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How does our home change?

Description of ecological succession.
by

Philip Lussier

on 20 December 2012

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Transcript of How does our home change?

How Does our Home Change over Time? Biomes have a characteristic group of plants and animals that are adapted to their characteristic conditions. That characteristic group doesn't change much over time. When a community of organisms is self-replacing over time that group is called a climax community. But climax communities can be disturbed. Then what? Remember? Ecosystem disturbances can be things like tornados, hurricanes, fires, landslides and even human activities like farming and land clearing. Land can be cleared all the way to bare rock or mineral (no organic matter) soil. In our area the biggest land clearing operation in recent geological time was the Wisconsin ice sheet (the "ice age"). Glaciers advanced from Canada and crossed our area. The ice was a mile thick in most places. Every surface was scraped down to bare rock but much debris was left behind when the glaciers began melting about 18,000 years ago. The bare rock and soil underwent a process of gradual revegetation called succession. Succession is a fairly predictable process in which one community of organisms replaces another until a climax community is established (at least temporarily). There are two starting places for succession. If you start from scratch (no soil - such as after the glaciers) the type of succession that occurs is called primary succession. If you start with soil that has already supported vegetation (such as a cleared forest or an abandoned farm field) it is called secondary succession. To understand how succession works it's necessary to learn some terms for forest layers. Light is a limiting factor in forest growth. All green plants require light to grow. Some require less light and are called shade tolerant. Others require more light and are called shade intolerant. The first plants to colonize a disturbed area are called pioneer species. Pioneer species are almost all shade intolerant and they thrive in disturbed areas because of that (plenty of light) and are quick to establish themselves by means of strategies such as an ability to grow quickly and produce numerous, easily transported seeds. Because they are shade intolerant their seeds do not sprout and grow on the ground once there is much shade. So here is why one group replaces another. The first colonizers in secondary succession (humus containing soil exists) are the annual plants. They sprout quickly because their seeds are always present in the soil. The next group of colonizers are the perennial plants like goldenrod. They take over because they grow taller than the annuals (shading them out) and sprout quicker in the spring (because they come up from roots that last through the winter -- annuals always sprout from seeds because their roots die out at the end of the summer). The next group of colonizers are the pioneer tree species. They are trees like gray birch, aspen and white pine. They replace the perennial plants because they grow taller and shade the ground. They shade the ground so much that their own seeds cannot get enough light to develop and grow. You rarely see white pine seedlings growing under a white pine canopy and any pines you see in the understory are usually dead. The final stage is reached when the shade tolerant species of trees come to dominate the canopy. Their seedlings and saplings could survive in the shade of the pioneer trees and grow tall enough to overtop them or at least share in the light of the canopy. Which community of plants has the greatest shade tolerance do you think? The climax community. So it very much makes sense. Each community in the succession series occupies the area for a time, during which that group's presence results in conditions to which its own descendents aren't adapted but those of another community of organisms are. A final stage is reached in which the offspring of the dominant group are able to replace their parents and so on, through the generations, until... A disturbance occurs that starts the process all over again.
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