Wolves and Prey Species use Green Bridges to cross over Busy Highway in Germany
Written By Manisha Bhardwaj
Swedish University of Agricultural Sciences (SLU), Department of Ecology
4th July 2022
Return of the Wolf
Gray wolves have returned to Germany from Poland after their local extirpation in the 19th century. As the wolf population grows, packs expand into new areas, and young animals disperse to find new and unoccupied territory. In a landscape dominated by human-modification, crossing roads can be challenging for this re-establishing population.
Wolf-vehicle collisions accounted for ~ 76% of total confirmed wolf mortality in Germany in 2020. Collisions were most numerous in February and November, possibly coinciding with wolves dispersing in winter. Wildlife overpasses, also known as “Green bridges,” are a commonly implemented mitigation strategy to try to offset the impacts of roads on wildlife, but prior to our research, the use of green bridges by wolves in the rewilding landscape of Eastern Germany has not been studied.
Dynamics between Predator and Prey
Green bridges were built in Germany with the purpose of facilitating all wildlife movement, and their use by wolves was particularly interesting due to the conservation implications. However, we hypothesized that high wolf presence may have a negative impact on prey species, such as red deer, roe deer, and wild boar. Some studies suggest that crossing structures can act as a “prey-trap” if predators wait at the site to ambush crossing prey. As a result, prey may avoid the bridge when wolves are active, which in turn may push prey to attempt to across on the highway itself, or deter them from crossing at all.
In this study, we used camera traps to investigate the seasonal and diurnal use of a green bridge by wolves, red deer, roe deer, and wild boar. We aimed to determine if the bridge functions successfully as a means for wolves to recolonize into their historical range, and if the bridge acts as a prey-trap limiting prey species from crossing.
Figure 1. A Gray wolf captured on the green bridge over the A12 HIghway in Brandenburg State, Germany.
Study area
The bridge studied was constructed in 2012 to provide connectivity and reduce the barrier effect of the A12 Highway in Brandenburg State, Germany. The highway runs east to west, and was challenging to cross for species travelling north-south (Figure 2). On average, highway A12 is 31 m wide and supports 30,000 to 40,000 vehicles per day with two lanes in each direction. It is completely fenced to reduce access of wildlife to the motorway. The green bridge is 50 m wide, and vegetated by shrub borders, to minimize traffic noise and light on the bridge. The green bridge connects a large mixed forest in the south to a small band of woodland followed by an open agricultural landscape in the north.
Figure 2 Location of the green bridge and camera trap positions. Left: Map of Germany. A12 shown in black within Brandenburg state (red), other German states shown by thin black lines. Middle: Aerial image of the landscape surrounding the green bridge, with A12 bisecting the middle of the panel. Right: Aerial image of the green bridge, with the wildlife camera (FF1/FF2) locations shown and arrows indicating the viewing angles of the wildlife cameras. The dotted line indicates the main wildlife game path. Map source: © GeoBasis-DE/LGB, dl-de/by-2-0
Green bridge use by wolves and prey
Wolves, red deer, roe deer, and wild boar all used the green bridge. While the bridge was used by prey species soon after it was constructed, it took almost 4 years before the first wolves were detected on the bridge, and another year before wolves used the bridge regularly. Wolves crossed the bridge most often in winter, whereas prey species crossed more often in spring and summer. Daily activity patterns for all four species remained relatively consistent throughout the year – activity on the bridge was highest during dawn and dusk and lowest during the day. We saw no direct incidents of wolves hunting on the bridge itself. We also did not find that prey avoided the bridge while wolves were active, suggesting no evidence of prey-trapping at crossing structures.
Further exploration of how wolves used the green bridge suggests that packs are using the bridge within their hunting territories. Packs tended to travel north during day and dusk and then southward again over night until dawn. Lone, dispersing wolves, showed little seasonal differences in crossing, so we were unable to explicitly demonstrate that young individuals used the bridge while searching for their own territory, as would be expected from known dispersal patterns. However, this may have just been a product of low sample size. We did find that lone wolves tend to use the bridge to move northward, away from the established territories in the south. This may suggest dispersal but further monitoring of the green bridge would be necessary to confirm this. Since we were unable to identify individual wolves in this study, we are unable to say with certainty whether particular animals crossed multiple times.
Figure 3. Red deer captured on the green bridge over the A12 HIghway in Brandenburg State, Germany.
From ‘use’ to ‘effectiveness’
To understand how well crossing structures aid in dispersal and recolonization by wolves, additional research is necessary to complement this study. Genetic analysis (e.g., from faecal samples), would allow researchers to determine how many wolves cross northwards over the green bridge and establish their own territory. In future, for new construction, effectiveness could be evaluated through reduction in mortality or reduction of barrier effects, for example by comparing mortality on the roads before and after structure construction, or by using GPS-collaring to compare the success of crossing attempts when wolves use the bridge and when they don’t.
Future directions
We found that wolves and prey species both use green bridges. Future investigations into the effectiveness of green bridges to reconnect landscapes and reduce wildlife-vehicle collisions will further improve our ability to rewild intact habitat in Europe.
Author information:
Manisha Bhardwaj
Swedish University of Agricultural Sciences (SLU), Department of Ecology, Grimsö Wildlife Research Station, Riddarhyttan, Sweden
Current address: University of Freiburg, Faculty of Environment and Natural Resources, Chair of Wildlife Ecology and Management, Freiburg, Germany
Source:
Plaschke, M, Bhardwaj, M, König, HJ, Wenz, E, Dobiáš, K, Ford, AT. Green bridges in a re-colonizing landscape: Wolves (Canis lupus) in Brandenburg, Germany. Conservation Science and Practice. 2021; 3:e00364. https://doi.org/10.1111/csp2.364
Editor:
Rodney van der Ree
Cite this summary:
Bhardwaj, M. (2022). Wolves and Prey Species use Green Bridges to cross over Busy Highway in Germany. Edited by van der Ree, R. TransportEcology.info, Accessed at: https://transportecology.info/research/wolves-prey-greenbridgeuse [Date accessed].
