What is a wildlife corridor, and do they work?
A strip of habitat connecting otherwise isolated areas, allowing animals to move between them — and the evidence that connectivity matters is strong, while the evidence about specific corridors is more mixed.
The problem: habitat fragmentation. Development, roads and agriculture divide habitat into patches. A population confined to a small patch faces:
Loss of genetic diversity, since individuals cannot breed outside the patch. Small isolated populations accumulate inbreeding and lose the variation needed to adapt.
No recolonisation. If a patch's population dies out through disease, fire or a bad year, nothing can move in to replace it. Local extinctions become permanent, and patches empty one by one.
Inability to track changing conditions, which matters increasingly as climate shifts species' suitable ranges — a species that cannot move cannot follow its habitat.
Edge effects, since small fragments are mostly edge, with different conditions from interior habitat.
What corridors provide: movement for dispersal, breeding and seasonal migration; gene flow between populations; and a route for range shifts.
The forms they take: hedgerows and field margins; river corridors and riparian strips; road crossings — green bridges and underpasses, which have measurably reduced vehicle collisions as well as improving connectivity; verges and railway embankments; and urban green networks.
What the evidence says. Studies generally find corridors increase movement between patches, and the underlying principle — that connectivity supports population persistence — is well supported. The more contested questions are about design: how wide, of what habitat, and for which species. A corridor suiting one species may be useless for another with different dispersal behaviour, and width requirements vary enormously.
The criticisms worth knowing:
They can spread disease and invasive species as readily as native ones.
Predators use them too, and narrow corridors can concentrate prey.
Cost-effectiveness is debated — whether money is better spent enlarging existing habitat than on connecting fragments.
They are used to justify development, as mitigation for fragmentation the development itself causes.
The wider framing in UK policy is the Lawton principles — "bigger, better, more, and joined" — in which connectivity is one of four elements rather than a substitute for habitat area.