Article by Ed Snell – Project Support & Development at NatureSpy
Habitat fragmentation can be disastrous for wildlife. Manmade structures such as roads can divide habitats, creating barriers for wildlife that needs to move through the landscape. In many instances wildlife will still try to cross these obstacles, but this sadly often leads to wildlife-vehicle collisions.
For the species that keep clear of roads or simply cannot cross them due to fencing that’s designed to prevent collisions, this can result in wildlife populations becoming isolated. This means it’s much harder for animals to disperse, find mates, in some cases migrate, and ultimately this impacts the genetic diversity of the species and the health of the population. In the worst cases, this leads to local extinctions of a species.
A green bridge in Croatia
Many species require large landscapes to roam across. This is especially important for large carnivores such as wolves, lynx and bears that need to range across vast areas to hunt, forage, disperse and find mates.
Wildlife crossings are designed to reconnect habitats for wildlife. These structures include green bridges, viaducts and tunnels that are built in carefully selected locations where they should be most effective. Structures such as green bridges can be impressive feats of design and engineering, often landscaped in such a way that makes an almost-seamless transition for wildlife from one area to another. Fencing is used in combination with many of these wildlife crossings to funnel wildlife to a safe area to cross a busy road.
A green bridge in Croatia blending into the surrounding habitat
Wildlife crossings are often designed to help a diverse range of species to move safely across the landscape. In some areas they’ve been found to reduce wildlife collisions with vehicles by around 85-95%. Sometimes there is a more specific need for a green bridge when a particular species is in trouble. For example, underpasses in Kenya are helping elephant conservation, green bridges in Sweden are enabling reindeer to roam further to access food, and crab bridges on Christmas Island are enabling red crabs to safely migrate across roads. When a species is recovering, green bridges can play a role in supporting their recolonisation of the landscape, as has been found with wolves in Germany.
Depending on the landscape and target species, there are cases where bridges or tunnels may work better. For example, in Banff National Park in Canada, it was found that black bears and cougars prefer narrow bridges, elk prefer wide, open bridges and smaller mammals have been found to prefer tunnels. The same study also found that there’s different reasons why different species use them. Small mammals were found to use wildlife crossings to avoid predators and large mammals use them to avoid humans. Does this mean that wildlife crossings become a trap used by predators? Another Canadian study found that there’s no evidence of predators using wildlife crossings as prey traps.
The view from a green bridge in Croatia
To learn about the success of wildlife crossings and the wider impact they have on a species, green bridges are often monitored with camera traps. Camera traps can reveal all kinds of information, such as how much a bridge is used, which species is using it and in some instances it’s possible to tell individual animals apart from unique fur markings.
A trail camera monitoring a green bridge in Croatia
At NatureSpy, we’ve been supporting the work of Bioterra since 2016, who have run extensive monitoring of green bridges using trail cameras in Croatia. This has led to various research projects to support the conservation of species including as wolves, lynx and wildcats. The video at the top shows just a snapshot of the diversity of wildlife recorded on a Croatian green bridge in a year, and it includes all large carnivores present in the area. Here are some of the studies that the Bioterra team have worked on…
Over a 5-year period, the team monitored 6 green bridges to learn about the abundance trends of 5 wolf packs, whose home ranges intersected a motorway. The study found a reduction in wolf abundance in the area between 2011-2016, particularly in the northern part of the study area, which highlighted the negative impact of the legal cull on wolves in the area at that time.
The monitoring of 10 green bridges and 8 viaducts with camera traps at various times over a 10-year period revealed the patterns of bridge use by wolves. Early mornings were found to be a peak time in wolves using the bridges, with less frequent crossings at other times of day. Least crossings were recorded in July and most crossings were recorded in October. Seasonally, less crossings were made in winter, and most were made in autumn. These timings coincide with human activity, with wolves appearing to use crossings less when there are more people in the area.
A study of wildcats in Croatia revealed the success of green bridges when compared to a motorway that had no green bridges. From 2005 to 2008, 23 wildcats were killed on the A3 highway, which had no wildlife crossings, compared to 2 wildcats killed on the A1 highway with wildlife crossings. Five green bridges on the A1 highway were monitored with camera traps, recording wildcats using 3 of the 5 monitored bridges. This study shows a clear benefit to wildcat conservation by improving the permeability of the landscape with green bridges.
Prior to 2015, there were no records of golden jackals crossing green bridges in the team’s study area in Croatia. However, from 2016-2018, camera traps revealed jackals at all 5 green bridges being monitored at that time. Analysis indicated that the population of jackals was dispersing, expanding their range and increasing in population density.
These studies provide insights into the tremendous knowledge that can be gained from monitoring green bridges, highlighting not only the conservation impact of the bridges themselves, but what the monitoring of wildlife crossings can reveal about a species at a larger scale.
Learn more about Bioterra’s conservation work over on our Project Pages.
