Secondary Succession Definition
Whilst primary succession takes place when pioneer species inhabit a newly formed substrate lacking in soil and biotic organisms (such as rock formed from lava flow or areas of glacier retreat), secondary succession occurs on a substrate that has previously supported vegetation but has been altered by processes such as fire, hurricanes, floods or human disturbances.
Secondary succession is usually faster than primary succession because soil and nutrients are already present due to ‘normalization’ by previous pioneer species, and because roots, seeds and other biotic organisms may still be present within the substrate.
Examples of Secondary Succession
Fire is one of the most common causes of secondary succession and is an important component for the renewal and vitality of many types of ecosystem. Fires may either take place naturally, for example when lightning strikes a dry habitat, or may involve controlled, systematic burning of a landscape by humans.
Both the abiotic and biotic components of an ecosystem can be drastically altered by the presence of fire. The most notable abiotic feature that is affected by fire is the soil; CO2, CO and CH4 stored within the organic material is released into the atmosphere during the combustion process; however, this initial loss of nutrients is often counterbalanced and then increased by the decomposition of leftover plant material which leaches N, P and K back in to the soil. The moisture retention of the soil also increases due to the reduction of transpiration by plants, and because more water is allowed to reach the soil surface where interception of rain by leaves is greatly reduced or non-existent. Soil pH often rises (more alkaline) after a fire due to the combustion of acids.
After a fire, species start to recolonize an area, beginning the secondary succession process. The first species to colonize are usually fast growing herbaceous plants, such as conifers or ferns, which require high levels of light. These species are often already present in the form of seeds within the soil, or are able to rapidly disperse from nearby areas. In time, slow growing, shade-tolerant, woody species begin to suppress the early successional species, which are in-turn replaced or shaded by large trees, eventually leading to the generation of forests and a climax community.
The physical and biotic characteristics of an ecosystem, as well as the level of disturbance (determined by the intensity and frequency of fires), create a mosaic of habitats within an area. This mosaic effect allows a more diverse range of species to colonize than in an area that is ecologically stable for a long period of time. The types of plants and animals able to recolonize an area after fire are dependent on the properties of the soil, as well as climate and topography.
Harvesting, Logging and Abandonment of Crop Land
The abandonment of land previously utilized for crops is a common cause of human-induced secondary succession. Land which has been intensively cultivated is often nutrient poor, with the nutrients having been repeatedly removed through harvest or logging. Agricultural processes also often leave the soil vulnerable to high levels of erosion. The abandonment of such land allows plants and animals that were previously unable to inhabit the area to colonize. Early succession of vegetation following the abandonment of farmland is responsible for increases in soil organic content, nutrient density and soil porosity. The addition of shrubs and of root systems within the soils, which follow in later succession, acts as a natural barrier against erosion, thereby allowing for restoration of degraded habitats.
The process of secondary succession on human altered landscapes differs to that of succession after a natural event due to the homogenization of soil type and nutrients, especially where artificial fertilizers have been applied. This can lead to the exclusive colonization of an area by generalist species, which slows the succession process and does not allow for such high biodiversity.
Renewal After Disease
If a disease affects all of a certain species within an area, the species is likely to experience a rapid die-off. Although the onset of disease can be a catastrophic event for a particular species, once the living crop has entirely died off and the disease therefore eradicated, if the roots or seeds remain in the soil, the crop can repopulate. Alternatively, the disease can kill enough of a species to allow for invasion by species which may have been previously unable to colonize, which in turn enables a more diverse range of species to inhabit an area.
Although secondary succession can happen on a large scale and have an intense effect on a habitat or ecosystem, it is most common on a small scale. The disturbance and subsequent secondary succession that occurs after a gap is created in a forest canopy, following the death and collapse of a single tree or the loss of a large branch, is known as gap dynamics; the effect is often most prevalent in dense forests. The creation of a gap in a canopy allows light to penetrate to the forest floor, giving herbs, shrubs, vines and seedlings an opportunity to exploit the new resource. After a few years, fast growing, taller plants begin to dominate the lower canopy, suppressing the growth of the shade-intolerant species of the lowest canopy level but allowing shade-tolerant species to thrive. Heliophilic (sun-loving) species begin to dominate the top of the canopy after around 75-150 years, while the shade-tolerant species of the lower canopies establish a stable community. This stable state is known as a climax community, and will remain in equilibrium until a new canopy gap is created.