Written by: Summer Graham
After a disturbance, ecosystems go through a progressive change of species composition that follows a relatively set pattern over time. This process is called succession, and understanding it can be key to the successful restoration of degraded and disturbed natural areas.
There are two main types of succession, primary and secondary. Primary succession will occur after a large disturbance (volcanic eruption, glacial formation and retreat, forest fire, etc.) resulting in exposed rock with little to no vegetation present. Primary succession is a very slow process (sometimes hundreds or thousands of years!) in which species like mosses, lichens, and fungi establish over bare rock and eventually enough soil is present to allow secondary succession.
Secondary succession can occur after primary succession, or after a less impactful disturbance occurs that sets an ecosystem back in terms of development. Imagine a clear-cut forest that removes most canopy trees but leaves shrub and ground layer vegetation, or a wind storm that creates a clearing in an established forest. Secondary succession will also occur in an anthropogenic setting such as an abandoned farm field. In the example of succession into a forest community, grasses, forbs and shrubs will give way to early pioneer tree species (e.g., Aspen, Cherry, and Pine) that usually tolerate high levels of sunlight and exposure. These species will eventually be replaced by more shade tolerant, intermediate species such as Beech and Maple.
Eventually succession may reach a climax community, or the final stage of succession. In an old-growth forest, canopy will be fully developed and the understory will be composed of shade tolerant species. The composition of these communities will vary based on geography and climate. Contrary to general thought, climax communities like old-growth forests are not static or devoid of disturbance, as the oldest trees will eventually die and be replaced by other species from the understory. Small disturbances may still occur throughout the system, creating a dynamic equilibrium.
Some types of ecosystems will form climax communities relatively quickly, such as tall-grass prairies, while others may face frequent, high levels of disturbance and never reach this final stage of succession. Regardless, there is value in all stages of succession. For example, both late and early successional habitats support a variety of wildlife species including Species-at-Risk like the Golden Winged Warbler (early successional shrub habitat) and Spotted Owl (old growth coniferous forests). Some species have adapted to only establish after certain disturbances, such as Lodgepole Pines (Pinus contorta) that require the high heat of forest fires to release and germinate their seeds.
Disruption of the natural successional cycle by human interference can greatly impact the amount and variability of habitats available to wildlife. Restoring natural disturbance, or mimicking it in restoration projects, may help to introduce these species to the landscape once more.
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