Wildlife Corridors

As roads, farms, and cities spread across the globe, they have carved the natural world into ever smaller fragments. A forest bisected by a motorway becomes two forests, each too small to sustain the wide-ranging animals that once roamed the whole. Populations marooned on these islands of habitat face a grim arithmetic: with fewer mates to choose from, they breed among close relatives, accumulating harmful mutations, while a single fire, disease, or bad winter can wipe out an isolated group that has no neighbours to replenish it. Conservationists have come to see this fragmentation as one of the gravest threats to biodiversity, arguably as dangerous as outright habitat destruction. A reserve that looks generous on a map may, if walled off by farmland and traffic, function biologically as little more than a spacious cage. The remedy they increasingly champion is deceptively simple: rather than merely protecting isolated reserves, we should physically reconnect them, stitching the torn landscape back together so that wildlife can once again （ 1 ）.

A wildlife corridor is any strip of habitat that links two larger areas, allowing animals to travel between them in relative safety. Corridors take many forms. Some are broad bands of restored forest or grassland stretching for hundreds of kilometres; others are ingeniously local, such as the vegetated overpasses that now span busy highways, letting deer, bears, and even amphibians cross without being crushed by traffic. In one celebrated Canadian park, a series of such crossings has dramatically cut collisions between cars and large mammals while permitting grizzly bears to mingle and breed across a once-impassable road. By enabling individuals to move, corridors （ 2 ）: they enlarge the effective size of a population, refresh its gene pool, and allow species to shift their ranges as the climate warms and their traditional habitats become inhospitable. In an era of rapid warming, this last function, letting creatures migrate toward cooler ground, may prove the most valuable of all.

Corridors are not a panacea, however, and their design demands care. A poorly conceived passage can do more harm than good, funnelling animals into the path of poachers, or serving as a highway along which invasive species, diseases, and fire spread from one reserve to the next. What benefits a bear may be useless to a butterfly, since different creatures perceive and traverse the landscape in utterly different ways, so planners must decide which species a given corridor is meant to serve. There are also formidable practical obstacles: corridors often must cross private farmland or densely settled country, requiring negotiation, compensation, and political will that are frequently in short supply. Persuading a farmer to surrender a strip of productive land so that bears may pass is rarely an easy sell, and the benefits, unlike the costs, are diffuse and slow to appear. For all these complications, the underlying logic is compelling, and as conservation increasingly thinks at the scale of whole landscapes rather than isolated parks, planners argue that connectivity （ 3 ） if the living world is to weather the pressures of the coming century.