Purpose: To compare ecological succession in a variey of biomes.
Procedure: After reading the passage about succession in different biomes, read and answer the questions about environmental change.
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Updated Klein ISD, 2008
Imagine a beautiful Saturday morning. Instead of sleeping late or hanging out with your friends, your mom insists that you weed the flowerbed. You unhappily pull on a pair of gloves, grab a bag, and get to work. Just before the door slams behind you, you hear your mother’s voice, “You know, Sweetie, there wouldn’t be weeds to pull if you’d covered the flower bed with a layer of mulch like I asked you to!”
Environments, whether as small as a yard or as large as a forest, can be disturbed or damaged in a variety of ways. Disturbances can be huge naturally occurring events like a volcanic eruption, forest fire, or flood. Disturbances can also be manmade events like building a road, digging a new flowerbed, or damming a river. When snorkeling, people are asked not to stand on or touch the coral. Even a footstep can be an environmental disturbance in a fragile ecosystem. The on-going process of environmental change is called succession.
Ecologists evaluate an environmental disturbance by asking several questions:
· Severity: Were all the trees removed as in slash and burn agriculture or were trees selectively harvested?
· Size: Did the flood or fire consume 1 or 100 hectares of land?
· Frequency: Do similar disturbances happen often?
· Timing: Did the disturbance happen during the height of the rainy season or during the dry season when creatures are more stressed?
An additional measure of environmental damage is determining how quickly plants and animals return to the area. If a disturbance occurs because of natural causes, such as fire after a lightning strike, or flooding of an unchanged creek, it is assumed that this type of disturbance happens fairly regularly. The creatures in the area are likely to have behavioral adaptations that allow them to survive.
In most forest ecosystems, small fires happen frequently. This process returns carbon and other nutrients to the soil. Following such a fire, buried seeds that were protected from the heat begin to sprout. In fact, there are some seeds, such as the jackpine, that rely on the heat of a fire in order to germinate. These young seedlings attract animals and insects. However, fires that destroy large areas cause great changes to forest ecosystems. After the fire, the soil is unprotected and is easily washed away by rain. In rainforests, the soil is baked by the intense sunlight and becomes brick-like. Seeds that were buried or were blown into the area by the wind cannot take root. The ground remains bare.
The succession or change of the environment over time happens in a predictable pattern. Plants and animals tend to return from the edges, slowly moving from less disturbed areas into the center. The first plants that thrive following a major disturbance are pioneer or colonizing species. These small plants, such as lichens and moss begin to break down the exposed rock and prevent soil from washing away. In general, these species grow rapidly and quickly produce offspring.
Following a minor disturbance, opportunistic species, such as grasses and plants with soft green stems, follow. There may be hundreds or thousands of seeds in one square meter of healthy soil waiting for the right conditions to sprout. Although these opportunistic plants are often thought of as weeds, they are important to the health of the ecosystem because they keep the soil in place and provide moisture to the soil. Insects in the area will often eat these small plants rather than young trees. At this point in succession, there is typically the greatest variety of plant and animal species.
The final stage of succession is called a climax community. In a forest community, the trees are mature and are the major vegetation. Climax communities are stable, but vary from biome to biome. The vegetation of a climax community can live for many years and create shelter and food for many species of animals. Since little light reaches the forest floor, there are fewer species of plants found here.
In rainforest ecosystems, a small disturbance such as a tree falling increases the amount of light that reaches the forest floor. This additional sunlight causes plants to grow quickly. Bushes and vines fill in the void of the fallen tree. However, more severe disturbances may permanently change rainforest communities. In many areas, since the rainforest soil is so poor, repeated clearing of the forest will lead to severe soil depletion, and the normal patterns of succession will no longer occur. Instead of returning to forest, the area will become a grassy area because there aren't enough nutrients remaining in the soil to support trees. Succession is continuous competition for limited resources.
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Updated Klein ISD, 2008
Consider the following scenarios:
The year is 1899. You are a Swedish immigrant who has bought a small farm in southeastern Nebraska. You spend your first year as a farmer chopping down the maple and oak trees and clearing the small bushes from a 10-acre plot of land. For the next thirty years, you grow corn on the 10-acre field. In 1929, you retire, move to town, and no longer plow the field and plant corn each spring. To celebrate your 101st birthday, your grandchildren bring you back to farm to reminisce.
1. What has happened to the 10-acre field?
2. What would have to be done if your great-grandchildren wanted to plant corn in this field?
3. Where are the tallest trees?
4. Why would there be more varieties of plants and animals in the center of the field?
The year is 2000. You are a middle school student who has been asked to dig a new flowerbed in the backyard of your home. It is late July and the temperature has hovered around 100 °F for the past two weeks. You dig out the grass, place bricks around the edge for a border and arrange plants from a local nursery according to your parents’ instructions. The next spring, your mother sends you out to weed the flowerbed.
5. Where did the weeds come from?
6. You notice grass has also grown into the flowerbed. Where is the grass thickest?
7. What would likely happen to the flowerbed if it were not weeded regularly?
8. Why didn’t the weeds emerge during the summer you planted the flowerbed?
9. How would covering the bare ground with mulch prevent the growth of weeds?
Interpretation:
What factors are unique to rainforests that affect succession?
1
Updated Klein ISD, 2008