An ecosystem represents a community of organisms interacting with the surrounding environment. That environment contains both abiotic and biotic factors.
Over time, those factors help shape the progression of the community. This series of changes is called ecological succession.
Ecological Succession Definition
Ecological succession describes a typically natural change over time of species within a community or ecosystem. These changes result in some species becoming more abundant while others may undergo a decline.
Types of Ecological Succession
Ecological succession progresses through primary and secondary succession. Eventually succession ceases, and the resulting, stable community is called a climax community. Even so, various factors can shift an ecological community into succession again.
Primary succession: This is a type of ecological succession that begins essentially on a blank slate. A new habitat forms either from a volcanic eruption flow or from glacial retreat, where there is new bare rock or glacial till. The resulting exposed substrate contains no soil or vegetation.
Once soil is made, new species called pioneer species move in. Over time, the landscape is altered by additional species that affect shade and other factors.
Secondary succession: An established community undergoes secondary succession due to a disturbance caused by natural disasters such as wildfires, tornadoes or hurricanes.
Human influences such as foresting, farming and development also lead to secondary succession. After the event, the community species are reestablished.
Stages of Primary Succession
Primary succession is a slow process because it starts as a new habitat where nothing lives. There are no plants, insects, animals or organic matter of any kind at this point. In the first stage, new rock is exposed either from lava flows, the retreat of glaciers, sand dunes, clays or other minerals.
As primary succession begins, there is no soil at all. This is because soil requires a mixture of organic material, living creatures and minerals.
Eventually, species such as lichen and moss move in and begin to break down exposed rock or build the soil. Additional abiotic factors such as wind and erosion can bring more materials to this landscape. Eventfully, after soil development takes hold, new plants arrive.
These new plants are called pioneer species. They enable the alteration of the environment by breaking down bare rock. This in turn leads to soil nutrient enrichment, more moisture capacity, temperature and wind moderation, and reduced light. Small animals move in to take part in eating the producers available for consumption.
These accumulated conditions make possible additional plant growth with deeper root systems. More shade-tolerant trees move in. This creates a layered community for organisms to thrive in. Eventually, the completed habitat reaches a status called a climax community.
Examples of Pioneer Species
Pioneer species tend to be fast-growing and sun-loving. Some examples of pioneer species include birches, aspens, grasses, wildflowers, fireweed and yellow dryas.
Examples of plants in primary succession in Alaska include shrubs and small trees like willows and alders, and occasionally actinorhizal plants that can help fix bacteria at the roots. Fertile soil results, leading to larger trees like Sitka spruce. As organisms die, they add organic matter to the soil as well.
In the drylands of Hawaii, originally new volcanic substrate played host to pioneer plant species such as the shrub Dodonaea viscosa and the grass Eragrostis atropioides. Over time, taller tress such as Myoporum sandwicense and Sophora chrysophylla moved in.
Interestingly, primary succession takes place more quickly on ropy, pahoehoe lava substrates, possibly because of water flow into cracks where new plants can take root.
Stages of Secondary Succession
Secondary succession occurs as result of a disturbance that greatly alters an ecological community. Fires, storms, floods and removal of timber by humans can cause either the complete or partial destruction of vegetation. The availability of resources affects species diversity for each trophic level undergoing secondary succession.
While damage has occurred following such events, soil still remains viable and usually intact. Pioneer species once again set the stage for the community to recover from the disaster. However, in this case, those pioneer species start over from the seeds or roots left over in the viable soil.
In Hawaii, fires (some ignited by volcanic eruptions) repeatedly swept the drylands of the region for thousands of years, before human settlement began. This created a stage for succession. Some of the species that grew in this environment proved to be adaptive to fire.
Secondary succession typically takes several years before a community is fully restored. An example of secondary succession would be the land use of tropical forests. Tropical forests that are cleared for timber or agricultural needs as their disturbance undergo reestablishment at varying speeds. The speed at which a community becomes reestablished varies based on the time and intensity of the disturbance.
Once an ecological community reaches its complete and mature form, it is called a climax community. At this stage, it contains fully grown trees and adequate shade, and it supports the surrounding biome. Both animals and plants can reproduce in these conditions. A climax community is considered the end of ecological succession.
An example of a climax community would be the Kenai Fjords, in which the willows and alders eventually make way for cottonwood trees, then Sitka spruce, and then finally mountain hemlocks after a period of 100 to 200 years.
Community Reversion to Succession
A climax community can, however, be reverted to a successional stages from new disturbances and environmental conditions. And if those disturbances are repeated, forest succession may not reach the point of a climax community.
Climate change, natural events such as forest fire, agriculture and deforestation cause this reversion. This sort of disturbance can lead to removal of key species in the community, and potentially extinction. Invasive species can induce a similar disruptive effect. Repeated, large disturbances favor homogeneous plant species and therefore decrease biodiversity.
Localized disturbances like tree falls from wind storms or animal damage to plants can also revert a community to succession. As climate change affects glacial melt, more areas will be exposed over time, leading to primary succession again.
Resilience in Ecological Communities
Ecologists are finding, however, that some resilience is built into ecological communities. Even with the constant threat of anthropogenic disturbances, tropical dry forests in Mexico begin to recover within 13 years of disturbance. Given the prevalence of agricultural fields and livestock pastures in the region, this resilience proves to be promising for long-term sustainability.
The functionality of the community can return sooner in secondary succession than once thought. This is true despite the complete recovery of the community’s structure. Animal species can return to something resembling a mature forest within 20 to 30 years post-disturbance. Some mutualistic animal and plant interactions prove to rebound despite the changes caused by forest fragmentation.
The Earth is a dynamic place, affected by natural and manmade causes that induce changes to plant communities over time. Any disturbance threatens species diversity. As ecologists learn more about the process of succession, they can better manage ecosystems to try and prevent environmental disturbances.
About the Author
J. Dianne Dotson is a science writer with a degree in zoology/ecology and evolutionary biology. She spent nine years working in laboratory and clinical research. A lifelong writer, Dianne is also a content manager and science fiction and fantasy novelist. Dianne features science as well as writing topics on her website, jdiannedotson.com.