What evidence is there that fragmentation reduces biodiversity?

Fragmentation is inevitably accompanied by a loss in total habitat area, and separating the effects of this loss of total area from the effects of increased fragmentation can be difficult.  Many scientific studies have shown an overall loss in biodiversity in different areas over time, but it is often difficult to attribute this loss directly to fragmentation.  However, a recent review of the scientific literature (Bailey 2007 - see link on the right) concluded that for woodlands at least, species occurrence was more limited in fragmented areas.

One way to tackle the impacts of fragmented habitats is to recreate connections between habitat patches in a network.  See the history of networks for some of the evidence for this approach and where the idea came from.

What evidence is there that creating habitat networks will work?

Many habitat networks are in their infancy and so there are relatively few studies on their success or otherwise.  There is also potential for a significant time lag before being able to measure any improvements in the field.

There's mixed evidence for the impact of structural connections between habitat patches - for example the creation of 'corridors'.  Although the concept of a structural corridor is easy to understand and intuitively 'feels' as if it should bring benefits, it's unclear whether purely structural connections are successful in creating a habitat network that works in practice.  For example, Wiens commented in 2002 that:

"Evidence for the efficacy of corridors is nowhere near as compelling as the enthusiasm with which corridors have been embraced."

Others are more positive and believe that structural corridors should be preserved where possible, in the absence of clear evidence to the contrary (e.g. Beier and Noss, 1998).   However, studies on certain species on hedgerows have shown clear benefits of structural connectivity.  Dover and Fry (2001) demonstrated modifications to hedgerow structure modified the behaviour of studied butterflies.

There are some studies which have reported species re-colonising areas after the restoration of connectivity between habitat patches - for example, Stouffer and Bierregaard (1995) - see link on the right - found some species of birds re-colonising a previously isolated fragment of Brazilian rainforest from which they had been lost.  

This research was one of the many hundreds included in a recent systematic review of scientific studies on connectivity for the Department for the Environment and Rural Affairs (see link to the right).  It found that:

"landscape features between habitat patches, such as corridors and matrix structure can have a role in enhancing connectivity"

though there was acknowledgement of a lack of research in this area for many species.  However, the balance of evidence was such to lead Defra to conclude:

"Given the magnitude of the threat posed by climate change, a precautionary approach would indicate that measures to enhance functional connectivity should be a priority."