Ecological succession describes the gradual process by which an ecosystem changes over time. It involves a series of progressive shifts in the species that make up a community. Communities of species replace one another, adapting to changes in their environment. This dynamic and continuous process shapes the structure and function of ecosystems.
Primary Succession
Primary succession begins in environments where no life or soil previously existed. These barren landscapes can be newly formed, such as volcanic islands emerging from the ocean, or areas exposed after a glacier retreats. Sand dunes also represent sites for primary succession. The first organisms to colonize these harsh environments are called pioneer species, often including hardy organisms like lichens, mosses, and certain microorganisms. Pioneer species play a role in breaking down bare rock through physical and chemical processes, gradually creating the initial layers of soil. As these organisms live, die, and decompose, they contribute organic matter, slowly building a substrate that can support more complex plant life.
Secondary Succession
Secondary succession occurs in areas where a disturbance has removed much of the existing vegetation but the soil remains intact. Common events that trigger secondary succession include natural disasters like wildfires, floods, and hurricanes, as well as human activities such as logging, farming, or abandoned agricultural fields. The presence of pre-existing soil is a key feature. This soil contains nutrients, organic matter, and a seed bank of dormant seeds and roots from the previous community. Pioneer species in secondary succession are fast-growing grasses and herbaceous plants that rapidly repopulate the disturbed area.
Comparing the Speeds of Succession
Secondary succession is much faster than primary succession. The main reason for this difference in speed is the presence of intact soil in secondary succession. In primary succession, the initial colonization of barren rock by pioneer species like lichens and mosses is a slow process, as it involves the creation of new soil from scratch. This soil formation can take hundreds or thousands of years.
In contrast, secondary succession benefits from an established soil base. This existing soil provides nutrients and organic matter, allowing for immediate plant growth. The soil also contains a seed bank of viable seeds and dormant roots from the previous plant community, which can quickly germinate after a disturbance. The presence of these surviving propagules and the proximity to nearby ecosystems that can serve as sources of new colonizers accelerate the recovery process. Primary succession requires organisms to build an ecosystem from a blank slate, while secondary succession involves the recovery and re-establishment of a community on a foundation that holds remnants of past life.