As you should know, ecosystems are dynamic systems (they change), and the process by which they change over time is called succession.
There are two types of succession: Primary succession and Secondary succession. Primary succession starts on new areas of land with NO organic material or soil (eg. bare rock/sand), for example, new land exposed when sea level drops, or a new surface formed by a volcanic eruption. Secondary succession is when the land has been destroyed or damaged, so succession must start again; the area has been cleared of all plants and must re-develop (eg. plants growing after a forest fire).
The succession occurs in stages called seral stages. The stages for primary succession are:
1) PIONEER SPECIES COLONISE THE LAND. The abiotic conditions are harsh, there’s no soil to retain water, so these species are specialised to tolerate and survive in these conditions.
2) PIONEER SPECIES CHANGE ABIOTIC CONDITIONS. As they die, the humus (dead organic material) is decomposed by microorganisms and create a thin, basic soil.
3) NEW ORGANISMS CAN MOVE IN AND GROW. The soil means more water csn be retained and more nutrients are available for plants. As each organism arrives, dies and is decomposed, more soil is built up and larger plants can live there. Once enough and large enough plants live there, habitats emerge and animals can move in and survive.
As more species move in and live there, the ecosystem becomes more complex and diversity increases.
The final seral stage is called the complex community, this is when the ecosystem is at its largest and most complex and won’t change much more; it’s at a steady state.
Secondary succession occurs the same way, except there is already soil present so succession starts at a later seral stage.