ECOLOGICAL SUCCESSION

Transcription

1 reflect What would happen if a city park or a garden were left unattended, with no mowing, weeding, watering, or any other human interference? At fi rst, weeds would grow and replace the cultivated plants that may have been present. What would this area look like after a month? What about after a year or after a decade? Ecological Succession The observed process of change in the species composition of an ecosystem over time is called ecological succession. In a park that is no longer tended, weeds would replace cultivated plants. Other plants may eventually replace these weeds. This process is called succession. When the process of succession begins in an area that is entirely uninhabited by living things, it is called primary succession. Primary succession is a slow process because the area in which it is occurring is barren. The fi rst organisms that are able to grow in such a location, called pioneer species, must be able to grow on bare rock. Lichens are an example of a pioneer species. They result from a mutualistic relationship between a fungus and algae or bacteria. The algae or bacteria are photosynthetic and provide the fungus with a source of food. The fungus absorbs other necessary nutrients from the environment and provides a structure in which the algae or bacteria can live. Lichens break down bare rock into small particles as they grow on it. This results in soil production and provides a foothold for the next group of organisms that moves in. The lichens change the physical environment in such a way that it is easier for other organisms Lichens and mosses (shown above) are common pioneer species. to live there. In this stage, and each subsequent stage of succession, the organisms that are best adapted to the current conditions will thrive and reproduce. They will be the most abundant organisms in the environment until conditions change, and another set of organisms move in that is more suited for the new conditions. Mosses use the products released by lichens and begin growing on pockets in the rock. Mosses continue the soil production begun by lichens and provide the necessary materials for small plant seeds to germinate. 1

2 The plants that fi rst take root in these areas must be adapted to full sunlight and little soil. Generally, small annual plants are best adapted to this environment. These annual plants are gradually replaced by perennial plants and grasses, which grow more slowly. These small plants provide the base of a food chain consisting of mostly small animals. In each of these stages, the organisms present add to the soil and alter the environment. These changes in the environmental conditions allow the next group of plants and other organisms to fl ourish. The soil and organic matter produced by smaller plants allow larger shrubs to begin growing. The shade from these shrubs will inhibit the growth of some of the small plants underneath them, though some small plants will continue to grow in the spaces between shrubs. Soon, the area will contain more shrubs. The shrubs will continue adding organic matter to the soil and will provide shade for germinating tree seeds. The soil, organic matter, and shade will allow larger trees to begin germinating and colonizing the area. Softwood trees such as pines often colonize an area first, followed by hardwood trees such as oaks and maples. These trees shade large areas below them. This alters the types of plants that grow in the area, as shade-tolerant plants replace the plants from earlier stages of succession. The larger plants provide food and habitats for larger animals. The interactions of all the organisms at each stage in this process change the environment and pave the way for the next group of organisms. 2

3 look out! Succession is a predictable shift in species from early stages to late stages. However, the plants present at each stage of succession vary based on the climate of the area. Not all areas can support large trees, and those that do may support different species of trees. In some areas the largest plants that can be supported are shrubs; in others, the environment limits the organisms to lichens and small plants. The organisms present at the peak of succession are part of the climax community for that area. A climax community occurs when the diverse species within the community can each maintain relatively stable populations over time. Temperature, water availability, food availability, human activity, and many other factors determine the organisms present in the climax community. Factors in the environment that limit the growth of plants and other organisms are called limiting factors. For example, a desert has very little water and does not allow for the growth of many large plants. In the desert, water is often a limiting factor and the climax community consists of drought-tolerant plants such as cacti. Secondary Succession Secondary succession occurs in locations that were previously inhabited but experienced a natural disaster such as a forest fi re, volcanic eruption, or fl ood. Secondary succession can also occur in the wake of human activity, such as clear-cutting a forest for timber. natural disaster: a force of nature that results in catastrophic consequences These events may kill many, but not all, of the organisms in an area. After the catastrophic event, a process similar to primary succession occurs. The main difference between primary and secondary succession is that in secondary succession the process begins with soil and some organisms already present, usually microscopic soil organisms. The catastrophic event generally results in a loss of large, shade-producing plants. The sudden loss of shade following a catastrophic event allows fast-growing annuals to thrive because these plants generally prefer full sunlight. Eventually, larger and slower-growing plants replace these smaller plants. Because secondary succession begins with soil and plants already present, it occurs much more quickly than primary succession. 3

4 Some natural disasters, such as forest fires, occur regularly in certain areas. Plants in these areas have adapted to these fires and some even require fi res to thrive. For example, some plant seeds will remain dormant in the soil until they are scorched by fire. Only then will they germinate. This adaptation allows these plants to succeed in areas that experience fi res on a regular basis. The ecosystems in these areas are even dependent on fi res to maintain the health of the community. Inhibiting fi res in these areas can cause lasting harm to the ecosystem. Naturally occurring fires tend to burn quickly and will mainly kill small plants and shrubs. These fi res clear the dead leaves and other matter from the forest fl oor. If humans extinguish these naturally occurring fires, the matter on the forest fl oor builds up over time. Then, when a fi re fi nally occurs that cannot be extinguished by humans, it burns hotter and longer. This type of fi re causes much more harm to an area and it will take longer to restore the climax community. Managing forest fi res, especially around areas of human inhabitation, is a challenging and important job that involves cooperation between many different local, state, and federal agencies. The images above show three stages of secondary succession after a fire destroyed the forest. what do you think? The image on the right shows lava from a volcanic eruption fl owing through a forest. Will the area affected by the lava fl ow likely undergo primary or secondary succession? Describe the steps that will likely occur. 4

5 Looking to the Future: The Importance of Biodiversity Later stages of succession tend to have increased biodiversity. Biological diversity, or biodiversity, is a measure of the variety of species and intricate interactions that support one another within a geographical region. Biodiversity includes the full variety of genes, species, communities, and ecosystems within a given space. For example, rainforests typically have high biodiversity. Many different species of organisms make their home in the rainforest, such as tree frogs, snakes, cats, trees, vines, fungi, and monkeys. In contrast, an industrial farm would be considered to have low biodiversity. Only a few species of plants are grown on the farm, such as wheat or corn. High biodiversity tends to add to the stability of an ecosystem. Biological diversity is impacted by catastrophic events, during which many species may be killed. The process of ecological succession can restore the biodiversity of an ecosystem. An area with high biodiversity is more likely to recover quickly from a catastrophic event. Human activities can lead to habitat destruction for organisms living in the ecosystem. The loss of habitat can decrease biodiversity in an area by making resources more scarce. Competition for dwindling resources usually results in fewer organisms. In some cases, loss of habitat may result in the permanent Habitat destruction decreases the biodiversity of an ecosystem. loss of species or genetic variety in species. Decreased biodiversity in an area also decreases the stability of the region and makes it less resilient to environmental changes. This is not only problematic for the plants and animal in the region, it also decreases resources available for humans. As the human population grows and expands farther into natural areas, habitat loss is increasing and biodiversity is decreasing. The careful balancing of human wants with environmental needs is essential for maintaining healthy ecosystems for people as well as other organisms. 5

6 What Do You Know? Put the following images in order for the process of secondary succession, starting with number 1 for the image that represents a natural disaster. Below the images, describe the typical organisms that grow in each step. 1: 2: 3: 4: 5: 6

7 connecting with your child Measuring Biodiversity Biodiversity can be challenging to measure and often requires complex calculations. However, one simplifi ed way to estimate biodiversity is to count the number of different types of organisms present in an area. This estimation can be used to compare different areas and identify which has a greater biodiversity. 1. Obtain the following materials: a. 10 stakes, posts, or twigs anything you can use to mark a point in the soil. b. Twine, yarn, rope any material that can measure a straight distance is fi ne. You should cut three pieces to the following lengths: 0.5 m, 3 m, 6 m. 2. Identify two different locations in a natural area, such as the bank of a stream, a desert ecosystem, or a forest fl oor community. If possible, identify two locations at different stages of succession. If not possible, any two locations can be used. 3. Use the 0.5-m length of twine and seven posts to make a circle with a radius of 0.5 m. a. Use one of the posts as the central point of the circle. Attach the twine to this post and mark six points on the outside of the circle with the remaining posts. 4. Identify and count all the herbaceous plants and seedlings in this circular area. Herbaceous plants and seedlings are defined as those having stems with a diameter less than 2.5 cm. a. If the plant is easy to identify, then do so; however, it is not essential that you identify the plant correctly. You should try to determine how many different species and how many of each species there are. These plants can be identified simply as herb 1, herb 2, shrub 1, etc. 5. Identify and count any animals in the circle. To do this, you may have to dig through the litter on the soil surface. You will likely fi nd many types of invertebrates living on or just under the soil, such as worms, ants, or beetles. 6. Using the original center post, mark an expanded circle with a radius of 3 m. 7. Identify and count the different types of shrubs, vines, and saplings present in the plot. These are defi ned as having a stem diameter of cm. 8. Using the original center point, mark an expanded circle with a radius of 6 m. 7

8 9. Identify and count the different types of trees present in the plot. The data can be entered in a table similar to the one below. Type Herb 1 Herb 2 Herb 3 Animal 1 Animal 2 Shrub 1 Shrub 2 Tree 1 Number present in Area 1 Number present in Area 2 Here are some questions to discuss with your child: Which area has greater biodiversity? What components of the environment may account for this variation in biodiversity? (For example, does one area have a more readily available water source?) If areas in different stages of succession were selected, what is the correlation between stages of succession and biodiversity? 8