Getting away from the relatives

The process of dispersal can allow species to become established in a new area. Individual animals or plant seeds move away from their birth area, or move from a zone that has a high population density to an area with a lower density. The overall ability of a species to disperse is likely to depend on a range of factors such as its mobility and reproductive ability. Dispersal ability will affect how sensitive a species it is to habitat fragmentation. Species which have low dispersal abilities are likely to be more affected by fragmentation, especially if they also require a large area of habitat to survive.

For example, dog's mercury (Mercurialis perennis) is a woodland plant species which often reproduces vegetatively (i.e. by producing new plants from underground rhizomes). That means it can only colonise new areas very slowly and it can't disperse across areas of hostile or unsuitable land. So if an area of woodland becomes fragmented, dog's mercury wouldn't necessarily be able to disperse between the different patches of woodland. If a population of dog's mercury is wiped out in one woodland patch, it's unlikely that a new population will be able to recolonise the patch.

In contrast, some species are quick dispersers, such as Rosebay willowherb (Epilobium angustifolium), which reproduces both vegetatively and by wind blown seeds. This means it can spread easily between patches of suitable habitat, even when those patches have become quite fragmented.

Population dynamics and metapopulations

Most people are familiar with the idea of populations of species - a group of individuals of one species living in a particular geographical area, for example, a population of foxes living in an urban area. However, some species also exist within what are known as meta-populations - a group of linked populations where individuals can move between each sub-population. This means that if one population is wiped out, it can potentially be replaced by individuals from other populations within the wider meta-population.

For example, water voles (Arvicola terrestris) are thought to function as a metapopulation with individuals moving between sub-populations in different parts of a river catchment. However, metapopulations rely on individuals being able to move from one sub-population to another.

If the area between the two sub-populations becomes too hostile or difficult to cross, exchange between populations can't take place, and the different populations will no longer be connected. That makes them more vulnerable to being wiped out by catastrophic events - for example, many populations of water vole in Scotland have been wiped out by the highly effective predation actions of American mink. Where habitat fragmentation has also occurred, the water voles are unable to travel from one sub-population to another, so they can't recolonise areas that have been decimated by the mink.