Ecological succession describes how the species structure of an ecological community changes over time. It occurs in stages, where communities gradually replace one another. Various factors initiate these changes, leading to new communities developing in an area.
Natural Environmental Disruptions
Naturally occurring, sudden events can disrupt existing ecosystems, clearing the way for new communities to develop.
Wildfires, for instance, destroy vegetation and displace animals, yet they return nutrients to the ground as ash. This creates conditions for pioneer species like annual plants and grasses to quickly colonize the area, followed by shrubs and trees over time. Fire can also break seed dormancy and stimulate germination for certain plant species, influencing which ones dominate an environment.
Volcanic eruptions are another force, initiating primary succession on new land formed from cooled lava or exposed rock. This process begins on surfaces devoid of soil, where pioneer species such as lichens and mosses break down rock to create the initial soil layers. On Mount St. Helens, erosion revealed older surfaces and water channels that supported seedling establishment.
Floods can also trigger successional changes by disturbing plant communities and altering soil composition. Floodwaters can remove vegetation and organic matter, initiating primary succession on new layers of silt or sand, or they can simply deposit new material over existing soil, leading to secondary succession. Landslides similarly cause disturbance, with their impact varying based on soil conditions and ground movement. These disturbances create environmental changes, such as altered sunlight and soil chemistry, allowing different species to thrive.
Human Activities
Human actions frequently initiate ecological succession, both directly and indirectly.
Deforestation, for example, removes forest cover, exposing soil and allowing different plant species to colonize. Agricultural practices, particularly the abandonment of cultivated fields, also lead to successional changes. These abandoned lands, often called “old fields,” transition from grasses and herbaceous plants to shrubs and trees, eventually reverting to forest.
Urbanization and land development transform natural landscapes into built environments, altering habitats and redirecting successional processes. Mining operations similarly disturb vast areas, removing topsoil and vegetation, necessitating long-term recovery. Pollution events, such as oil spills, impact ecosystems. Oil spills can kill marine life, coat coastlines, and contaminate food sources, leading to changes in species composition and long-term recovery. These human-induced changes create conditions that favor the establishment of new communities or alter the trajectory of existing ones.
Climatic and Geological Transformations
Broader, often slower, environmental changes also play a role in initiating ecological succession.
Long-term climate shifts, including changes in temperature and precipitation patterns, can alter environmental conditions, making an area more or less suitable for certain species. These gradual changes can lead to shifts in species composition over extended periods.
Glacial retreat is a geological transformation that exposes new land, providing a blank slate for primary succession. As glaciers recede, they leave behind barren rock or gravel devoid of soil, allowing pioneer species like lichens and mosses to colonize and begin soil formation. Over time, these newly exposed areas develop complex ecosystems.
The formation of new landmasses, such as volcanic islands or river deltas, also initiates primary succession. Volcanic islands emerge from the sea as new terrain, which is then gradually colonized by organisms. Similarly, the deposition of sediments in river deltas creates new surfaces for plant growth and successional development. These large-scale geological processes create entirely new environments, leading to the long-term establishment of biological communities.
Biological Influences
Certain biological factors can act as direct drivers for ecological succession.
The introduction of invasive species is a major influence, as these non-native organisms can outcompete native species for resources like space, light, and nutrients. Invasive species can alter successional pathways, leading to a dominance of non-native species and a shift in the ecosystem’s balance.
Large-scale disease outbreaks can also initiate successional changes by decimating dominant species within an ecosystem. The loss of a widespread species creates openings and alters the competitive landscape, allowing other species to colonize and flourish. These biological interactions change an ecosystem’s structure, initiating a new successional process. The presence or absence of key species can thus directly influence the trajectory and speed of ecological community development.