Meeting the needs of Arizona’s growing population requires upgrading existing transportation and service corridors as well as constructing new transportation infrastructure. Large-scale projects like the proposed I-11 corridor, which will connect Las Vegas to southern Arizona, and smaller scale improvements such as widening or paving county roadways are constantly being planned for and implemented across the state. Considering the impact these projects may have on wildlife and their habitats, and making design or siting modifications to reduce these impacts where possible, can streamline the environmental compliance and approval process while also contributing to the conservation of Arizona’s wildlife and the safety of motorists traveling roadways.
All animals move across the landscape to varying extents in order to acquire the resources necessary for survival such as food, water, protective cover, and mates. Large animals such as mountain lions, black bears, elk, and mule deer may roam over vast expanses that can encompass thousands of acres, while smaller animals such as tassel-eared squirrels and northern leopard frogs engage in essential movements on a much smaller scale (Van Riper and Ockenfels 1998). Some animal movements occur on a daily basis, while seasonal migrations may occur annually, and the dispersal of young from their natal sites to secure new breeding territories happens only once in an individual’s lifetime. Man-made infrastructure can affect each of these movement patterns and may pose a threat to the long-term persistence of wildlife populations (Watson and Klingel 2006). Anthropogenic infrastructure can create both physical barriers and behavioral barriers to wildlife movement. Physical barriers encompass a wide range of features such as roadways, irrigation canals, residential and commercial development, fencing, and changes to habitat. Behavioral barriers may occur when wildlife species avoid areas near roads and railways related to the degree of human disturbance, such as traffic density and secondary development (Luell et al. 2003).
Connecting communities and providing goods, services, and economic opportunities are essential functions of our transportation infrastructure. As Arizona’s population grows, more efficient connections are required to meet our needs and those of our visitors. With this increase in highway, road, and urban development, we’ve also seen increased interactions with wildlife and fragmentation of their habitat. Each year in the United States more than 200 motorists are killed and thousands more are injured in animal-vehicle collisions (Huijser et al. 2008). Countless animals are injured or killed while property damage is measured in the millions of dollars.
While human and wildlife safety concerns are of highest priority, construction and modification of transportation corridors present additional ecological challenges. Ground disturbance associated with roadway construction, and vehicles traveling on the roadways once operational, can introduce or spread invasive plant species across right-of-ways and onto adjacent lands. Pollution by toxins, nutrients, and noise from the transportation corridor can create edge effects on adjacent hydrology and microclimate, reducing the suitability of the remaining habitats (Murcia 1995; Reijnen et al. 1995; Boarman and Sazaki 2006; Eigenbrod et al. 2009; Parris and Schneider 2009). These effects spread into the surrounding landscape and contribute to the loss and degradation of natural habitat several times larger than the area of the road footprint itself. The effects are influenced by road and traffic characteristics, landscape topography and hydrology, wind, and vegetation. In addition, the resulting impacts on wildlife also depend on the sensitivity of the species in the vicinity.
Table 2: Examples of AZGFD-led wildlife crossing projects completed in recent years.
Installation of Wildlife Crossing Structure
11 crossing structures and 6 bridges that function as crossing structures installed along State Route 260
Roadside animal detection system on State Route 260
2 wildlife underpasses along State Route 260
3 overpass structures along US 93 just south of Hoover Dam and the Nevada state line
2 crossing structures (1 overpass and 1 underpass) north of Tucson on State Route 77
6 crossing structures (1 overpass, 5 underpasses) along the Boulder City Bypass portion of I-11
Road and bridge construction can also directly influence stream characteristics, such as channel and floodplain configuration, substrate embeddedness, riparian condition, relative prevalence of woody debris, stream flow rate, and temperature regime (Furniss et al. 1991). The timing, quantity, quality and location of surface water runoff can change as roadways and related drainage structures act to intercept, collect, and/or divert water. These factors can accelerate water delivery and surface flow, thereby increasing the potential for higher magnitude of runoff in watersheds having roadway developments as compared to those not having such developments (Wemple et al. 1996).
The negative effects associated with road construction and improvements can often be reduced and/or mitigated by identifying and accommodating fish and wildlife movement areas during early stages of roadway design. The Federal Highway Administration’s (FHWA’s) Wildlife Crossing Structure Handbook (FHWA 2011) and ADOT’s Integrated Roadside Vegetation Management Guidelines and Clean Water Act Guidance Manual provide general guidance on designs that minimize roadway impacts to wildlife movement.
Identifying important wildlife movement areas during the construction of new roads or improvements allows for the informed siting of wildlife-friendly over- and underpasses and exclusionary fencing that can greatly reduce the likelihood of wildlife-vehicle collisions and decrease the roadway’s habitat fragmentation impact. AZGFD conducts numerous studies on wildlife movement to identify movement corridors that can then be incorporated into the planning process for roads and highways (Table 2). Through partnerships with FHWA, ADOT, and Nevada Department of Transportation, and others, crossing structures have been installed in several high traffic areas throughout the state, providing safe movement corridors for many species of wildlife and greatly reducing the number of wildlife-vehicle collisions.
Low-level flights by fixed-wing and rotary aircraft can startle and change behavior in some mammal and bird species. Those species thought to be particularly susceptible include pronghorn, bighorn sheep, raptors, and waterfowl (Efroymson et al. 2001). However, studies of species responses to low-level flights are generally lacking for several taxonomic groups including reptiles, bats, and invertebrates.
The visual image and sound of aircraft can stress wildlife, and may interfere with feeding, mating, nesting, fledging, and predation. Direct mortality of wildlife due to physical collision has been reported especially where flight paths are in close proximity to habitat and migration routes (Efroymson et al. 2001). The Federal Aviation Administration (FAA) maintains a database of aircraft/wildlife strikes (FAA Wildlife Strike Database) and documented more than 1,500 between 2010-2020 in Arizona (FAA 2021). The vast majority of these reports involved birds, although other taxa including bats and rabbits have also been documented. In addition to the direct stress imposed on wildlife by these aircraft/wildlife collisions, they represent a serious threat to human safety for both civilian and military aircraft.
Lighted communications and transmission towers, which attract a variety of insect species, have the potential to attract and kill night-flying migratory birds and bats (Longcore et al. 2008). Collision and electrocution may both cause wildlife mortality, with some research estimating that up to 57 million individual wildlife are killed by collisions and another 11.6 million individuals die from electrocution each year in the United States (Loss et al. 2014). Raptors are particularly vulnerable due to their tendency to perch or nest on transmission lines.
The Avian Power Line Interaction Committee (APLIC) developed guidelines for protecting birds on power lines. The suggested practices are aimed at either providing the birds with an alternative safe place to perch or discouraging birds from landing on potentially unsafe areas of the pole (APLIC 2006). Modifying conductor separation and grounding procedures are two primary considerations that can be applied to new and existing transmission lines. Additionally, retrofitting pole-tops of existing transmission lines is a commonly used technique to prevent electrocution. Collisions can be reduced by using devices to increase line visibility (Eccleston and Harness 2018).
Utility corridors also impact other wildlife species by fragmenting habitat, creating disturbance from authorized and unauthorized use of access roads, contaminating areas due to application of herbicides in right-of-way maintenance, and the introduction of non-native plant species. Power companies like Arizona Public Service (APS) have adopted standards for right-of-way management that allow for low-growth vegetation rather than mowing and broadcast-spraying (Right-Of-Way Stewardship Council 2016). As human populations continue to encroach upon and fragment wildlife habitat, appropriate planning and management of utility corridors will likely become increasingly important.