LWB: Lesson - Ecological Succession (Topic 2.7) đź“–

⏳ Estimated Reading/Watching Time: 7 - 9 minutes

Learning Objective

Describe ecological succession and the effect of ecological succession on ecosystems.

 

Succession


grass growing in cracks in concrete
Some people consider succession that occurs on areas of concrete or asphalt as primary succession because the concrete and asphalt are not soil.

Ecological succession is a transition from one biotic community to another after a disturbance.

Some anthropogenic disturbances are...

  • 🏢 urbanization
  • 🪓 deforestation
  • 🚬 air pollution

Some natural disturbances are...

  • 🔥 fire
  • đź’¨ wind
  • 🏜 drought
  • 🌨 blizzards

 

Primary Succession


Primary succession is the process of initial invasion and progression from one biotic community to the next. This type of succession USUALLY begins in places with no soil. 

This could happen in areas where new islands are forming, where glaciers are receding, or after a volcanic eruption, where the only thing left is bare rock in the form of hardened and cooled lava. 

Primary succession depicted in a six-stage graphic over centuries. It begins with bare rock and progresses through stages of colonization by hardy pioneer species, soil formation from decomposed organisms, growth of grasses and herbs enriching the soil, increased plant diversity with shrubs and small trees, culminating in a diverse forest ecosystem. The graphic spans from “bare rock” to “mature forest” over “hundreds of thousands of years.
The process of primary succession usually occurs as shown in the diagram. This process takes many thousands of years, especially the beginning stages that allow the rock to decompose into soil but does provide a mechanism for new land or areas that lack soil to become colonized with plants, and, eventually, animals.

Secondary Succession


Secondary succession occurs when an area has been affected by a disturbance, either human or anthropogenic.  If an area is cleared and then left alone plants and animals from the surrounding ecosystem may gradually invade the area.  This process begins with pre-existing soil, so it occurs much more quickly than primary succession.

A diagram illustrating how secondary succession can restore an ecosystem over time, depending on the type and severity of the disturbance, the availability of resources, and the interactions among species.
If a fire occurs in an area, it might kill many of the large plants and trees in the area.  This might leave bare soil in the ecosystem.  Pioneer species would begin to colonize this bare soil.  After this, shrubs may begin to grow in this area.  Then, small trees would begin to colonize the area.  Finally, large trees and a climax ecosystem would occur in the area. If another disturbance were to occur in the area, this process would start all over again.

Secondary succession is continually occurring all around us.  You may see it on the side of the highway, in a neighbor’s yard if they aren’t mowing and weeding it regularly, in a farmer’s uncultivated land, or any number of places.

 

Essential Knowledge

There are two main types of ecological succession: primary and secondary succession.

 

Types of Species


Certain organisms serve particular roles in ecosystems. Click through the tabs to learn more about keystone, indicator, and pioneer species.

 

Keystone Species

a black and white drawing of an archway with the keystone in the center highlighted red
The keystone is the stone that holds the arch in place. If you remove the keystone, the arch will fall.

The term "keystone species" comes from the materials and process used to build a stone archway. The keystone is the stone that holds the arch in place. If you remove the keystone, the arch will fall. The same often happens in ecosystems when the keystone species is removed – the ecosystem collapses or suffers a huge, catastrophic change.

Keystone species are species whose impact on the community or ecosystem is much larger than its population numbers would indicate.

Typically, producers are not keystone species. Usually, keystone species are heterotrophs. This is because producers tend to be in the ecosystem in large numbers already, and the keystone species often has a smaller population size. A keystone species could be a predator that keeps an herbivore in check.

A keystone species could also be a predator that keeps other predators in check or an herbivore that keeps the producers from growing out of control.

Review the presentation below to discover some classic examples of keystone species. Did you know that these species played such a pivotal role in their ecosystem?

 

 

Essential Knowledge

A keystone species in an ecosystem is a species whose activities have a particularly significant role in determining community structure.

An indicator species is a plant or animal that, by its presence, abundance, scarcity, or chemical composition, demonstrates that some distinctive aspect of the character or quality of the ecosystem is present.

Pioneer members of an early successional species commonly move into an unoccupied habitat and over time adapt to its particular conditions, which may result in the origin of new species.

 

Effects of Succession


a climax forest in Tongas National Forest
 Disturbances, small and large, are constantly taking place in ecosystems, so succession is constantly happening on small and large scales within ecosystems.

Succession usually begins with a climax ecosystem.  A climax ecosystem is an ecosystem that appears to be balanced between all of the species and the physical environment.  This implies more stability than actually occurs in nature, but it gives us a place to start.  Then, there is usually some type of disturbance, whether natural or anthropogenic, that causes the ecosystem to lose many of the climax species (large trees, etc.).  The process of succession then begins to take place.  Pioneer species begin growing in the gaps left by the large, climax species and then shrubs begin growing, and then eventually trees replace the shrubs, creating a new climax ecosystem…until the next disturbance.

Essential Knowledge

Succession in a disturbed ecosystem will affect the total biomass, species richness, and net productivity over time.

 

AP Exam Tip

Primary succession occurs on bare rock and secondary succession occurs where soil is present.

 

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