Ecological Succession

Examples of Changing Ecosystems A forest could have been a shallow lake a thousand years ago. Mosses, shrubs, and small trees cover the concrete of a demolished building.

Ecological Succession Gradual process of change and replacement of the types of species in a community. May take hundreds or thousands of years.

Newer communities make it harder for the older ones to survive. Example: Younger birch trees will have a harder time competing with taller, older birch trees for sun, but a shade loving tree may replace the smaller birch trees.

Primary Succession Type of succession that occurs where there was no ecosystem before. Occurs on rocks, cliffs, and sand dunes.

Primary succession is very slow. Begins where there is no soil. Takes several hundred years to produce fertile soil naturally. First species to colonize bare rock would be bacteria and lichens.

Lichens Do not require soil. Colorful, flaky patches. Composed of two species, a fungi and an algae. The algae photosynthesize and the fungi absorbs nutrients from rocks and holds water. Over time, they break down the rock.

Lichens and Moss

As the rocks breaks apart, water freezes and thaws on the cracks, which breaks up the rocks further. When the lichens die, they accumulate in the cracks. Then mosses begin to grow and die, leading to the creation of fertile soil. Fertile soil is made up of the broken rocks, decayed organisms, water, and air.

Mosses on rocks

Primary succession can be seen happening on the sidewalks. If left alone, even NYC would return to a cement filled woodland.

Secondary Succession More common Occurs on a surface where an ecosystem has previously existed. Occurs on ecosystems that have been disturbed or disrupted by humans, animals, or by natural processes such as storms, floods, earthquakes, and volcanoes.

Secondary Succession: Mt. St. Helens Erupted in 1980. 44,460 acres were burned and flattened. After the eruption, plants began to colonize the volcanic debris. Pioneer species: the first organism to colonize any newly available area and begin the process of ecological succession.

Over time, the pioneer species makes the area habitable by other species. Today, Mt. St. Helens in the process of secondary succession. Plants, flowers, new trees and shrubs have started to grow. If this continues, over time they will form a climax community.

Climax community: the final and stable community. Climax community will continue to change in small ways, but left undisturbed, it will remain the same through time.

Fire and Secondary Succession Natural fire caused by lightening are a necessary part of secondary succession. Some species of trees (ex: Jack pine) can only release their seeds after they have been exposed to the intense heat of a fire. Minor forest fires remove brush and deadwood.

Fire and Secondary Succession Some animals depend on fires because they feed on the newly sprouted vegetation. Foresters allow natural fires to burn unless they are a threat to human life or property.

Fire and Secondary Succession

Old-field Succession Occurs in farmland that has been abandoned. Grasses and weeds grow quickly, and produce many seeds that cover large areas.

Over time, taller plants grow in the area, shading the light and keeping the pioneer species from receiving any light. The longer roots of the taller plants deprive the pioneer species from water. The pioneer species die.

Taller trees begin to grow and deprive the taller plants of water and light. Followed by slow growing trees (oaks, maples) takeover the area. After about a century, the land returns to a climax community.

Your Turn: Create two flowcharts. One illustrating the steps of primary succession, one illustrating the steps of secondary succession. You may use either pictures or words. Use the following terms in your charts: