Primary Succession

Succession is the gradual change in the community of an area over time. Primary succession starts on bare ground where there is no soil and no previous life, such as bare rock after a glacier retreats, cooled volcanic lava or newly formed sand dunes.The stages are:
• Pioneer species colonise first. On bare rock these are usually lichens and algae. They are adapted to harsh conditions: they tolerate drying out and extreme temperatures, need few nutrients and spread by spores carried on the wind. Lichens slowly break down the rock surface (weathering).
• When pioneers die, they are decomposed and form humus. Mixed with weathered rock particles, this makes a thin, simple soil that holds water and mineral ions.
• Mosses, then grasses and small flowering plants, can now grow. They add more organic matter, so the soil becomes deeper and more fertile.
• Larger plants such as shrubs, then trees, grow. Each new group shades out and outcompetes the species before it.Each stage is called a seral stage. The key idea is that each community changes the abiotic conditions, especially the soil, making them more suitable for other species and less suitable for itself.As succession proceeds, soil depth, humus and water retention increase; the number of species and the biodiversity increase; food webs become more complex; and the biomass increases. The community becomes more stable.Succession ends in a stable climax community, covered in the next lesson.