Green bridge

Green bridges are designed to restore ecological connectivity by allowing animals to cross linear infrastructure safely, thereby reducing mortality from vehicle collisions and enabling gene flow between populations. They are particularly important for large mammals such as deer, elk, and bears, which have large home ranges and are prone to roadkill. Studies have shown that wildlife crossings can reduce collisions by up to 83% and increase gene flow among otherwise isolated populations. The effectiveness of a green bridge depends on its location, width, and the surrounding habitat; animals are more likely to use structures that align with their natural movement corridors and that are planted with native vegetation providing cover.¹ Beyond mammals, green bridges also benefit reptiles, amphibians, and invertebrates, though these smaller species may require additional features like logs or rocks to facilitate passage.

The design of a green bridge must balance engineering constraints with ecological requirements. Width is a critical factor: wider structures (typically over 50 meters) are more effective for large mammals, while narrower crossings may suffice for smaller species. The substrate is usually a layer of soil and native vegetation, sometimes including fallen trees and rocks to mimic natural ground cover. Fencing along the road is essential to funnel animals toward the crossing, and noise barriers can reduce disturbance. Drainage systems prevent erosion and waterlogging. In the Netherlands, the 'ecoduct' design has evolved to include underpasses and overpasses integrated with existing landscape features, such as the Veluwe region's crossings that connect forest fragments.² Innovative designs also incorporate wildlife cameras for monitoring and adaptive management.

Green bridges have been implemented worldwide, with notable examples in Europe, North America, and Asia. The Netherlands leads in ecoduct density, with over 600 crossings, including the famous 'Wildlife Crossing' over the A50 highway. In Canada, Banff National Park has installed 44 crossings (overpasses and underpasses) that have dramatically reduced wildlife-vehicle collisions and maintained connectivity for grizzly bears and wolves. In the United States, the Liberty Canyon wildlife crossing in California, currently under construction, will be the largest in the world, spanning 10 lanes of Highway 101. Monitoring studies using camera traps and GPS tracking show that animals quickly adapt to these structures, with some species using them within weeks. However, effectiveness varies by species; for example, some small mammals and amphibians may still avoid crossings if not properly designed.

Beyond large mammals, green bridges serve as habitats for pollinators and plants, acting as 'stepping stones' for seed dispersal. In some regions, they are designed to support specific endangered species, such as the European badger in the UK, where underpasses are tailored to their burrowing habits. The first green bridge in the world is often attributed to France, built in the 1970s in the Parc National des Cévennes. A lesser-known fact is that some green bridges are built for railway lines, not just roads, such as the 'wildlife overpass' over the high-speed rail in Germany. Additionally, green bridges can have social benefits, providing green spaces for human recreation and improving mental health, though their primary purpose remains ecological. The cost of construction can be high, but the economic savings from reduced collisions and ecosystem services often justify the investment.

Ecoduct

A green bridge specifically designed for wildlife crossing over a road or railway.

Habitat fragmentation

The breaking apart of continuous habitat into smaller, isolated patches, often due to infrastructure.

Wildlife-vehicle collision

An accident where a vehicle strikes an animal, often causing injury or death to both.

Gene flow

The transfer of genetic material between populations, essential for maintaining genetic diversity.