Disturbance and succession

Human disturbance is constantly creating habitats. Over time, the plants and animals that live in the habitat will change and develop. This process is known as succession.

What is succession?

Human disturbance is constantly creating habitats. Plants and animals will recolonise these habitats and together with the non-biological part of the environment form a new ecosystem. These colonising species have an impact on the environment themselves and so the ecosystem will continue to change and develop.

The ecological process by which an ecosystem changes and develops over time is known as succession. The box above defines some of the terms used in describing an ecosystem and helps you to revise your knowledge of how nutrients are recycled.

Primary and secondary succession

When succession starts from a bare habitat e.g. bare rock it is called a primary succession.

When disturbance has not resulted in the loss of soil or even all species a secondary succession will take place. On a disturbed urban site e.g. waste land where buildings have been removed, it may well be possible for both primary and secondary succession to be observed.

The stages of succession

In a succession the following stages can be recognised:

1. A pioneer community forms

The first colonisers are likely to be certain species of algae, lichens and mosses. These are producers, making their own food through the process of photosynthesis. They are also able to survive without soil, taking up rainwater and mineral salts through the whole of their body surface.

Pioneer species have to be hardy individuals likely to be able to put up with extreme conditions e.g. large humidity ranges.

Twisted moss (Syntrichia ruralis) is a moss which often forms part of the pioneer community on walls, roof tiles and stony and sandy ground. It shows the following characteristics.

• Thread-like structures (rhizoids) anchor the shoots onto the surface of the habitat.
• Water and mineral salts can be absorbed over the whole body surface as there is no waxy cuticle.
• The moss has a cushion or turf growth form which helps to hold water between the shoots.
• The shoots twist as they dry out slowing down water loss.
• The moss is also able to tolerate severe drying but recovers immediately in the first shower of rain.

See the recovery of this moss in action (video).

2. Changes occur in the habitat

The pioneer species begin to have an impact on the site.

• Food is present for consumers and decomposers which now arrive.
• Soil begins to form as the dead remains of plants and animals accumulate and are broken down by the decomposers.
• Water is held in the habitat both in the living organisms themselves and in the soil.

3. Vascular plants (ferns, their allies and seed plants) colonise These plants have a waxy covering (the cuticle) which protects them from water loss. They have to get most of their water and mineral salts from the soil. These are transported through the plant body by the vascular system. Once soil has formed these plants begin to establish themselves causing further changes to the habitat. Conditions become unsuitable for the pioneers, which disappear as a result.

4. Shrubs and trees arrive The larger woody plants such as shrubs and trees need more soil than the smaller non-woody plants and are also much slower growing, so they usually appear in the later stages of the succession.

5. A climax community forms

If left for long enough a more or less stable community can develop. This is known as the climax community. A good example is undisturbed tropical rain forest. Unfortunately all over the world humans are gradually destroying these communities and, because they take a very long time to develop (often hundreds of years), such communities are becoming increasingly scarce

Two other features are characteristic of succession - the quantity (biomass) and variety (biodiversity) of living organisms increases as the succession proceeds

Few species can tolerate the environmental conditions at the pioneer stage but as succession continues ecological conditions become less extreme and more habitats develop so a greater variety and number of organisms are able to colonise. When shrubs and trees begin to arrive the increase in biomass become especially noticeable.

The highest biodiversity is not always at the climax community stage. For example in the dense yew woodland, which forms a climax community on parts of the North Downs, toxic chemicals in the leaves and low light intensity means that the number of species in this sort of woodland is relatively low.