Secondary Succession Definition
Secondary succession is a type of ecological succession in which the natural succession has been disrupted and must restart with a smaller number of species. As opposed to primary succession, secondary succession happens after a basic ecosystem and nutrient-rich soils have been established, but some accident has wiped many species out. As is often the case after natural disasters, only a basis for an ecosystem and some small survivors remain in place.
Secondary succession proceeds as these smaller ecosystems build a basis for larger, more diverse organisms. Secondary succession is largely predictable by previously observed patterns, and often happens in identifiable steps, depending on the ecosystem. There are many factors which can affect the rate and progression of secondary succession, which are discussed below.
Factors of Secondary Succession
Secondary succession is a process that largely depends on the types of organisms which existed before the ecological disaster. Their remains will provide food for organisms in the soil. Whatever organisms remain in the soil are the organisms which will serve as a basis for what life can establish itself on the surface. Due to the high exposure to sunlight and limited nutrient content of the soil, this is often small plants like grasses and shrubs.
These plants reproduce, and their seeds are distributed across the fresh ground. With limited competition they grow rapidly and after several generations have greatly added to the nutrient content of the soil. The seeds of larger plants then have the ability to sprout and thrive, and soon grow tall on the nutrients from the grasses. These larger plants then drastically change the conditions of the ground below them, increasing the shade. In deciduous forests, this can lead to shade-tolerant trees again sprouting. As they grow above the faster-growing trees, these die off and the entire ecosystem again changes. This basic scenario is shown in the image below.
Which species end up occupying the final community is largely determined by the effects of the species which came before them. A plant which needs a lot of sunlight is unlikely to do well in an area blocked by taller species. This may seem like a change in one plant to another, but the implications are much deeper. Certain animal species and even bacteria in the soil depend on certain plant species through the food web.
If the plant disappears, it alters the whole structure of the ecosystem as it can literally change which heterotrophic organisms can survive there. These types of chain reactions are common in secondary succession and can lead to vastly different ecosystems in places that are similar.
Primary or Secondary Succession?
Primary succession is classified as the introduction to life in a barren environment. This slow progression of basic microscopic ecosystems must proceed before a climax community can be reached. This progression is considered primary succession. But, most ecosystems do not reach a climax before they are befallen by some sort of disaster. Among these are hurricanes, disease, fire, and other things which wipe out a majority of the species, but not the basic foundation of the ecosystem.
Secondary succession takes over when the ecosystem must recover from one of these disasters. It is different than primary succession because, with primary succession, there is no basis on which to start. Secondary succession is less of a blank canvas, and more like a paint-by-color. Though the environment can ultimately have a lot of different colors (or species) there is a precise foundation from which it must start. The conditions present naturally favor species present in the area, and they are the first to take advantage of the open, rich soil.
Secondary Succession Examples
In a Forest
In a forest, secondary succession takes place in 3 basic, well-documented steps. After a fire or other disaster destroys the trees and other plants, the community is reduced to the small organisms and insects present in the soil. These organisms continue to break down detritis in the soil, leaving nutrients deposited for future plants. The first step of secondary succession takes place when early opportunists jump on the open patch of ground, as seen in the image below.
These plants establish a new dynamic in the soil, creating more opportunities and niches for organisms like insects, mice, and other small animals to thrive. Birds often visit these short grass patches to collect seeds and insects. These animals often help disperse other seeds onto the ground, such as pinecones and seeds from berry-producing shrubs. These intermediate species are seen in the second stage of secondary succession.
These plants also change the basic forms of life that exist, blocking out sunlight for grasses below. This may also expel some small animals which depended on the grasses. But it also brings in other occupants, such as squirrels and birds which are dependent on the taller plants for food and shelter. In time, the slow-growing deciduous trees eventually outgrow the fast growers. This advanced stage of secondary succession is known a climax community. However, this community could easily be disrupted again and secondary succession would start over.
In a Kelp Forest
The climax community, in a kelp forest, consists of many different and biologically dependent species. A simple act of disrupting any of these species can cause massive effects on the entire community. Four of the most important species are otters, starfish, sea urchins, and the kelp itself. Kelp is a plant-like alga which anchors itself to the seafloor. Many species of fish reproduce and live in kelp forests. Sea urchins like to eat the base of the kelp. This can easily wipe out a kelp forest, but the urchins are held in check by their predators: the otters and the starfish.
Throughout the 1800s and into the 1900s otters were hunted to near extinction for their pelts. This disruption slightly upset the balance of kelp forests, and the kelp was thinned. Without the otters, the starfish increased in number feeding on the abundance of sea urchins. However, this abundant density of starfish led to a terrible starfish virus that wiped out a majority of their population. As such, many kelp forests are currently being destroyed by sea urchins. “Urchin barrens”, as they are called, have no more kelp and therefore are like underwater deserts when it comes to other marine life.
These ecosystems have undergone a massive disruption and now must go through secondary succession to again reach a climax community. Luckily for the kelp forests, otters are returning in some numbers. They will be aided by the fact urchins are abundant, and eventually, the starfish will return. The kelp will be able to reestablish itself on the seafloor, and a wealth of life will come back to the underwater ecosystem.