Use of highway undercrossings by wildlife in southern California

1. What they did

Highways can block wildlife movement and kill animals that try to cross them, which matters most for wide-ranging animals such as carnivores and deer. Drainage culverts, tunnels and road underpasses already run beneath many highways, so the researchers asked whether wildlife use them to cross, and which features of a passage make it more or less likely to be used.

They studied 15 passages beneath three major highways near Los Angeles, California, around the Simi Hills. None had been built for wildlife: they were square culverts or tunnels, round drainage culverts, livestock tunnels, and underpasses where a road or wide stream passes under a bridge. Each passage was checked for four days in a row every month for a year, from July 1999 to June 2000.

• Track stations (11 passages): where cameras were likely to be vandalized, the team sifted three strips of agricultural gypsum powder (about 3 mm thick and 1 m wide) across the floor, one at each entrance and one in the middle. Each day they identified, recorded and then erased all tracks with a feather duster.
• Cameras (4 passages): where vandalism was unlikely, or where water flow in the rainy season made track stations impossible, they used infrared-triggered flash cameras, at least three per passage.
• Classing each record: tracks or photos at both ends and the middle counted as a verified crossing; at both ends but not the middle, or at the middle and one end, as a probable crossing; at one end only as an animal checking the entrance. Individual animals were not told apart, so some were probably counted more than once.

For each passage they measured length, width, height and cross-sectional area, the share of natural, landscaped and developed land within 250 m of each entrance, and human activity (people on foot, horseback, bicycles and vehicles). They then tested which of these features went with use by each species.

2. What they found

Who used the passages

• 2723 detections in the year: 1640 (60.2%) were human activity, 397 (14.6%) small mammals, 531 (19.5%) native large and mid-sized mammals, and 155 (5.7%) domestic animals.
• Wild mammals that passed through at least once: deer mice, woodrats, ground squirrels, cottontail rabbits, opossums, striped and spotted skunks, raccoons, coyotes, bobcats, mountain lions and mule deer.
• Of the 531 native large and mid-sized mammal records, 391 (73.6%) were verified or probable crossings and 140 (26.4%) were animals checking an entrance.
• Raccoons were the most common, with 265 records, and used 14 of the 15 passages. Coyotes, bobcats, domestic dogs and house cats each used about half the passages. Mule deer used 3 passages and a mountain lion only one (a single record, at passage 1).

What made a passage more likely to be used (P < 0.10 counted as significant)

• Coyotes: used passages with more human activity more, and passages surrounded by more developed land less. Coyotes were often recorded in passages with trails or roads that people used regularly.
• Bobcats: used passages with more natural habitat around them more, and those with more developed land less. Bobcats and coyotes used passages of many sizes, from large bridge underpasses to small pipe culverts.
• All large carnivores together: used passages with more natural habitat more.
• Mule deer: used shorter passages with larger cross-sections more, and passages with more human activity more. Use was not significantly related to habitat. The authors report that deer used only the three large passages and never small culverts, and that all three passages had significant amounts of natural habitat nearby (see the detailed keywords for two points where the tables differ).
• Raccoons: used longer passages and those surrounded by developed land more, and natural habitat less. Many raccoons were found in culverts holding water, sometimes whole families photographed foraging there, so they used the passages as feeding places and not only to cross.
• Opossums and skunks: no significant relationships, probably because of small samples.
• Domestic dogs and cats: used shorter passages, with larger cross-sections and more human activity, more. Their use was not related to habitat.
• People: human activity was mostly in the shorter, larger underpasses and tunnels and rarely in culverts.

Checking the track identifications

At one passage, monitored with both cameras and gypsum, every track identification agreed with the photographs.

3. What they concluded

• Underpasses and drainage culverts beneath highways are used regularly by wildlife, including species of conservation concern, even when they were not built for animals.
• Suitable habitat on either side of a passage was a particularly important predictor of use, especially for larger carnivores and deer. A culvert or underpass of any size is of little value as a wildlife corridor if it does not connect suitable habitat.
• For deer and coyotes, passage size was also important (abstract) and should be considered along with habitat when passages are planned or assessed.
• Coyote use near people probably reflects how common coyotes are on the urban fringe and their willingness to use areas of human activity, not an attraction to people. Encounters are probably few, because coyotes most likely use the passages when people are least likely to be there.
• The single mountain lion record shows passages can help these animals move, and more information on mountain lions in the region is critical.
• Passages also let house cats move into wild areas, where they may harm native birds and small mammals.
• The study shows use, not the benefits of use. Because these species are sensitive to fragmentation and roads kill many of them, the authors believe the benefits of highway undercrossings outweigh possible harm in their study area.

4. How tracking was or was not used

Tracking was one of the two main methods. At 11 of the 15 passages, animal use was recorded entirely from footprints in strips of gypsum powder at both entrances and in the middle. Tracks were identified with Murie’s field guide to animal tracks. To help identification, the team measured track length and width, and for dog and cat family tracks (canids and felids), stride (the distance between two consecutive tracks) and straddle (the distance across the outermost sides of the left and right tracks). They also recorded the direction of travel. In most probable crossings all tracks pointed one way, which suggested the animal had not turned back and that the missing tracks had been lost to wind or people. Track identifications were checked against photographs at one passage, where all agreed.

5. Tracker or observer certification

Not reported. The paper does not say who identified the tracks or how experienced they were. The acknowledgements thank interns and volunteers of the National Park Service. Track identifications were checked against camera photographs at one passage (all agreed), but no test of the observers’ skill is described, and the results at 11 of the 15 passages rest on their identifications.

6. Other technologies, skills or innovations

• Gypsum powder track stations, sifted through a mesh colander, at entrance and middle positions so that crossings could be told apart from animals only checking the entrance.
• Passive infrared camera units (TrailMaster TM550), set to take no more than one photograph a minute.
• Habitat around each passage measured from aerial photographs (Digital Orthophoto Quarter Quadrangles) and SPOT satellite images, then checked in the field along 24 bearings.

7. Implications for conservation (from the paper)

• Keeping or modifying existing passages beneath highways can be important for protecting native species where fast roads cut through their habitat. Building passages only for wildlife may also be justified where no other passages exist and the habitat is suitable.
• Protecting suitable habitat near crossing points is especially important, particularly for larger carnivores and deer. Passage size matters too, especially for deer.
• Animals were often killed trying to cross road surfaces. The authors recommend animal-proof fencing to guide animals away from the road and into crossing structures, together with habitat restoration near crossing points.
• More effective structures could be designed if wildlife movement were the main aim.