Main Difference – Primary vs Secondary Succession
Before analyzing the difference between primary and secondary succession, let us first see what is succession. There are two types of changes that can happen in a community as spatial changes and temporal changes. Temporal changes refer to the changes at a given point in space in a community (at a single location). These changes include diurnal changes, seasonal changes, and other changes over a long period of time. Temporal changes are also of two types as, directional changes and non-directional changes. The directional changes in a community are called succession (ecological succession). There are two main types of succession namely, primary succession and secondary succession. The main difference between Primary and Secondary succession is that primary succession occurs in the land where there is no initial vegetation whereas secondary succession occurs in a land that has primary vegetation.
What is Primary Succession
Primary Succession is an ecological succession that takes place following an opening of uninhabited, infertile habitat, or it can also occur in an environment that is devoid of vegetation and usually lacking topsoil. Primary succession can be divided into two types based on the primary physical source of change.
- Autogenic succession
- Allogenic succession
Autogenic succession
When the community itself changes its physical environment, that is called as autogenic succession. Biological actions on the environment (is included as degradative succession as) is a type of autogenic succession.
Ex:. fallen leaves
Fallen leaves are first colonized by fungi. Due to the secretion of enzymes by fungi, leaves get digested and soften. Then other fungi and mites penetrate into these leaves. Subsequently microorganisms in the soil feed on leaves and fungi. When the organic substrate is completely metabolized degradative succession terminates.
Allogenic succession
Allogenic type of succession refers to the succession in response to the changes in the external environment.
Ex: formation of woodlands after a volcanic eruption (volcanic islands)
During a volcanic eruption, heat and fire destroy life in the relevant areas. Then a new layer of rock covers the existing land. This becomes the new landscape for the primary succession to take place. Then pioneer colonists such as blue-green algae establish on this land initially. Dispersal methods promote algae and lichens to establish on the land. The moist land favors the establishment of mosses and ferns in the area. The accumulation of dead and decomposing organisms and erosion of rock by weathering lead to soil formation. By this time, seed plants may reach the land by the wind, water, or animals such as birds or bats. Finally, more nutrient-demanding seed plants including grasses and shrubs and trees occupy the land.
What is Secondary Succession
Secondary succession occurs when the primary succession land is disturbed by external or biological cause. Disturbances create gaps and openings within the area. Gaps can be either small scale or large scale. The death of a single tree or group of trees can create a gap in the forest. This creates a gap in the canopy and thereby in the substrate. In a gap, the light intensity is high. As a result, soil temperature and nutrients increase while the soil moisture and relative humidity are low. The final result is an abundance of resources. Small gaps stimulate more shade tolerant species to grow whereas large gaps favors suppressed seedling growth and invading opportunistic to grow.
Furthermore, extensive land clearing and fire can result in severe damage to the vegetation that already exist and therefore completely different community can be established in the gap/area created by such devastation.
There are similarities and differences among primary and secondary succession. The main similarity is that both primary and secondary succession are unidirectional. The difference between primary and secondary succession can be summarized as below.
Difference Between Primary and Secondary succession
Definition
Primary Succession is an ecological succession that takes place following an opening of uninhabited, infertile habitat. It can also occur in an environment that is devoid of vegetation and usually lacking topsoil.
Secondary Succession is the ecological succession that takes place on a preexisting soil after the primary succession has been disrupted or destroyed due to a disturbance that reduced the population of the initial inhabitants.
Location
Primary succession occurs in the land where there is no initial vegetation.
Secondary succession occurs in a land that has primary vegetation.
Cause
Primary succession can be initiated due to a biological factor or an external factor.
Secondary succession starts due to an external factor such as gap formation by falling trees, fire, or land clearing.
Soil Cover
Surface soil cover is absent when primary succession starts.
Secondary succession occurs in places where soil cover is present.
Growth
Primary succession land favors the growth of all forms of flora (mosses, ferns, shrubs, trees).
Secondary succession mostly favors the invading opportunities to grow.
Pioneer Species
In primary succession, pioneer species come from outside environment.
In secondary succession, pioneer species develop partly from existing environment.
Time
Primary succession needs more time to complete. (1000 or more years)
Secondary succession takes comparatively less time. (50 – 200 years)
Image Courtesy:
“AP Biology – Primary Succession Drawing” by Joshfn – Own work. (CC BY-SA 3.0) via Wikimedia Commons
“Secondary Succession” by Katelyn Murphy – Own work. (CC BY-SA 3.0) via Commons
