How Does Succession Change a Community Over Time?

• Bare rock, no plants yet
• Moss on the rock
• Low shrubs
• Tall trees

What you will learn

• Explain what ecological succession is and what it changes.
• Tell apart the two starting points of succession.
• Describe how stages follow one another and why the process never fully stops.

Before you start

No prior knowledge is needed. You only need to know that plants and other living things share a place and form a community.

What is ecological succession?

Succession is the natural, predictable change over time of the species that inhabit an ecosystem. Kenai Fjords National Park's page on plant succession says that certain species become less abundant or disappear entirely, while others thrive.

The point to hold on to is that a community does not stand still. Species change in a recognisable order, so an observer can tell which ones tend to arrive early and which come later.

Where does succession start?

Succession can start from bare ground or from ground that still has soil. According to the Kenai Fjords page, primary succession follows the formation of a totally new habitat. That means new land devoid of soil or vegetation, created or revealed by a lava flow or a receding glacier.

Secondary succession differs. The same page says it is triggered by a disturbance that resets the plant community but leaves the soil relatively intact. A wildfire, a hurricane or a timber harvest are the examples it gives.

• Brand-new ground, no soil: primary succession
• Soil left after a disturbance, new sprouts
• Burnt stumps
• The disturbance

How do the stages follow one another?

Each group of species arrives after the previous one has changed the place. In a chapter of the Encyclopedia of Ecology, summarised on the University of Illinois research portal, pioneers are the species that first colonize new habitats created by disturbance.

For primary succession, the Kenai Fjords page explains that organisms from nearby habitats begin to build soil by breaking down exposed bedrock. Once a sufficient substrate has developed, new vegetation can take root.

• Moss on rock
• Thin soil and a small shrub
• Thick soil and a large tree

Later, small trees and shrubs grow. The page notes that species suited to one set of conditions gradually alter their own environment until other species that thrive in the new conditions outcompete them. So each stage prepares the ground for the next.

Why does succession never fully stop?

Because new disturbances and changes keep arriving. Kenai Fjords writes that the process of succession never truly stops. Localized disturbances, such as a tree blown over in a storm, can cause small areas to revert to an earlier stage. Geologic processes and changes in climate also prevent any ecosystem from remaining the same forever.

• Late stage: tall tree
• A local disturbance
• Return to an earlier stage
• Early stage: low shrub

Real-world example

At Kenai Fjords National Park a retreating glacier leaves bare rock behind. The park's page describes mosses and lichens beginning to colonize and break down the rock. After that, small trees and shrubs such as willows and alders begin growing. Walking along the road away from the glacier's end, a visitor can see these stages laid out in sequence, from bare ground to tall trees.

Common misunderstandings

Succession stops once the forest is mature. The process never truly stops. Small disturbances still send patches back to an earlier stage.

Succession always starts from bare rock. Only primary succession starts where there is no soil. Secondary succession starts while the soil is still there.

Later species replace earlier ones because they are stronger. According to the Kenai Fjords page, earlier species gradually change their own environment until species suited to the new conditions outcompete them.

Frequently asked questions

What is ecological succession?

It is the natural, predictable change over time of the species living in a place.

How does primary succession differ from secondary succession?

Primary starts on brand-new land with no soil or vegetation. Secondary starts after a disturbance that leaves the soil relatively intact.

What is a pioneer species?

A species that is among the first to colonize a new habitat created by disturbance.

Does succession ever end?

Not truly. Local disturbances, geologic processes and climate change keep it going.

Summary

• Succession is a change in which species live in a place, in an order that can be predicted.
• It starts either from land with no soil or from land whose soil remains after a disturbance.
• Stages follow one another because each group changes its environment, and the process never fully stops.