SELF-STUDY GUIDE TO ECOLOGICAL SUCCESSION (for Cunningham & Saigo, 10th edition) Introduction The purpose of this guide is to provide a tutorial on ecological succession. The only resource you will need is your textbook. You may work through the guide alone, or with a group of other students. This material will not be covered in lecture, so study it carefully and ask me if you have any questions about it. Ecological Succession Ecosystems are damaged by many impacts, some natural, some caused by humans. Examples of natural disturbances are volcanic eruptions (such as Mt. St. Helens) or forest fires started by lightening. Humans disturb pre-existing ecosystems when we clear land for farms or when we dam rivers and create artificial lakes. Such disturbances kill or remove many of the ecosystem’s plants and animals, and alter environmental conditions such as soil and light. The result can be a very different ecosystem or, in severe cases, no surviving system at all. Our skin has a built-in ability to repair damage. Can ecosystems repair damage done to them, or are they permanently altered by disturbances? The answer lies in the process of ecological succession. How does your textbook define ecological succession? (try the Glossary) This definition is a good general one, but it leaves out the idea that ecological succession often enables a disturbed ecosystem to return to its original state. It is a process that repairs damage. Let’s see how this occurs. There are two types of ecological succession, primary and secondary. Read about them and then define them in your own words: Primary succession: Secondary succession: What do they have in common? What is the key difference between them? Think of an example of each that is not discussed in your textbook. Is the disturbing factor natural or human? Bio. 112 Ecological Succession, p. 2 Successional plant communities The first plant species to establish themselves in a region undergoing succession are termed pioneer species. The term is taken from human society, where pioneers are people who establish themselves in an area that was previously unsettled. Imagine that you are a plant trying to grow in a recently disturbed site - what is the environment like for you if the site is: the top of a boulder: a sand dune: an abandoned farm field: In cases of primary succession, the initial environment is harsh, mostly because of the absence of soil. Pioneer plants there suffer from lack of moisture, nutrients, and a way of anchoring their roots. According to your textbook, what types of plant might begin primary succession on bare rock? How do you think they cope with the harsh conditions present there? Now, here is the key idea of primary succession: pioneer species alter the environment so that a different set of species can invade. How, according to the textbook, do lichens and mosses change a bare rock site so that other plants are capable of living there? In cases of secondary succession, the soil is usually intact, so that conditions are not so difficult. The pioneer species are normally those that get there first. Many plants that we call “weeds” are species adapted to invading open sites of secondary succession rapidly. They use two different techniques: Seed dispersal: think of dandelion seeds - how are they adapted for reaching open patches of ground? Resistant seeds: many weeds have seeds that can remain viable while buried in the soil for long periods of time. In either primary or secondary succession, there’s plenty of sunlight, so the pioneer species are (1) sun-loving and (2) fast-growing. Later species take longer to get there and longer to grow to full size. Bio. 112 Ecological Succession, p. 3 Species replacement during succession You should understand now that pioneer species change the environment so as to favor the growth of new plant species (call them mid-successional species). But why do these species replace the pioneers? Why can’t the pioneers continue living there? The main answer to this question is simple: shade. Each succeeding group of plants is taller than the previous species and out-competes them for light. In fact, one author has described ecological succession, particularly secondary succession, as a “race for the sun.” Here’s a sequence of plants for a typical example of primary succession, a beach dune in northern Indiana: beach grasses -> shrubs -> cottonwoods -> pines -> oaks -> beech/maples (pioneers) (climax) Complete the table below during the class field trip: list a typical sequence of species for secondary succession in an abandoned field in western New York State. What is the approximate timing of each stage? How does the site and its environment change during the successional series? Dominant Plant Species Time (years) Community Characteristics Other notes: Bio. 112 Ecological Succession, p. 4 Characteristics of pioneer and climax species The characteristics of the plants change predictably during both primary and secondary sequences: Pioneer species Climax species Adapted to dry soils with few nutrients Require moist, fertile soils Sun-loving; can’t grow in shade Shade-loving; seedlings can grow under shade of adults Full-grown plants short, cast little shade Full-grown plants tall, cast dense shade Rapid growth; little permanent (woody) tissue Slow growth; trees with large woody trunks Most energy put into growth and reproduction Most energy put into maintaining adult plants Describe, in a few sentences, how these characteristics adapt each type of species to the conditions present during their position in the succession process: Why do mid to late-successional species put so much of their energy and nutrient resources into woody tissue? The “endpoint” of succession Once the climax vegetation for a given region takes over, the pace of succession slows way down. The ecosystem enters a period of stability or steady state. There’s a simple explanation for this fact: once the area is taken over by a plant species which is adapted to the environment and has seedlings capable of reproducing under the shade of adults of the same species, that species can block further replacement. Climax species in the eastern U. S., such as oak, maple and beech trees, can live to ages of several hundred years, so any remaining changes are slow. At the same time, a climax is not permanent. New disturbances can remove the climax vegetation and begin the process of succession over again. For example, a maple forest growing on an old sand dune can be damaged by severe storms, toppling trees and “blowing out” the soil. A patch of the forest is thereby returned to bare sand and primary succession begins again. Or, a forest fire may kill mature hardwoods and trigger new secondary succession. Mosaic landscapes Imagine for a moment two extremes: (1) Humans take an extreme position of environmental preservation and treat all disturbances as evil. Ecosystems are protected from all changes. Now wait 100+ years. What has happened to pioneer and mid-successional species, and why? (2) Human disturbance of ecosystems is frequent and widespread. Forests are cut and land cleared for farming or development every 20-30 years. What sort of species are left in the region, and why? Bio. 112 Ecological Succession, p. 5 The pace of ecological succession It might seem as if we can damage ecosystems without concern, because they have this marvelous ability to renew themselves through succession. There are two reasons to be cautious, however. First, ecosystems take a long time to recover from disturbances. How long would it take a hardwood forest in the eastern U. S. to recover from being completely cleared? In cold or dry climates the pace of succession is even slower; in some parts of Alaska which are recovering from glaciers, fully fertile soils and climax communities take over 400 years to appear. Second, some disturbances are so severe or widespread that the ecosystem cannot recover. To go back to the analogy of human skin, not all wounds heal. What is the message of Figure 14-23 (p. 321)? Human management of ecosystems Ecological succession is more than a fact about ecosystems: it affects our lives in several ways. For example, what would happen if you stopped mowing your lawn for a few years? The time and energy required to maintain a lawn or golf course in the eastern U. S. is partly due to an attempt to fight nature, to keep ecological succession from occurring. Our fight against weeds is also an example of succession at work in our lives. Weeds are pioneer plant species. Weeds can be thought of as plants that (1) are good at dispersing their seeds rapidly into freshly disturbed soil (gardens or farm fields); (2) have strong root systems; and (3) grow rapidly and choke out the plants we want to grow. How are these the adaptations of pioneer species? We tend to maintain our gardens and farms in early successional stages for a good reason: early successional species grow rapidly and put most of their energy into root, leaf and seed production, rather than woody growth. Roots, leaves and seeds, of course, are what we mainly use for food. Even when we grow trees, why do we prefer fast-growing, mid-successional species such as pines to climax species such as oaks and maples? There is even some evidence that the development of cities may follow a path analogous to ecological succession. Pioneer societies exhibit rapid population and economic growth. As cities “age”, more of their resources (tax dollars??) go into maintaining the “infrastructure” (roads, utilities, police…) and less is available for economic growth. Study the following graph of the costs of services (=maintenance) per capita (person) for different size cities in Ohio. Is it possible that large cities are reaching a “climax stage” in which they cost so much to run that there is no potential for future growth? Bio. 112 Ecological Succession, p. 6 Use the space below to summarize the major points you’ve learned about ecological succession in a brief outline. Are there any unclear points?
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