Ecological succession is the gradual and predictable change in the composition of species in an area over time. It can occur following the creation of a new, lifeless area or after a disturbance to an existing ecosystem.
Primary succession is a slow process, often taking hundreds or thousands of years, that occurs in lifeless areas where no soil initially exists, such as after a volcanic eruption or a glacial retreat. The initial colonizers, pioneer species, include lichens, algae, mosses and fungi, which can thrive in harsh conditions where other species cannot survive, including on bare rock surfaces and with wide temperature fluctuations, periods of extreme dryness and high levels of ultraviolet radiation.
These hardy organisms contribute to soil formation, and thus to the gradual development of more complex plant communities, by breaking down bare rock through physical and chemical weathering and by leaving behind organic matter and nutrients when they die. Of particular importance is the release of acids by lichens that helps disintegrate rock.
Secondary succession occurs in areas where a disturbance has removed an existing plant community but has left the soil largely intact, such as after a forest fire or the abandonment of agricultural land. It is generally much faster than primary succession because the soil and some organisms remain, allowing for quicker recolonization by plants and animals. The sequence typically begins with fast-growing species such as grasses, dandelions and fireweed, followed by shrubs and eventually trees, leading back to a climax community.
Ecological succession is crucial for maintaining biodiversity because it creates or recreates diverse habitats that support a variety of species. Understanding it is of great importance for conservation efforts and for habitat restoration projects, including for managing ecosystems that have been affected by human activity, natural disasters and climate change.