The problem with patches

The fragmentation of habitat has several impacts on wildlife, including:

• loss of available habitat area;
• individual habitat patches become too small to support some species;
• small areas of habitat and the species they contain are more vulnerable to destruction in catastrophic events;
• 'edge effects' start to have a greater impact on the habitat quality within each patch.

Loss of habitat area

Changing land uses can reduce the total area of a habitat, which puts pressure on species that need those habitats. For example, the total area of lowland raised bogs in Scotland declined by 44% between 1947 and 1988 - that's an average annual loss of 2.5km2 of raised bog habitat every year. The plants and animals dependent on that habitat have to make do with a smaller total area of habitat each year.

Reduced patch size

As habitats become increasingly fragmented, the remaining habitat patches can become too small to support some species which need a large area to survive. So although there may be some suitable habitat left, it may not be of sufficient size to support all the species that are characteristic of that habitat type. For example, red squirrels are thought to need at least 6 hectares of suitable habitat in order to survive and reproduce successfully.

Catastrophic effects

With increasing fragmentation, the distance between habitat patches increases, making it harder for individuals to move from one patch to another. Potential movement routes can also be fragmented, for example when a road cuts through a strip of woodland. Fragmentation can leave the remaining population in each habitat patch very isolated, resulting in in-breeding and a loss of genetic diversity. And if there is a catastrophic event which wipes out a particular population, individuals from other patches will not be able to migrate in to replace the lost population. Populations in small, isolated habitat patches are therefore more vulnerable and can be more easily lost.

Edge effects

The edge of a habitat patch is always adjacent to a different land use, and as a result it's often affected by 'edge effects'. These can include things like increased light penetration and higher wind speeds as well as greater impacts from what's happening in the adjacent land area. For example, the edge area may be affected by drift from chemicals being sprayed in a neighbouring agricultural area or by unsuitable species spreading in from the adjacent land use.

As habitat fragmentation takes place, the remaining habitat patches get smaller and the relative amount of habitat edge in each patch increases and becomes more significant. This means that a greater proportion of the habitat area is influenced by 'edge effects'. Some species respond well to those changing conditions and they can be considered as 'edge species', whereas other species respond badly to an increase in edge area. These 'interior species' need to be further away from edge effects and often need a large habitat patch in order to survive. For example, wild clematis (Clematis vitalba) is usually found on the edge of woodland or in narrow hedges, so it could be described as an edge species. In contrast bluebells (Hyacinthoides non-scripta) are more frequent in the interior of a woodland and are adversely affected by edge effects.

In the UK, changing land-uses and habitat fragmentation has taken place over a relatively long period of time, so we don't have many interior species left. Many UK species are tolerant of edge effects, so although we may need to try and reduce the fragmentation of habitat patches, we don't necessarily need to recreate large individual patches of habitat to allow most species to thrive. Edge effects are also important in creating a mix of habitat types within an area - very large patches of a single habitat type aren't always good for biodiversity. A mosaic of smaller patches, with good interconnection between the patches, may be preferable.