Utilizing a multi-technique, multi-taxa approach to monitoring wildlife passageways in southern Vermont

1. What they did

Roads take away and break up habitat, kill animals and get in the way of their movement. Many wildlife crossing structures have been built in North America, but few are monitored, and most monitoring only records which animals use the structure, not those that cross the road instead or turn away. The authors set out to monitor a whole range of animal movements, for many kinds of animals, around two wildlife passageways on a newly built highway, the Bennington Bypass in southern Vermont.

They drew a conceptual model of nine possible movements an animal can make near a road and passageway, from crossing the road safely or being hit, to being guided by fencing through the structure, to approaching and turning away. They then used five methods together and sorted their records by these movements:

• Track beds and track plates. Boards covered with a fine layer of marble dust were laid across both passageways, and sooted aluminium plates were set in the drainage culvert. Track sets were identified to species (or at least family) with their direction of travel. Each track set counted as a track bed crossing, used as an index of how much the structure was used.
• Camera traps. Film and infrared digital cameras at the track beds confirmed species, and digital cameras along the streams recorded animals passing where there were no track beds.
• Snow-tracking. After snowfalls, observers walked a grid of transects parallel and perpendicular to the highway, covering 500 m beyond each passageway, and recorded every set of tracks and what the animal did in relation to the road.
• Small mammal trapping. Live traps on lines beside both structures, with ear tags, to see whether small mammals moved through the structures or over the road.
• Road kill surveys. Three drives a week along the whole 7 km, recording every carcass by size group and location.

2. What they found

Track beds and cameras

• The track beds recorded 786 track sets from at least 26 kinds of animals. Of these, 110 could only be called small or medium-sized mammals, and 62 could be identified only to family or genus.
• The stream cameras recorded 90 crossings by six species: white-tailed deer (53), wild turkey (12), bobcat (9), raccoon (9), woodchuck (6) and domestic cat (1).
• Adding the camera records raised the number of structure crossings detected for these six species by 38% in 2006 and 41% in 2007. Bobcats were detected only by cameras, and deer sometimes walked around the track beds (for example at East Airport Brook in 2006, with no deer on the track bed but 21 on camera).

Snow-tracking

• 162 track sets over 24 surveys, from 10 species: coyote, bobcat, mink, fisher, long-tailed weasel, river otter, gray fox, raccoon, white-tailed deer and domestic cat.
• Nine of the 10 species used the structures and seven crossed over the road. Mink and otter, which travel along streams, only used the structures.
• Overall, 68 crossings were through the structures and 57 over the road (Table 2 and Discussion; the Results text gives these the other way round).
• White-tailed deer made the most structure crossings, and these rose by 75% from the first winter (12) to the second (21), even with fewer track nights.
• Coyotes mostly crossed over the road rather than through the structures, but 15 of their 54 identified movements showed them turning away from the road or the structures.
• Only 17% of animals that came to the structures (25 of 143) turned away from them.

Small mammals

• 690 small mammals were ear-tagged over 48 trapping sessions; 92% were Peromyscus mice. Just over half (55%) were recaptured at least once.
• 15 individual Peromyscus mice made 26 crossings through the structures, and one possibly crossed the road. No other species was recorded crossing.
• More than 36% of recaptured Peromyscus mice moved 65 m or more, the shortest distance from one forest edge to the other through a crossing structure.

Road kill

• 1,289 road-killed animals were counted in 148 surveys. Three quarters were small animals, and at least 31% of the small animals were frogs and toads.
• Very few deer were killed (about 0.05 to 0.07 per survey), though many deer lived in the area.
• Traffic and the road kill rate both rose each year, and road kill did not drop closer to the crossing structures (no significant correlation with distance).

3. What they concluded

• The Bennington Bypass structures are used often and by many species, but animals still cross over the road and risk being hit. Bobcats and coyotes seem to cross where pre-existing game trails meet the road at least as often as they use the structures.
• The large structures suit medium and large animals, especially deer, but their lack of vegetation cover may hold back small mammals. This may change as the area grows back.
• Whether the structures “work” depends on the goal: if it is to keep animals off the road, they are not fully effective; if it is to let part of each population cross, they seem effective for some species, such as deer and mink.
• The lack of high fencing along the highway may explain the many road crossings.
• Track beds and cameras used together give a good index of structure use but cannot tell individuals apart. Mark and recapture or telemetry is needed for that. Snow-tracking shows movement in the wider landscape but only in winter.
• Monitoring a broad range of movements and taxa, ideally combined with population modelling, gives a more accurate picture of whether mitigation succeeds.

4. How tracking was or was not used

Tracking was central to two of the five methods.

• Track beds and plates: fresh marble dust on boards inside the passageways, and sooted plates in the culvert, recorded the footprints of animals passing through. For each track set the team recorded species (or family) and direction of travel. Difficult tracks were photographed and measured (foot width and length, stride and straddle) for later identification. Track beds could not be read after rain or wind, and small mice running up and down the beds made true crossings hard to judge.
• Snow-tracking: 48 hours after a snowfall of more than 1.3 cm, observers walked the transect grid and recorded species, track and gait measurements, gait pattern, direction of movement, scat and scent marking, where animals crossed the highway, weather, days since the last snowfall and snow depth. Each track set was then classed by the movement it showed in relation to the road. Deer and domestic cat tracks were so many that individual movements could not be told apart, so only their crossings were counted.

The authors list the strengths and weaknesses of track beds (cheap, working round the clock, simple and easy to spread over wide areas; but high upkeep, weather-sensitive, often hard to tell species apart, unable to tell individuals apart, and needing a flat surface) and describe snow-tracking as a low-cost alternative to telemetry.

5. Tracker or observer certification

Not reported. The paper does not say who identified the tracks or what tracking experience they had, although the track bed and snow-tracking results rest on track identification and on reading the direction and type of each movement. Four field technicians are thanked in the acknowledgements.

6. Other technologies, skills or innovations

• Palm Pilot handhelds running CyberTracker software linked to Global Positioning System (GPS) receivers, used to record snow-tracking data in the field.
• TrailMaster TM1050 active infrared 35 mm film camera, and Reconyx Silent Image PM35M13 motion-sensing infrared digital cameras, set to take 10 images per trigger.
• Track beds of oriented strand board (OSB) sheets covered with about 2 mm of marble dust; track plates of aluminium sheet sooted with a propane torch, with white contact paper.
• Sherman live traps (226) and metal ear tags for small mammals.
• A conceptual model of nine possible movements near a road and passageway, used to compare results across methods.

7. Implications for conservation (from the paper)

• Monitoring only what uses a crossing structure misses animals that cross the road or turn away, so it can overstate how well mitigation works.
• Clear goals and success criteria are needed when planning mitigation: keeping large animals such as moose or elk off the road calls for different measures than letting enough animals cross to keep populations connected.
• Highway fencing is likely needed if the goal is to prevent collisions and steer animals into the structures.
• Amphibians made up a large share of road kill, so monitoring should cover many taxa, not only large mammals.
• More information on population size and trends is needed to judge how many crossings each species needs and how road kill affects its survival.