Colonisation

When a building is demolished in an urban area, the land is quickly colonised by plants and animals. Where do these plants and animals come from?

In any habitat, including urban habitats, a successful plant or animal needs to to be able to go through two stages

1. Dispersal - the plant or animal needs to be able to move around (or disperse) to find new sites
   
2. Establishment - the plant or animal needs to be able to compete with other living organisms to become established.

Stage 1: Dispersal

Dispersal of plants

Although plants are not obviously mobile, they are still able to spread to new sites. They can do this EITHER by spreading their seeds and spores OR by vegetative reproduction.

Seeds and spores are dispersed in several different ways. They can be self dispersed or use the help of animals, wind and occasionally water.

(i) Wind-dispersed seeds can travel long distances with the aid of parachutes or wings and are often the first to colonise disturbed ground.

The Rosebay willowherb (Chamerion angustifolium) is a good example. Rosebay willowherb produces large numbers of small seeds, each with a parachute formed of tufts of hairs.

When the seeds are ripe, they are released by the plant so that they are carried by the wind. In this way, seeds can float long distances from the parent plant.

Rosebay willowherb was introduced to Britain from North America in the 19th century. After World War II, bomb sites provided ideal conditions for its growth and it was able to spread rapidly by means of its tiny airborne seeds. Today it is very common and has been recently been voted London's favourite flower (organised by the charity Plantlife).

(ii) Animal-dispersed seeds can often hitch a ride on an animal using sticky substances or hooks to hang on with. Animals collect surprisingly large numbers of seeds and fruits in a relatively short time (Bebbington 1993).

(iii) Dispersal of plants by vegetative reproduction may be important in helping plants to spread particularly within a site. For example, the stinging nettle can spread rapidly by means of long underground stems (rhizomes).

Dispersal of animals

Most animals are mobile but their success in colonising new sites is heavily dependent on there being a network of corridors linking suitable habitats.

These wildlife corridors can provide:

• protection (e.g. from traffic, humans and predators)
• shelter from weather conditions
• sources of food
• a link to breeding areas.

In an urban area hedgerows, gardens, canals roadside verges and railway embankments can all form valuable wildlife corridors.

Stage 2: Getting established on site

Once plants and animals have arrived, they need to become established on site. Both plants and animals must avoid being eaten and compete for resources. Competition in plants will be mainly for space, light, water and minerals whilst animals will be competing with other species for food, shelter and nest sites.

To become part of an ecosystem the species must find its position both in terms of space and its relationship to other organisms. See the section on opportunism for more information about niche.

Case study: what makes bramble a successful coloniser?

Bramble (Rubus fruticosus agg.) is the group name for a large number of closely related species. They are found in a wide range of habitats throughout Britain.

Stage 1: Dispersal

Brambles produce a large amount of fruit (blackberries). This is eaten and dispersed by animals. Once on site, brambles can spread rapidly by vegetative reproduction. When the arching branches touch the ground they produce new shoots which form new bramble bushes.

Stage 2: Getting established on site

Bramble competes successfully. It can grow in full sun, where it produces characteristic small reddish sun leaves, or in shade. Prickles on the leaves deter large herbivores from grazing.

Prickles on the stems and leaves are also used to scramble over other plants. When the arching branches touch the ground they produce new shoots. These large arching branches shade out other species.