In search of the ideal underpass for wild animals
Written by Alex Jensen
North Carolina Museum of Natural Sciences
November 27th, 2023
Which crossings are most effective?
One of the central challenges in road ecology is keeping wildlife off roads while still allowing them to cross safely. Wildlife exclusion fencing has been shown to be effective at reducing wildlife-vehicle collisions, but likely at the cost of reduced ecological connectivity across roads. However, when crossing structures are integrated into fencing, connectivity can be maintained while still reducing collisions. But what makes an ideal crossing structure?
Several factors influence the effectiveness of crossing structures for wildlife. First, habitat in the surrounding landscape influences the probability that a given species would be nearby in the first place. Second, habitat at the local scale (e.g., vegetative cover near the entrance) has been shown to influence their use. Third, attributes of the crossing structure itself – like how big and open it is – have been shown to be important for some species. Yet because species vary in habitat use and behavior, it remains a challenge to design crossing structures which can accommodate the most species.
Figure 1. Our study area was along highway 101 (shown in red) on the Central Coast of California. This area was identified as an important movement corridor for large mammals like pumas.
Here we focused on medium- and large-sized mammal use of crossing structures along Highway 101 on the Central Coast of California (Figure 1). This highway bisects an important linkage between protected areas and our focal stretch was identified as a roadkill hotspot. A wildlife exclusion fence was installed to reduce these collisions, but we needed a better understanding of how permeable the highway was via existing crossing structures, including nine medium-large culverts and two bridge underpasses. Note: none of these crossing structures were installed for wildlife – rather they served a variety of purposes including carrying water underneath the highway and connecting private land on either side.
Using wildlife cameras, we quantified 5 years of mammal activity in 11 crossing structures and tested which factors best explained their use. We had several focal species (black bear, mule deer, puma, bobcat) but also investigated which crossing structures accommodated the highest diversity of species.
Large open crossing structures along natural travel routes are best
We detected 12 species of medium and large-sized mammals in our crossing structures, with medium-sized carnivores (e.g., foxes) accounting for 81% of detections (Figure 2). Of our focal species, we detected bobcats the most (n = 1,221), followed by mule deer (n = 397), black bear (n = 146), and puma (n = 32).
We found that mule deer were the most picky of our focal species, almost exclusively using the two large, open underpasses. Indeed – the openness of the crossing (height x width / length) best explained deer activity. Deer were rarely detected even in relatively open 3m x 3m box culverts, highlighting the importance of underpass-like crossings for ungulates.
We also found that both black bear activity and species richness were greater at crossing structures closer to streams. Riparian corridors are known to serve as travel routes for wildlife, so this finding highlights the importance of crossing structures serving as a link between natural corridors on either side of roads. Black bear activity was also greater in crossing structures in fenced sections of the highway, suggesting that the fence was working as intended.
We found no significant predictors of bobcat activity, perhaps because they were the least picky of our focal species in terms of their use of crossing structures. Unfortunately, we did not detect pumas enough to reliably model their activity, but a GPS-collar study in the area showed that one regularly used one of the underpasses in our study area.
Indeed, more information about mammal activity in the surrounding landscape (either from GPS collars or cameras) would have given us a more holistic understanding of how the highway was altering their activity. Regardless, we come away with two conclusions: 1) given how abundant (and large) ungulates often are, crossing structures should be large and open when possible; 2) crossing structures along or near streams will likely promote connectivity for the most mammal species.
Figure 2. Average number of detections and several of our ‘greatest hits’ from undercrossings. Note how open (b) is compared to the 4x8 foot culverts the gray foxes (d) were detected in.
Author information
Alex Jensen
Postdoctoral Associate, North Carolina Museum of Natural Sciences
Source citation
Jensen A, J Perrine, A Schaffner, R Brewster, A Giordano, M Robertson, N Siepel. 2023. Mammal use of undercrossings is influenced by openness and proximity to riparian corridors. Wildlife Research https://www.publish.csiro.au/WR/WR21183
Editor:
Rodney van der Ree
Cite this summary:
Jensen, A. (2023). In search of the ideal underpass for wild animals. Edited by van der Ree, R. TransportEcology.info, Accessed at: https://transportecology.info/research/ideal-underpass-for-wild-animals
