect of direct mortality resulting from vehicle collisions (e.g., cars and trucks) on paved roads (e.g., asphalt or concrete).
The following sources were used to evaluate whether direct road mortality is a major threat to the survival probability of threatened and endangered species: (1) documents that provided a rationale for the listing of threatened and endangered species (Federal Registerpublications), (2) the 2006 International Union for Conservation of Nature and Natural Resources Red List of Threatened and Endangered Species, (3) other sources, including publications on individual species or species groups and expert opinions (appendix A).(112) If an expert opinion was the sole source of information that direct road mortality is among the major threats to the survival of a certain species, additional quantitative information was sought out on the importance of road mortality before the species was added to table 7. In addition, speculations alone about the potential impact of direct road mortality were not sufficient for a species to be listed in this chapter.
The list presented in this chapter is not necessarily complete because the required information was difficult to access and the time available for this effort was limited. Furthermore, some species have been listed for decades and circumstances have changed or more and better knowledge about the threats to individual species has become available since the original listing documents were published. For these reasons one cannot only rely on the original listing documents. Other sources have to be included in determining whether the survival probability of a species is substantially impacted by road mortality.
Even though the information available was carefully evaluated, the process of including and excluding species from the species listed in this chapter was at least partially subjective. Because of the diverse and inconsistent nature of the sources and data available, the inclusion or exclusion from the list could not be based on a simple definition. The inclusion or exclusion of the species listed relied, at least to a certain extent, on expert judgment that is open to debate. Furthermore, just as the status of species and circumstances have changed since the original listing documents were published (discussed above), the status and circumstances will continue to change and the list presented in this chapter will become less applicable over time.
For the 21 species listed in table 7, direct road mortality is considered a major threat to the survival of the species. The table includes three amphibian species, seven reptile species, three bird species, and eight mammal species. A brief discussion for each species follows table 7.
The California tiger salamander (Ambystoma californiense) is affected by habitat loss due to urbanization and agriculture, unnatural hydrology, predation by nonnative species (bullfrogs, crayfish, various fish species), reduced availability of burrows as a result of rodent control programs, vehicle collisions, reduced food availability through the use of pesticides for mosquito control, hybridization with nonnative tiger salamanders, and storm water road runoff (Dave Johnston, California Department of Fish and Game, CA, personal communication). (See references 112, 113, 114, 115, and 116.)
The flatwoods salamander (Ambystoma cingulatum) was listed because of habitat loss and habitat alteration.(117,150) However, direct mortality (road mortality during migration, capture by bait collectors) is also a potential or a major threat to this species (Bruce Means, Coastal Plains Institute and Land Conservancy, Tallahassee, FL, personal communication; John Palis, Palis Environmental Consulting, Jonesboro, IL, personal communication).(112,117) At one location where a substantial population decline has been observed, road mortality was not considered substantial. In this case, habitat loss and habitat degradation (agriculture, silviculture, urbanization, and changes in hydrology, predation by nonnative fish species) are thought to be the primary cause of the decline.(151) Silviculture is the cultivation and management of forest trees or woodlands for producing timber and other wood products.
The Houston toad (Bufo houstonensis) is affected by habitat loss and habitat alteration, mostly through urbanization, recreational development, and agriculture.(112,118) However, direct road mortality through increased habitat fragmentation by road construction has also been identified as a major threat to the survival probability of the species.(112,118) Other threats include predation by nonnative species (e.g., Brazil fire ants).(112)
The American crocodile (Crocodylus acutus) is affected by changes in hydrology and consequent changes in salinity levels.(121) In addition, direct mortality of adult American crocodiles is considered higher than the population can sustain.(121) Of the deaths recorded between 1971 and 2001, the majority were hit by cars.(119,120,121) Warning signs and fences were installed along the major highways throughout crocodile habitat in south Florida.(119) However, it appears that some or all of the planned underpasses may not have been built (U.S. Highway 1) and that some of the fencing that was installed (State Route 905) was not flush with the ground so that American crocodiles could enter but not exit the right of way. Some of these fence sections have now been removed (Frank Mazzotti, Department of Wildlife Ecology and Conservation, Fort Lauderdale Research and Education Center, Davie, FL, personal communication).
The desert tortoise (figure 22) is affected by habitat loss (agriculture, landfills) and habitat degradation (e.g., through off-road vehicle use, overgrazing, invasive plant species). (See references 96, 123, 152, and 153.) Substantial direct mortality occurs on highways as well as off highways (nonintentional and intentional crushing by off-road vehicle operators, trampling of their burrows by off-road vehicles and livestock, shooting). (See references 96, 122, 123, 124, 125, 152, and 153.) Other mortality causes are disease, drought, mining, wildfires, garbage and litter, handling by humans, collection by humans, and predation by common ravens (Corvus corax). (See references 96, 123, 125, 152, 154, and 155.)
Figure 22. Photo. Desert tortoise (copyright: Marcel Huijser).
The gopher tortoise (Gopherus polyphemus) is affected by habitat loss (urbanization, agriculture, silviculture, mining) and habitat degradation (silviculture, fire suppression, nonnative plant species).(126,127,128) Collection by humans and road mortality also affected the species substantially. (See references 125, 126, 127, and 128.) Furthermore, the species is affected by predation, including by nonnative fire ants.(128) Fences and culverts were installed along a section of Highway 63 in Green County, south of Leakesville, MS (Matthew J. Aresco, Nokuse Plantation, Bruce, FL, personal communication; Claiborne Barnwell and Chuck Walters, Mississippi Department of Transportation, personal communication). The aim of the mitigation measures is to reduce gopher tortoise road mortality and to allow for gopher tortoises to cross under the road (Claiborne Barnwell and Chuck Walters, Mississippi Department of Transportation, personal communication) (figure 23). Highway 63 has 24.1 km ( 15 mi) of road length with gopher tortoise fencing, and, because of the nature of the terrain, there is only one culvert that was specifically designed for the gopher tortoise (between Lucedale and Leakesville, MS) (Chuck Walters, Mississippi Department of Transportation, personal communication). At the site of the culvert, the fence stretches out about 914 m ( 3,000 ft) to either side of the culvert (Chuck Walters, Mississippi Department of Transportation, personal communication). Some of the fencing was installed as early as 1998, and along those road sections the number of reported road-killed gopher tortoises was reduced from one to two per year to zero (Chuck Walters, Mississippi Department of Transportation, personal communication).
Figure 23. Photo. Fences lead gopher tortoises towards a culvert along Highway 63 in Green County, south of Leakesville, MS
(copyright: Chuck Walters, Environmental Division, Mississippi Department of Transportation).
The Alabama red-bellied turtle (Pseudemys alabamensis) is affected by egg predation, human disturbance, and road mortality (David Nelson, Department of Biological Sciences, University of South Alabama, personal communication; Matthew J. Aresco, Nokuse Plantation, Bruce, FL, personal communication).(125,129,156) The small population size and low recruitment rates of the species make recovery a difficult process. A weekly road mortality survey along the Mobile Bay Causeway ( 6.5 mi from Spanish Fort to Mobile, AL) between 2001 and 2004 reported 324 Alabama red-bellied turtle carcasses (David Nelson, Department of Biological Sciences, University of South Alabama, personal communication) (figure 24 and figure 25).(129) In a typical year, 12–15 adult females, most of them with eggs, are found dead on the Mobile Bay Causeway (David Nelson, Department of Biological Sciences, University of South Alabama, personal communication). In addition, several dozen juveniles and a few males are killed by vehicles each year as well (David Nelson, Department of Biological Sciences, University of South Alabama, personal communication).
Figure 24. Photo. A section of the Mobile Bay Causeway that has relatively many road-killed Alabama red-bellied turtles (copyright: Marcel Huijser).
Figure 25. Photo. Road-killed Alabama red-bellied turtle (copyright: Marcel Huijser).
The northern population of the bog turtle (Clemmys muhlenbergii) is affected by habitat degradation and fragmentation from agriculture and development, habitat succession due to invasive exotic and native plants, and illegal trade and collecting.(130) In addition, roads contribute "significantly" to mortality, especially where roads are adjacent to or within wetlands.(130)
The copperbelly water snake (Nerodia erythrogaster neglecta) is affected by habitat loss and habitat fragmentation, primarily because of agriculture, drainage and damming of wetlands, coal mining, channelization, damming and diversion of streams and rivers, and residential and commercial development.(131) In addition, predation by pets and vehicle-caused mortality are a concern.(131,132) Traffic mortality may account for mortality of 14–21 percent of the population per year.(132) The species seems especially vulnerable as it frequently crosses overland to different wetland sites.(132)
The eastern indigo snake (Drymarchon corais couperi) is affected by habitat loss due to development, collection and commercial trade, intentional killing, vehicular traffic, and residual pesticide exposure.(135,157) In addition, gopher tortoise burrows that are gassed to kill rattlesnakes also unintentionally kill indigo snakes.(157) Bolt reported that road mortality was the highest cause of death in a study where 81 individuals were followed, some for more than three consecutive years.(133) At least 15 of the 38 known mortalities (39 percent) in the field were due to vehicles. In that study, twice as many males were killed on the road as females (M. Rebecca Bolt, The Dynamac Corporation, Kennedy Space Center, FL, personal communication). In another study, of the 31 indigo snakes documented, 5 were found dead on a road (16 percent of total number of individuals followed), accounting for 55 percent of all known mortalities.(134)
The crested caracara in central Florida (Polyborus plancus audubonii) is affected by habitat alteration for agriculture and housing, illegal killing, and vehicle collisions.(136,137) In a 3-year study, 52 percent of all fledgling mortality (14 out of 27 deaths) was caused by vehicle collisions.(137) The crested caracara spends substantial time close to roads as it searches for and feeds on road-killed animals (Dan Smith, Western Transportation Institute, Montana State University, personal communication).
The Hawaiian goose, or nene (Branta sandvicensis), is affected by habitat loss, predation by the nonnative small Indian mongoose (Herpestes auropunctatus), dogs, and perhaps rats and cats.(112) Poaching and roadkills are also important causes of mortality (Kathleen Misajon, National Park Service, personal communication).(112,138) Road mortality is the most common known cause of mortality in adults.(158) The species may also be affected by diseases and parasites, inbreeding depression, loss of adaptive skills in captive-bred birds, and dietary deficiencies.(112)
Haleakala National Park reported 35 road-killed Hawaiian geese between 1973 and 2006, and Hawai'i Volcanoes National Park reported 33 road-killed Hawaiian geese between 1996 and 2006 (Kathleen Misajon, National Park Service, personal communication). The population size of the Hawaiian goose fluctuated between 140 and 200 between 1996 and 2006 (Kathleen Misajon, National Park Service, personal communication). In Hawai'i Volcanoes National Park, five adult Hawaiian geese have been killed on the road in 2006 between January 1 and August 28, out of a total of 160 individuals (Kathleen Misajon, National Park Service, personal communication).
In Hawai'i Volcanoes National Park, the Hawaiian goose is attracted to roads because of feeding by park visitors, especially around parking areas (Kathleen Misajon, National Park Service, personal communication) (figure 26). This practice habituates the birds to roads and cars, and it encourages them to spend more time on and alongside roads, increasing their exposure to vehicles. Furthermore, some road sections in the park split roosting habitat from feeding habitat. When they have young, Hawaiian geese walk between roosting and feeding sites for 3–4 months and cross the road frequently, mostly at dawn or dusk, with relatively low visibility (Kathleen Misajon, National Park Service, personal communication). Pairs with goslings are basically pedestrian until the goslings fledge at 3–4 months of age.
Permanent warning signs have been installed in known Hawaiian goose kill areas (Kathleen Misajon, National Park Service, personal communication) (figure 27). In addition, temporary warning signs can be installed at new or unexpected locations. Nonetheless, all five individuals that were killed by vehicles between January 1 and August 28, 2006 were within signed crossing zones (Kathleen Misajon, National Park Service, personal communication). There are also indirect effects of roadkills to the Hawaiian goose population. For example, mates are left without partners, often for at least one breeding season, resulting in one less nesting attempt that year (Kathleen Misajon, National Park Service, personal communication). In addition, goslings without one or both parents have substantially reduced survival probability (Kathleen Misajon, National Park Service, personal communication).
Figure 26. Photo. "Do Not Feed Nene" sign (copyright: Hawai'i Volcanoes National Park, National Park Service).
Figure 27. Photo. Hawaiian goose (nene) warning sign (copyright: Haleakala National Park, National Park Service).
The Florida scrub jay (Aphelocoma coerulescens) is affected by habitat loss (housing developments, citrus-groves) and reduced habitat quality (disrupted fire regimes, human disturbance), predation by nonnative species (feral cats) and roadkill. (See references 112, 139, 140, and 159.) Annual mortality rates of the Florida scrub jay have been recorded to be 65 percent higher in road territories compared to nonroad territories.(140) Furthermore, roadside territories are a population sink, and the high mortality rate appears to be caused by vehicle collisions rather than other factors associated with a roadside environment.(140)
The Lower Keys marsh rabbit (Sylvilagus palustris hefneri) is or has been affected by wetland drainage for residential, commercial and military purposes, habitat destruction associated with road building, hunting, predation by feral house cats, road mortality, mowing practices, and off-road vehicle use.(117,160) In a combination of a field and modeling study, almost one-third of all mortalities were caused by vehicle collisions, and modeling showed that theoretical removal of road mortality would eliminate the chance of extinction for the Big Pine metapopulation.(141) Dispersing subadult males seem especially vulnerable to traffic mortality.(117)
The Florida Key deer (Odocoileus virginianus clavium) is affected by vehicle collisions, habitat loss, and human disturbance.(110,143) Vehicle collisions account for more than 50 percent of the total deer mortality, mostly on U.S. Highway 1.(143)
The bighorn sheep, peninsular California population, (Ovis canadensis) is affected by a range of issues including disease, low recruitment, habitat loss, habitat degradation, habitat fragmentation, residential and commercial development and high predation rates.(144) This population, especially small groups that have low recruitment, is also threatened by road mortality.(144)
The San Joaquin kit fox (Vulpes macrotis mutica) is threatened by habitat conversion (agriculture, urban development, industrial development), habitat fragmentation, loss of prey species (e.g., eradication of prairie dog towns), predation (coyotes, bobcats, nonnative red foxes, and domestic dogs), and vehicle mortality.(112,145,161) In the San Joaquin Valley of California, habitat conversion for agriculture has slowed, but habitat loss, reduction of habitat quality, and habitat fragmentation are still a primary threat. Road mortality varies between studies: 20 out of 225 adult deaths (9 percent), 11 out of 142 juvenile deaths (8 percent), 1 out of 60 deaths (2 percent), 1 out of 22 deaths (5 percent), 2 out of 49 deaths (4 percent), 2 out of 17 deaths (12 percent), 15 out of 23 adult deaths (65 percent), and 6 out of 12 juvenile deaths (50 percent).(145) After predation, vehicle collisions are likely to be the second most common cause of mortality.(145)
Canada lynx (Lynx canadensis) is likely impacted by urbanization and forestry practices (including fire suppression) and trapping.(162,163) In addition, its population size fluctuates with the availability of its main prey species, the snowshoe hare (Lepus americanus).(163) In the United States, road mortality may limit the reestablishment of the Canada lynx in Wisconsin and Michigan.(146) A total of 218 adult lynx were released between 1999 and 2006, and there were 80 known mortalities as of June 30, 2006.(147) Starvation was a substantial cause of mortality in the first year of the releases only. About 31.3 percent of the known mortalities were human induced (including collisions with vehicles or shooting by humans).(147) Malnutrition and disease or illness accounted for 21.3 percent of the deaths, while 32.5 percent of the deaths were from unknown causes.(147) Closer and more recent analyses showed that road mortality accounted for a minimum of 44 percent (11 out of 25) of human-caused mortalities (Alison Michael, U.S. Fish and Wildlife, personal communication). This percentage may be higher as this estimate only included confirmed vehicle-caused mortality and excluded suspected vehicle-caused mortality (Alison Michael, U.S. Fish and Wildlife, personal communication). In Maine, 11 road-killed Canada lynx have been reported since 1999; nine on two-lane logging roads that are also accessible to the public and two on paved public roads.(164) Recent data from Minnesota show that Canada lynx have died from shooting, trapping, collisions with trains, and road mortality (Phil Delphey, U.S. Fish and Wildlife Service, personal communication). Road mortality on paved highways amounted to 17 percent (5 out of 30) of all known mortalities since the species was listed in 2000 (Phil Delphey, U.S. Fish and Wildlife Service, personal communication).
The ocelot is affected by habitat loss (loss of dense thorn shrub habitat), mortality vehicle collisions, and genetic erosion.(107,148,165) Vehicle collisions constituted 35 percent of all mortality.(148)
The Florida panther is affected by habitat loss (agriculture, urbanization), habitat fragmentation, road mortality, and loss of genetic diversity .(166,167,168) Road mortality is substantial, 25 out of 73 deaths were caused by vehicles.(149)
The red wolf (Canis rufus) went extinct in the wild by 1980 and was reintroduced in 1987 in North Carolina.(112) After reintroduction, the species was affected by hybridization with coyotes (Canis latrans) .(112,169,170) Direct mortality (vehicle collisions, shooting) can be substantial.(112)
In addition to the species listed in table 7, the authors of this report recognize that other federally threatened and endangered species may be substantially affected by road mortality too. However, species that had insufficient data available, at least to the authors of this report at the time of publication, were excluded from table 7.
This chapter identified 21 federally listed species from four species groups (amphibians, reptiles, birds and mammals) for which direct road mortality is among the major threats to the survival of the species. However, road mortality is typically only one of the major threats to these species. Habitat loss (e.g., due to agriculture, urbanization, mining, and changes in hydrology), reduced habitat quality (e.g., due to agricultural and silviculture practices such as livestock grazing, logging, fire suppression, introduction of nonnative plant species, and water contamination with pollutants, and the use of pesticides, in general), habitat fragmentation (e.g., due to roads or other unsuitable habitat), competition and predation by nonnative species, other sources of natural and unnatural mortality (e.g., off-road vehicles, poaching, direct killing or collection by humans, disease), and low recruitment and loss of genetic diversity due to small populations also threaten the survival of the species listed in this chapter. This implies that a substantial reduction in road mortality is not necessarily sufficient for the recovery of the species listed in this chapter. For successful species recovery, including mitigation for effects related to roads and traffic, it is advisable to use an integrated approach.(171)