Florida State Route 46 was elevated over this underpass. Notice the channeling fences on either side of the crossing.
Wildlife crossings are structures that allow animals to cross human-made barriers safely. Wildlife crossings may include underpass tunnels or wildlife tunnels, viaducts, and overpasses or green bridges (mainly for large or herd-type animals); amphibian tunnels; fish ladders; canopy bridge (especially for monkeys and squirrels), tunnels and culverts (for small mammals such as otters, hedgehogs, and badgers); and green roofs (for butterflies and birds).
Wildlife crossings are a practice in habitat conservation, allowing connections or reconnections between habitats, combating habitat fragmentation. They also assist in avoiding collisions between vehicles and animals, which in addition to killing or injuring wildlife may cause injury to humans and property damage.
Similar structures can be used for domesticated animals, such as cattle creeps.
Roads and habitat fragmentation
Camel Crossing in Kuwait
Habitat fragmentation occurs when human-made barriers such as roads, railroads, canals, electric power lines, and pipelines penetrate and divide wildlife habitat (Primack 2006). Of these, roads have the most widespread and detrimental effects (Spellerberg 1998). Scientists estimate that the system of roads in the United States affects the ecology of at least one-fifth of the land area of the country (Forman 2000). For many years ecologists and conservationists have documented the adverse relationship between roads and wildlife. Jaeger et al. (2005)
identify four ways that roads and traffic detrimentally affect wildlife
populations: (1) they decrease habitat amount and quality, (2) they
increase mortality due to wildlife-vehicle collisions
(road kill), (3) they prevent access to resources on the other side of
the road, and (4) they subdivide wildlife populations into smaller and
more vulnerable sub-populations (fragmentation). Habitat fragmentation
can lead to extinction or extirpation if a population's gene pool is
restricted enough.
The first three effects (loss of habitat, road kill, and
isolation from resources) exert pressure on various animal populations
by reducing available resources and directly killing individuals in a
population. For instance, Bennett (1991)
found that road kills do not pose a significant threat to healthy
populations but can be devastating to small, shrinking, or threatened
populations. Road mortality has significantly affected a number of
prominent species in the United States, including white-tailed deer (Odocoileus virginianus), Florida panthers (Puma concolor coryi), and black bears (Ursus americanus) (Clevenger et al. 2001).
In addition, habitat loss can be direct, if habitat is destroyed to
make room for a road, or indirect, if habitat quality close to roads is
compromised due to emissions from the roads (e.g. noise, light, runoff,
pollution, etc.) (Jaeger et al. 2005).
Finally, species that are unable to migrate across roads to reach
resources such as food, shelter and mates will experience reduced
reproductive and survival rates, which can compromise population
viability (Noss et al., 1996).
In addition to the first three factors, numerous studies have
shown that the construction and use of roads is a direct source of
habitat fragmentation (Spellerberg 1998).
As mentioned above, populations surrounded by roads are less likely to
receive immigrants from other habitats and as a result, they suffer
from a lack of genetic diversity. These small populations are particularly vulnerable to extinction due to demographic, genetic, and environmental stochasticity because they do not contain enough alleles to adapt to new selective pressures such as changes in temperature, habitat, and food availability (Primack 2006).
The relationship between roads and habitat fragmentation is well
documented. One study found that roads contribute more to fragmentation
in forest habitats than clear cuts (Reed et al. 1996).
Another study concluded that road fragmentation of formerly contiguous
forest in eastern North America is the primary cause for the decline of
forest bird species and has also significantly harmed small mammals,
insects, and reptiles in the United States (Spellerberg 1998).
After years of research, biologists agree that roads and traffic lead
to habitat fragmentation, isolation and road kill, all of which combine
to significantly compromise the viability of wildlife populations
throughout the world.
Wildlife-vehicle collisions
In
addition to conservation concerns, wildlife-vehicle collisions have a
significant cost for human populations because collisions damage
property and injure and kill passengers and drivers. Bruinderink & Hazebroek (1996) estimated the number of collisions with ungulates in traffic in Europe at 507,000 per year, resulting in 300 people killed, 30,000 injured,
and property damage exceeding $1 billion. In parallel, 1.5 million
traffic accidents involving deer in the United States cause an estimated
$1.1 billion in vehicle damage each year (Donaldson 2005).
On a larger scale, research indicates that wildlife-vehicle collisions
in the United States result in 29,000 injuries and more than 200
fatalities per year.
The conservation issues associated with roads (wildlife mortality
and habitat fragmentation) coupled with the substantial human and
economic costs resulting from wildlife-vehicle collisions have caused
scientists, engineers, and transportation authorities to consider a
number of mitigation tools for reducing the conflict between roads and
wildlife. Of the currently available options, structures known as
wildlife crossings have been the most successful at reducing both
habitat fragmentation and wildlife-vehicle collisions caused by roads (Knapp et al. 2004, Clevenger, 2006).
"Animals' Bridge," on the Flathead Indian Reservation in Montana, used by grizzly and black bears, deer, elk, mountain lions, and others
Wildlife crossings are structural passages beneath or above roadways
that are designed to facilitate safe wildlife movement across roadways (Donaldson 2005).
In recent years, conservation biologists and wildlife managers have
advocated wildlife crossings coupled with roadside fencing as a way to
increase road permeability and habitat connectivity while decreasing
wildlife-vehicle collisions. Wildlife crossing is the umbrella term
encompassing underpasses, overpasses, ecoducts, green bridges,
amphibian/small mammal tunnels, and wildlife viaducts (Bank et al. 2002).
All of these structures are designed to provide semi-natural corridors
above and below roads so that animals can safely cross without
endangering themselves and motorists.
History and location
Written
reports of rough fish ladders date to 17th-century France, where
bundles of branches were used to create steps in steep channels to
bypass obstructions. A version was patented in 1837 by Richard McFarlan
of Bathurst, New Brunswick, Canada, who designed a fishway to bypass a dam at his water-powered lumber mill.[8] In 1880, the first fish ladder was built in Rhode Island, United States, on the Pawtuxet Falls Dam. As the Industrial Age advanced, dams and other river obstructions became larger and more common, leading to the need for effective fish by-passes.
The first overland wildlife crossings were constructed in France during the 1950s (Chilson 2003).
European countries including the Netherlands, Switzerland, Germany, and
France have been using various crossing structures to reduce the
conflict between wildlife and roads for several decades and use a
variety of overpasses and underpasses to protect and re-establish wildlife such as: amphibians, badgers, ungulates, invertebrates, and other small mammals (Bank et al. 2002).
The Humane Society of the United States reports that the more than 600 tunnels installed under major and minor roads in the Netherlands have helped to substantially increase population levels of the endangered European badger. The longest "ecoduct" viaduct, near Crailo in the Netherlands, runs 800 m and spans a highway, railway and golf course.
A terrapin crossing sign and a highway barrier designed for crossing at the end of the F.J. Torras causeway at St. Simons Island, Georgia, US (2015)
Wildlife crossings are becoming increasingly common in Canada and the United States. Recognizable wildlife crossings are found in Banff National Park in Alberta, where vegetated overpasses provide safe passage over the Trans-Canada Highway for bears, moose, deer, wolves, elk, and many other species (Clevenger 2007). The 24 wildlife crossings in Banff were constructed as part of a road improvement project in 1978 (Clevenger 2007).
In the United States, thousands of wildlife crossings have been built
in the past 30 years, including culverts, bridges, and overpasses.
These have been used to protect mountain goats in Montana, spotted salamanders in Massachusetts, bighorn sheep in Colorado, desert tortoises in California, and endangered Florida panthers in Florida (Chilson 2003).
The first wildlife crossing in the Canadian province of Ontario was built in 2010, along Ontario Highway 69 between Sudbury and Killarney, as part of the route's ongoing freeway conversion.
Costs and benefits
The
benefits derived from constructing wildlife crossings to extend
wildlife migration corridors over and under major roads appear to
outweigh the costs of construction and maintenance. One study estimates
that adding wildlife crossings to a road project is a 7-8% increase in
the total cost of the project (Bank et al. 2002).
Theoretically, the monetary costs associated with constructing and
maintaining wildlife crossings in ecologically important areas are
trumped by the benefits associated with protecting wildlife populations,
reducing property damage to vehicles, and saving the lives of drivers
and passengers by reducing the number of collisions caused by wildlife.
A study completed for the Virginia Department of Transportation
estimated that underpasses for wildlife become cost effective, in terms
of property damage, when they prevent between 2.6 and 9.2 deer-vehicle
collisions per year, depending on the cost of the underpass.
Approximately 300 deer crossed through the underpasses in the year the
study took place (Donaldson 2005).
Effectiveness
A number of studies have been conducted to determine the effectiveness of wildlife corridors
at providing habitat connectivity (by providing viable migration
corridors) and reducing wildlife-vehicle collisions. The effectiveness
of these structures appears to be highly site-specific (due to
differences in location, structure, species, habitat, etc.) but
crossings have been beneficial to a number of species in a variety of
locations. Some of the wildlife crossing success stories are detailed
below.
Banff National Park
Banff
National Park offers one of the best opportunities to study the
effectiveness of wildlife crossings because the park contains a wide
variety of species and is bisected by a large commercial road called the
Trans-Canada Highway
(TCH). To reduce the effects of the four-lane TCH, 24 wildlife
crossings (22 underpasses and two overpasses) were built to ensure
habitat connectivity and protect motorists (Clevenger 2007). In 1996, Parks Canada
developed a contract with university researchers to assess the
effectiveness of the crossings. The past decade has produced a number
of publications that analyze the crossings' effect on various species
and overall wildlife mortality (see Clevenger & Waltho 2000, Clevenger et al. 2001, and Clevenger 2007).
Wildlife overpass in Banff National Park, Canada
Using a variety of techniques to monitor the crossings over the last 25 years, scientists report that 10 species of large mammals (including deer, elk, black bear, grizzly bear, mountain lion, wolf, moose, and coyote) have used the 24 crossings in Banff a total of 84,000 times as of January 2007 (Clevenger 2007). The research also identified a "learning curve"
such that animals need time to acclimate to the structures before they
feel comfortable using them. For example, grizzly bear crossings
increased from seven in 1996 to more than 100 in 2006, although the
actual number of individual bears using the structures remained constant
over this time at between two and four bears (Parks Canada, unpublished
results). A similar set of observations was made for wolves, with
crossings increasing from two to approximately 140 over the same 10-year
period. However, in this case the actual number of wolves in the packs
using the crossings increased dramatically, from a low of two up to a
high of over 20 individuals. In continuation with these positive
results, Clevenger et al. (2001)
reported that the use of wildlife crossings and fencing reduced
traffic-induced mortality of large ungulates on the TCH by more than 80
percent. Recent analysis for carnivores showed results were not as
positive however, with bear mortality increasing by an average of 116
percent in direct parallel to an equal doubling of traffic volumes on
the highway, clearly showing no effect of fencing to reduce bear
mortality (Hallstrom, Clevenger, Maher and Whittington, in prep).
Research on the crossings in Banff has thus shown mixed value of
wildlife crossings depending on the species in question.
Parks Canada is currently planning to build 17 additional
crossing structures across the TCH to increase driver safety near the
hamlet of Lake Louise.
Lack of effectiveness of standard fencing in reducing bear mortality
demonstrates that additional measures such as wire 'T-caps' on the fence
may be needed for fencing to mitigate effectively for bears (Hallstrom,
Clevenger, Maher and Whittington, in prep).
Collier and Lee counties in Florida
Twenty-four
wildlife crossings (highway underpasses) and 12 bridges modified for
wildlife have been constructed along a 40-mile stretch of Interstate 75 in Collier and Lee counties in Florida (Scott 2007).
These crossings are specifically designed to target and protect the
endangered Florida panther, a subspecies of mountain lion found in the southeastern United States.
Scientists estimate that there are only 80-100 Florida panthers alive
in the wild, making them one of the most endangered large mammals in
North America (Foster and Humphrey, 1995). The Florida panther is
particularly vulnerable to wildlife-vehicle collisions, which claimed 11
panthers in 2006 and 14 in 2007 (Scott 2007).
The Florida Fish and Wildlife Conservation Commission
(FWC) has used a number of mitigation tools in an effort to protect
Florida panthers and the combination of wildlife crossings and fences
have proven the most effective (Scott 2007).
As of 2007, no panthers have been killed in areas equipped with
continuous fencing and wildlife crossings and the FWC is planning to
construct many more crossing structures in the future. The underpasses
on I-75 also appeared to benefit bobcats, deer, and raccoons, and
significantly reduced wildlife-vehicle collisions along the interstate
(Foster and Humphrey, 1995).
Underpasses in southern California
Wildlife crossings have also been important for protecting biodiversity in several areas of southern California. In San Bernardino County, biologists have erected fences along State Route 58 to complement underpasses (culverts) that are being used by the threatened desert tortoise.
Tortoise deaths on the highway declined by 93% during the first four
years after the introduction of the fences, proving that even makeshift
wildlife crossings (storm-drainage culverts in this case) have the
ability to increase highway permeability and protect sensitive species (Chilson 2003). Additionally, studies by Haas (2000) and Lyren (2001)
report that underpasses in Orange, Riverside, and Los Angeles Counties
have drawn significant use from a variety of species including bobcats, coyotes, gray fox, mule deer, and long-tailed weasels. These results could be extremely important for wildlife conservation efforts in the region's Puente Hills and Chino Hills links, which have been increasingly fragmented by road construction (Haas 2000) Los Angeles County's first wildlife-purpose built underpass is at Harbor Boulevard.
It was built in partnership between Los Angeles County, California
State Parks and the Puente Hills Habitat Preservation Authority.
Ecoducts, Netherlands
One of the two wildlife crossings spanning the A50 highway on the Veluwe in the Netherlands
The Netherlands has over 66 wildlife crossings (overpasses and ecoducts) that have been used to protect the endangered European badger, as well as populations of wild boar, red deer, and roe deer. As of 2012, the Veluwe,
1000 square kilometers of woods, heathland and drifting sands, the
largest lowland nature area in North Western Europe, contains nine
ecoducts, 50 meters wide on average, that are used to shuttle wildlife
across highways that transect the Veluwe. The first two ecoducts on the
Veluwe were built in 1988 across the A50
when the highway was constructed. Five of the other ecoducts on the
Veluwe were built across existing highways, one was built across a two
lane provincial road. The two ecoducts across the A50 were used by
nearly 5,000 deer and wild boar during a one-year period (Bank et al. 2002).
The Netherlands also boasts the world's longest ecoduct-wildlife
overpass called the Natuurbrug Zanderij Crailoo (sand quarry nature
bridge at Crailo) (Danby 2004). The massive structure, completed in 2006, is 50 m wide and over 800 m long and spans a railway line, business park, river, roadway, and sports complex (Danby 2004).
Monitoring is currently underway to examine the effectiveness of this
innovative project combining wildlife protection with urban development.
The oldest wildlife passage is Zeist West - A 28, opened in 1988.
Slaty Creek Wildlife Underpass, Calder Freeway, Black Forest, Australia
Another
case study of the effectiveness of wildlife crossings comes from an
underpass built to minimize the ecological effect of the Calder Freeway as it travels through the Black Forest in Victoria, Australia. In 1997, the Victorian Government Roads Corporation built Slaty Creek wildlife underpass at a cost of $3 million (Abson & Lawrence 2003).
Scientists used 14 different techniques to monitor the underpass for
12 months in order to determine the abundance and diversity of species
using the underpass (Abson & Lawrence 2003).
During the 12-month period, 79 species of fauna were detected in the
underpass (compared with 116 species detected in the surrounding forest)
including amphibians, bats, birds, koalas, wombats, gliders, reptiles, and kangaroos (Abson & Lawrence 2003).
The results indicate that the underpass could be useful to a wide
array of species but the authors suggest that Slaty Creek could be
improved by enhanced design and maintenance of fencing to minimise road
kill along the Calder Freeway and by attempting to exclude introduced
predators such as cats and foxes from the area.
The ARC International Wildlife Crossing Infrastructure Design Competition
In
2010, ARC Solutions - an interdisciplinary partnership - initiated the
International Wildlife Crossing Infrastructure Design Competition for a
wildlife crossing over Interstate 70 near Denver, Colorado.,
and designers had to account for many challenges unique to the area,
including snow and severe weather, high elevation and steep grades, a
six-lane roadway, a bike path, and high traffic volumes, as well as
multiple species of wildlife, including lynx.
After receiving 36 submissions from nine countries, a jury of
internationally acclaimed experts in landscape architecture,
engineering, architecture, ecology and transportation selected five
finalists in November 2010 to further develop their conceptual designs
for a wildlife crossing structure. In January 2011, the team led by HNTB with Michael Van Valkenburgh & Associates (New York) were selected as the winners.
The design features a single 100 m (328 ft) concrete span across the
highway that is planted with a variety of vegetation types, including a
pine-tree forest and meadow grasses, to attract different species to
cross. A modular precast concrete design means that much of the bridge
can be constructed offsite and moved into place.
Canopy Bridge in Anamalai Tiger Reserve
Many endangered lion-tailed macaques used to be killed while crossing the highway at Puduthotam in Valparai,
South India. Thanks to the efforts of NGOs and the forest department,
several canopy bridges were installed, connecting trees on either side
of the road. This helped to lower the numbers of lion-tailed macaques
killed in the region. The Environment Conservation Group
had initiated a national mission to increase awareness on the
importance of adopting roadkill mitigation methods through their mission
PATH traveling more than 17,000 kilometers across 22 states.
