Bridging The Gap Bat Use of Bridges, Tunnels and Culverts Shawn F. Lowery Arizona Game and Fish Department Wildlife Contracts Branch Arizona Bat Working Group - Researchers Management Agencies Private Consultants Non-Profit Groups Educators Arizona Game and Fish Department Town of Marana Environmental Engineering Division
Bat Basics: Benefits Bats Pollinate Desert Plants Bats Help to Control Pests Bats are a vital component in the Sonoran Desert ecosystem 28 Species of bats in Arizona, many of which are covered under the SDCP Bats are primary predators of insect pests Some small insectivorous bats can consume up to 2,000 mosquito- sized insects in one night Across the western United States 25% of bridges and culverts are occupied by bats as day roosts or night roosts
Where do Bats Roost? Bat Roosting Habitat - Maintain specific Microclimate Proximity to habitat and forage resource. Low human disturbance and colony space. Appropriate structures Caves Mines Natural Vegetation (Trees) Man made structures Buildings Bridges
Why Bridges? Bridges and other massive structures provide many of the basic requirements for roost selection by bats: Food: They are often associated with edge habitats (higher insect populations) Habitat: Many span water or open areas and washes (forage and/or migration pathways) Shelter: Concrete structures moderate high and low temperatures and provide protection from terrestrial predators and weather. Morse Flat Bridge Methow Valley, Washington
Basic Bridge Types: The older bridge designs provided crevices for bats Parallel box beams are most suited to crevice dwelling bat species (day roosting) Cast-in-place and pre- stressed girders provide open areas for night roosting bats Flat slab designs have limited bat use, but can be retrofitted with special bat houses ( Oregon Wedge )
Concrete bridges mitigate temperatures extremes and provide shelter. Large concrete bridges provide thermal mass and provide unique microclimates for maternity sites for developing young. Older bridges designs that provide more bat roosting areas are deteriorating and creating safety concerns The newer, flat-bottom bridges do not provide suitable places for most bat species to roost - So, as bridges are replaced, habitat is lost regionally.
Bats and the RTA Wildlife Linkages Wildlife Linkages Committee The Regional Transportation Authority plan Scope This element will provide funding for (1) design and construction of wildlife crossing improvements within future planned roadways and highways and (2) retrofitting existing roadways and highways with wildlife crossing improvements. Wildlife crossing improvements include, but are not limited to, expanded culverts or underpasses, overpasses, fences and signage. Proposals for wildlife crossings associated with roadways or highways that divide existing parks or preserves will be given highest priority. In order to make informed decisions on where and what type of wildlife crossings are needed, research on wildlife migration patterns, types of crossings, and locations of crossings will be actively encouraged and supported.
Bats and the RTA Wildlife Linkages Committee Migratory bats are part of wildlife movement patterns in Pima County Bridges provide roosting locations that connect large blocks of foraging areas Transportation projects can adversely affect bats when older structures are removed and replaced with less bat-friendly designs. Increasingly limited roosting habitat range wide. Retrofitting existing roadways and highways with bat boxes are bricks and mortar -ready projects
Bridging The Gap? Congress Avenue Bridge Austin, Texas Amid widespread urban expansion, suitable bat roost sites are often limited More than 50% of American bat species are in severe decline or listed as Endangered (Bat Conservation International) Many species only raise 1 or 2 pups per year Bridges, tunnels, and culverts can mitigate loss of natural roosting sites
Waiting for the bats to emerge at 40 th Street and Camelback Rd. in Phoenix, AZ Urban bat roosts can be great Watchable Wildlife sites that serve to educate the public about bats.
Tucson is an Important Area for Bats in Bridges At least 8 species of bats have been found in Tucson area bridges In Tucson, 92% of parallel beam bridges with appropriate sized crevices are occupied by bats The 3 most common species are: Mexican free-tailed bats (Tadarida brasiliensis), Photo: Merlin D. Tuttle, Bat Conservation International W. pipistrelles pipistrelles (Parastrellus hesperus), Photo: Bruce Talburt, AZGFD Cave myotis (Myotis velifer) Photo: Bruce Talburt, AZGFD
What Makes a Bridge Bat-Friendly? Characteristics of bridges or culverts most suitable as bat roosts: Crevices at least ¾ 1 ¾ wide and at least 12 deep (preferably 18-24 deep) Sealed at the top Bridges/arches at least 10 ft tall, over natural washes or rivers but not susceptible to flooding Box culverts at lest 5 ft tall and at least 300 ft long
Evidence of Bat Use: Day Roosts Day roosting bats often seek out niches between parallel box beam girders or expansion joints Day roosts are where bats sleep or rear young Guano piles line up with overhead girder gaps A large bat colony during the summer usually indicates a maternity roost Most bridge summer residents disperse to other locations in the fall (September November)
Evidence of Bat Use: Night Roosts Heavy accumulation of urine, guano staining on night roosts Evidence is often found in open corners formed by girders and piers Night roosts serve as rest stops between foraging bouts They also provide a hang- out for social interaction or mating McFarland Bridge Methow Valley, Washington
Recommendations for Transportation Projects 1. Identify bridges scheduled for work 2. Bridges with existing bat roosts should be considered as candidates for providing roosts when renovated or replaced 3. If roosting crevices can t be incorporated into the design, bat roosts or houses can be attached 4. Consult a qualified biologist for bridges known to harbor bats and time maintenance, repaving, and inspections for fall or winter when less bats are present 5. Post-construction monitoring is recommended to document use of new or replaced bat roosts
Mitigation example: brick and mortar project Ina Road Bridge Replacement over the Santa Cruz River
Artificial Bridge Bat Abodes
Bat Bridge Lodges
Another Potential Bat Refuge.
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