Green bridges – structures built over road and railway infrastructure to enable wildlife to cross safely, otherwise known as eco-bridges – could address the problem of habitat fragmentation. Plants, such as shrubs and grasses, are situated on these bridges, acting as inviting extensions of surrounding landscapes. These structures enrich biodiversity and are aesthetically pleasing features of many modern landscapes, once the plant life within is fully established.
Highways and railway lines often split up areas of natural beauty, at best, restricting territories, or at worst, producing pockets of land that are ecologically impoverished. Building green bridges for large mammals over roads and railway lines reduces accidents, which often occur as a result of vehicles careering into them. Green bridges enable a vast range of wildlife, from large mammals to small insects, to find water, food, shelter and partners (preventing genetic isolation) without braving oncoming traffic or trains.
These bridges aren’t necessarily custom made for wildlife, as existing grey bridges can be retrofitted with green features. Sophisticated green bridges could create a truly heterogeneous environment by incorporating elements that satisfy a wide range of regional wildlife. For instance, wet zones are sometimes created for amphibians and dragonflies. Wildlife fences, for large mammals, or habitat corridors are useful to direct wildlife towards the entrances to the bridges. Eco-bridges can be designed so that humans can also use them for walking, cycling and horse riding, but, if constructed for both wildlife and human use, care must be taken to ensure human use does not conflict with the wildlife aims of the structure in question.
So far, only a few green bridges have been constructed in the UK, such as the one constructed over the A11, which avoids the fragmentation of Mile End Park in London by roads and railway lines. In contrast, many green bridges have been built across the rest of Europe and North America. The Netherlands has an impressive history of green bridge construction, starting in 1988 when the first was built. Now there are at least 60 green bridges across the country, giving safe passage to a whole array of wildlife, such as the European badger and the great crested newt. In Canada, green bridges enable numerous animals, including bears, moose, deer, wolves and elk, to cross the Trans-Canada Highway, in Alberta, safely and reportedly reduce road traffic collisions by 80 per cent.
Close collaboration between architects, engineers and ecologists is needed to create rewarding green bridges. For certain localities, constructing underpasses or a combination of underpasses and green bridges is appropriate. Underpasses are generally cheaper to build and better for animals that prefer cover, so listening to those who have good ecological knowledge before commencing with building crossings is paramount. Decisions about what sort of flora to grow on green bridges are dependent on economic factors and the species of animals that are likely to use them. A double row of hedges, for example, is ideal for bats because they serve as good guiding structures. Established green bridges can be of interest to a wide spectrum of people, such as those fascinated by architecture or horticulture, and enrich, or help maintain, the cultural splendour of the areas where they are located.
Installing more green bridges may help give environmental organisations the confidence to reintroduce more animals, as these bridges will enable wildlife to have the space many such species require to survive and thrive. Adding new animals to the ecological mix can do far more than just add a little extra interest to the landscape, because many of the species of animal that conservations are keen to reintroduce control invasive species. For example, it would be worthwhile to try to reintroduce pine martins, which have territories of up to 25 squared kilometres, to more locations in the UK if more habitats were linked together. Pine martins are useful because they help to control grey squirrel numbers. Burgeoning populations of grey squirrels can be troublesome because these animals eat bird’s eggs, destroy the seeds of various trees and strip bark off trees, thereby jeopardising their health.
Green bridges and the habitat corridors that tend to lead up to them could perhaps become symbolic of the modern recognition that nature should be given considerable space to prosper. Even though the UK isn’t a megadiverse country we can still show a good example to other countries by increasing the biodiversity we have by connecting habitats together through green bridges, underpasses and wildlife corridors, which are all excellent ways of providing extensive areas of uninterrupted habitat.
We should be aware that green bridges can be effective solutions to the problem of road deaths, both human and animal, and consult professionals on where it would be sensible to situate them. Eco-bridges have the potential to radically enhance our post-industrial landscapes and could, in time, come to illustrate a step-change in our collective relationship with the natural world.