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Succession in Freshwater and Terrestrial Ecosystems

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  • 0:05 What Is Succession?
  • 1:41 Succession in…
  • 4:16 Succession in…
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Lesson Transcript
Instructor: Margaret Cunningham

Margaret has taught many Biology and Environmental Science courses and has Master's degrees in Environmental Science and Education.

The world's many freshwater and terrestrial ecosystems are constantly changing. This lesson will discuss how these ecosystems change over time and how they recover after disturbances occur.

What Is Succession?

If a volcano erupted or a tsunami hit land, how do you think the environment would be affected? You might expect that after the initial problem that plants and animals, along with the entire ecosystem, would be destroyed. In fact, over time, the ecosystem would recover and be restored to a functional environment. Ecological succession is the term used to describe how an ecosystem gradually changes in species composition and community structure over time after a disturbance has occurred.

Succession can occur after a variety of different types of disturbances, such as volcanoes, earthquakes, floods, fires and human activities. In ecological succession, the first species to colonize the area after the disturbance is called the pioneer species. Pioneer species are often small species that are very good at adapting to adverse conditions. Once the pioneer species are established, they facilitate the recovery of the environment by creating conditions that are more favorable for larger and less adaptable species.

Over time, the ecosystem will begin to stabilize as more species inhabit the area, and eventually the region will become a climax ecosystem. A climax ecosystem is when the plants and animals of the ecosystem are in a stable relationship with the environment and they remain relatively unchanged until another disturbance occurs. The steps of succession can vary by the type of ecosystem, but overall, they include the introduction of pioneer species and the end result of a climax ecosystem.

Succession in Terrestrial Ecosystems

Ecological succession can happen in any type of environment, including both terrestrial and freshwater ecosystems. Although succession occurs in both of these types of ecosystems, the stages of change are very different due to the variation in the species and community structures of the two different types of ecosystems. Terrestrial succession is better understood and is often divided into primary and secondary succession.

Primary succession is the type of succession that occurs when an area experiences a disturbance so severe that none of the original species survive. During the process of primary succession, the disturbance causes the exposure of bare rock and the ecosystem is rebuilt from scratch due to the lack of organisms that remain. Primary succession is often a slow process because it requires many steps to convert bare rock to a functional ecosystem. The process normally starts with the invasion of lichens and mosses, which create soil, and then the process moves on to allow for the establishment of larger vegetation.

An example of primary succession would be the recovery of an ecosystem after lava covers the ground or a glacier retreats. Both of these disturbances would leave the ecosystem void of species that previously inhabited the area and would leave bare rock exposed. In order for the ecosystem to recover, pioneer species would have to establish and create soil for other vegetation to utilize.

In contrast, secondary succession is the type of succession that occurs when an area experiences a disturbance that alters the existing ecosystem but does not destroy all of the original species. During secondary succession, the soil and species that remain after the disturbance are used as the building blocks that help facilitate the recovery of the ecosystem.

Secondary succession is often quicker than primary succession because the process does not have to start from scratch. The soil and organisms that remain establish a healthy ecosystem and larger plants and animals can begin to reestablish the environment more rapidly. An example of secondary succession would be the recovery of an ecosystem after humans cut down all of the trees.

After the disturbance, small plants would recover quickly, and eventually larger trees would be able to establish. The ecosystem would still have a variety of small plant species and many animal species left after the harvest, and these species would begin to grow and recolonize the area. Eventually, the larger trees would grow back, and the ecosystem would return to a stable state.

Succession in Freshwater Ecosystems

Ecological succession in freshwater ecosystems has not been studied as much as terrestrial succession, and therefore, more is unknown about freshwater succession. It is known that succession in freshwater ecosystems, such as lakes and streams, happens when a disturbance occurs that results in the aquatic area filling with sediment or organic matter.

Common disturbances that result in freshwater succession are floods, droughts, erosion and the construction of dams. These types of disturbances often cause land around the freshwater ecosystem to erode and sediment settles into the water. Disturbances can also cause organisms in the water to die, and the organic matter then settles to the bottom of the water.

During succession of a freshwater ecosystem, eventually the sediment or organic matter builds up and the dynamics of the ecosystem begin to change. In many cases, the freshwater ecosystem fills with so much sediment that the shoreline advances to the center of the water and the aquatic area is replaced with a semi-aquatic or a terrestrial environment.

For a freshwater ecosystem, the climax community can actually be the terrestrial ecosystem that ultimately remains after the water is gone. Throughout this process of the water filling in with sediment, some species die off, such as fish, and others, such as plants near the shore, may flourish in their new environment.

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