Fragmentation and hedges

I saw this interesting article on the BBC website a couple of days ago. It’s about a study in Thailand that found that small mammals become extinct from an area of forest within 5 years, when the area was less than 10 hectares (25 acres), and separated from other areas of suitable habitat. Even in bigger fragments of up to 56 hectares, small mammals were still extinct within 25 years. Large and medium mammals also died out in these areas. The areas in question in Thailand really were unconnected pockets of forest, surrounded by water from the creation of a reservoir. But it’s also a timely reminder for other settings.

Habitat loss and fragmentation has been blamed for the decline of species such as the hedgehog and dormouse in the UK. It’s obvious how the loss of habitat could damage a population, but why does a species become extinct if it has an area of habitat to live in, but that is small and separated from other suitable areas?

Habitat fragmentation is bad news for mammals. Lack of genetic diversity and opportunities for repopulation from other areas make populations vulnerable to disease, predation and competition from other species.

A major factor in the fragmentation of woodland habitats in the UK has been not just the destruction of woods for building or farming, but also the disappearance of proper hedges. In the post-war drive to increase agricultural production many hundreds of miles of hedges were pulled up to allow for bigger fields with less bulky boundaries. This was a tragedy for many wildlife species, who use hedges as safe highways between areas of suitable habitat. Hedges are used by hedgehogs (as the name suggests), dormice, otters, bank voles and many other species. The loss of hedges for these animals was like one day waking up and finding there’s no longer a road from your house to the shops or your work.

Where I grew up, in south Devon, seems so far to have escaped from the worst of the uprooting of hedges. Perhaps that’s partly why Devon has been a stronghold for otters and dormice, even when the former were completely wiped out of most of the rest of the country. I love the patchwork of small fields and species-rich hedgerows you get in the South Hams.

It’s not enough to protect small pockets of land for our wildlife. Small populations in small, isolated areas are just not sustainable. We need to be thinking at a landscape level – how can we join up areas for our wildlife. This will involve replanting hedges, building wildlife bridges over roads, protecting some areas from development, making sure river banks can be climbed by aquatic mammals and making sure there are gaps in our fences that hedgehogs can get through. It was also involve rethinking how we manage other areas of land. The Wildlife Trusts’ Living Landscapes programme is trying to address this at a national level.

As one of the co-authors of the study in Thailand said, “The bottom line is that we must conserve large, intact habitats for nature. That is the only way we can ensure biodiversity will survive.”