As the human population grows, the harder it is for wildlife to thrive. Sprawling cities, towns, roads, factories and farms use up open forest, prairies, meadows and wetlands, driving plants and animals into ever-decreasing areas. Even in places where there are pockets of green space, sometimes known as habitat islands, these spaces are cut off from each other, making it difficult for animals – insects, birds, mammals, amphibians – to move around in search of food, shelter and breeding grounds.
Isolating animal species in a small area makes them more vulnerable to predation, disease and inbreeding. And if one species disappears, it can affect the entire local ecosystem. In addition, animals in search of food or mates can wander on to roads, endangering both themselves and drivers, or into human settlements, bringing them into conflict with people. In Africa and Asia, elephants and tigers that have been backed into forest parcels too small to sustain them sometimes enter villages and farms, destroying houses, crops and livestock.
So how can we offer wildlife safe passage to help preserve and promote biodiversity?
The road to recovery
While roads are often part of the fragmentation problem, they can also offer a solution. Roadside verges planted with native wildflowers help support important pollinators such as bees and butterflies, while ponds become homes for amphibians and a watering hole for birds and other small animals. So there’s now a whole new movement in roadside landscaping – planted and maintained as purpose-built living spaces and green corridors for plants and animals. In the USA, for example, conservationists are encouraging roadside planting of milkweed, on which monarch butterflies depend, to help the recovery of this magnificent but endangered species. And near Oxford, UK, a green corridor linking two woodlands has been created along the side of a motorway, allowing species to migrate back and forth.
Some animals need something larger and more elaborate than a wildflower-planted verge to cross from one habitat to another, meaning that man-made structures are needed.
In the Netherlands an 800-metre “ecoduct” serves as a green superhighway across a motorway, railway and golf course. In Canada, large mammals in Banff National Park – bears, coyotes, elk, mountain lions and wolves – are protected by six wildlife bridges and 38 underpasses spanning the highways.
Not all wildlife crossings are for mammals. Fish ladders, for example, help migrating fish to get past dams blocking their way to upriver spawning grounds. The structures differ for different species, but they are typically a series of pools arranged in ascending steps so that fish can leap or swim from one to another. In Florida, USA, underpasses allow crocodiles to avoid road traffic. And each year on Christmas Island, north of Australia, rangers build bridges to allow more than 100 million terrestrial red crabs to migrate to the sea to lay their eggs.