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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.
• 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.
• 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.
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
Lets go outside at determine where the woods are in the succession process.
Did they form from Primary or Secondary succession?