Community Stability. Ecological Succession

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1 Community Stability Guiding Question: How do communities respond to a disturbance? LESSON 4 Describe what happens to a community after a disturbance. Explain the conditions necessary for a species to become invasive. Reading Strategy As you read, create a flowchart that summarizes the steps of both primary and secondary succession. Vocabulary succession, primary succession, pioneer species, secondary succession, invasive species Disturbances are common in nature. A disturbance is any change in a community s environment, large or small. Over time, a given community may experience natural or human-caused disturbances ranging from gradual phenomena like climate changes to sudden events such as storms, floods, or fire. Disturbances can modify the composition, structure, or function of an ecological community. How communities respond to disturbances is a measure of their stability or lack thereof. Ecological Succession Following a disturbance, communities may undergo succession. A community described as being in equilibrium is generally stable and balanced. Normally, species interactions and limiting factors hold their populations at or around carrying capacity. Sometimes, however, disturbances throw a community into disequilibrium. Limiting factors shift, altering carrying capacities. Population sizes change, and the community enters a period of adjustment. Some communities return to their original state following a disturbance, but other communities are changed permanently. Usually, permanent change happens in response to severe disturbances that eliminate all or most of the species in a community. When this occurs, a community experiences a somewhat predictable series of changes over time that ecologists call succession. Ecologists recognize two traditional types of succession: primary and secondary. Although usually described in terms of plant species, animals and other members of the community also change over the course of succession. 5.4 LESSON PLAN PREVIEW Differentiated Instruction Pairs stop and discuss lesson passages as they read. Real World Students consider the consequences of introducing exotic species as they read. 5.4 RESOURCES In Your Neighborhood Activity, Invasive Organisms Near You Lesson 5.4 Worksheets Lesson 5.4 Assessment Chapter 5 Overview Presentation GUIDING QUESTION FOCUS Tell students that a disturbance is any change in an ecosystem. Have students brainstorm a list of disturbances that can occur in ecosystems. Record their responses on the board. Then, for each disturbance on the list, discuss how it might impact the communities in the ecosystem. Figure 28 Disturbances A disturbance, such as a fire, can disrupt a community and start a series of changes called succession. Evolution and Community Ecology 149

2 BIG QUESTION How do organisms affect one another s survival and environment? Application Use the concept of primary succession after a glacial retreat to talk about how the environment can affect organisms survival and vice versa. Have students suggest at least two ways the environment can shape the communities that grow in the area and two ways organisms can alter the environment during primary succession. Figure 29 Primary Succession When a glacier retreated from Glacier Bay, Alaska, barren rock was exposed. For more than 200 years, primary succession has been occurring. Today, a hemlock and spruce forest grows in the area. 15 years 150 Lesson 4 Primary Succession When a disturbance is so severe that no vegetation or soil life remains, primary succession occurs. In primary succession, a community is built essentially from scratch. Primary succession takes place after a bare expanse of rock, sand, or sediment is exposed for the first time. This can occur when glaciers retreat, lakes dry up, or volcanic lava or ash spreads across the landscape. Species that colonize the newly exposed land first are called pioneer species. Pioneer species are well adapted for colonization. For example, they often have spores or seeds that can travel long distances, helping them spread quickly across the land. Lichens are particularly successful pioneers of bare rock. Lichens are formed by a mutualistic relationship between algae and fungi. The algae provide food and energy via photosynthesis, while the fungi take a firm hold on rock and capture the moisture that both organisms need to survive. As lichens grow, they release acids that break down the rock surface into the beginnings of soil. Once soil begins to form, small plants, insects, and worms can move in. As new organisms arrive, they change the environment by providing more nutrients and habitat for future arrivals. As time passes, larger plants establish themselves, the amount of vegetation increases, and species diversity increases. An example of primary succession is shown in Figure years Time 80 years 115+ years

3 Secondary Succession Secondary succession, unlike primary suc- cession, begins when a disturbance, such as a fire, logging, or farming, dramatically alters an existing community but does not destroy all living things or all organic matter in the soil. In secondary succession, at least the soil from the previous ecosystem remains. As a result, secondary succession usually occurs faster than primary succession. Consider the abandoned agricultural field in eastern North America, shown in Figure 30. In the first few years after farming ended, the site was colonized by grasses and herbs that were already in the area. (In comparison, it can take more than fifteen years for lichens to colonize bare rock in primary succession.) As time passes, shrubs and fast-growing trees such as aspens rose from the field. Pine trees then moved in, forming a pine-dominated forest. This pine forest developed an understory of hardwood trees that grow well under a canopy. Eventually, the hardwoods outgrew the pines, creating the hardwood forest that grows there today. Reading Checkpoint hy does secondary succession usually happen faster than W primary succession? 3 years 5 years Time Reading Checkpoint Secondary succession is usually faster than primary succession because the soil, and possibly some living things from the previous ecosystem, remain. Figure 30 Secondary Succession Secondary succession occurs after a disturbance, such as fire, flood, or farming, removes most but not all vegetation from an area. The typical series of changes in a plant community of eastern North America is shown here. Secondary succession began in many areas when farm fields were abandoned. 40+ years Evolution and Community Ecology 151

4 Successful Succession? 1 Obtain a clean jar with a cover and place a handful of dried plant material into the jar. 2 Fill the jar with boiled pond water or sterile spring water. 3 Cover the jar and place it in an area that receives indirect light. 4 Examine the jar every day for the next few days. 5 When the jar appears cloudy, prepare microscope slides of water from various levels of the jar. Use a pipette to collect the samples. 6 View the slides under the low-power objective microscope and record your observations. Analyze and Conclude 1. Infer Why did you use boiled or sterile water? 2. Infer Where did the organisms you saw come from? 3. Draw Conclusions Was ecological succession occurring? Give evidence to support your answer. 4. Evaluate and Revise Check your results against those of your classmates. Do they agree? Give possible explanations for any differences. Quick Lab 1. To make sure there weren t any living organisms already in the water 2. From the dried plant material 3. Yes. The community changed over time. 4. Answers will vary. Succession in the Water Succession occurs in aquatic systems, too. Primary aquatic succession takes place when an area fills in with water for the first time. This can happen, for example, when glaciers retreat and leave depressions in the ground. Over time, aquatic communities become established as the water becomes richer in nutrients. Disturbances to aquatic communities, such as floods or excess runoff, can lead to secondary succession. One classic example of secondary aquatic succession is shown in Figure 31. As algae, microbes, plants, and zooplankton (small floating animals) grow, reproduce, and die, they can gradually fill the pond with organic matter. Organic matter and nutrients may also enter the pond through streams and rivers. Eventually, the pond may fill in completely, and a terrestrial ecosystem can establish itself. Figure 31 Aquatic Succession Secondary succession in lakes and ponds usually occurs over a number of years. 1 Algae and other organisms add nutrients to the lake. These nutrients support more plant growth. 2 Soil, fallen leaves, and decaying matter pile up on the bottom of the lake. The lake becomes shallower and marshy. 3 Eventually the lake fills in, creating a grassy meadow. 152 Lesson 4

5 Climax Communities In the traditional view of succession that we have described, the transitions between stages of succession eventually lead to a climax community. A climax community is a stable community that completes the succession process. Ecologists used to think that each region had its own characteristic climax community determined by the region s climate. However, ecologists now know that not only climate but soil conditions and other factors influence a community s composition. Further, various conditions can promote or inhibit a community s progression between succession stages. Many ecologists now view communities as temporary, ever-changing associations among individual species not as cohesive and predetermined units. Once a climax community is disturbed, there is no guarantee that the community will ever return to that climax state. Though communities, such as the beech-maple forest in Figure 32, may appear stable over long periods, they are often not as uniform as they seem. Disturbances, small and large, are constantly affecting them. Invasive Species Without limiting factors, species introduced to a new area can become invasive. Traditional concepts of succession involve sets of organisms native to an area. A pine forest, for example, would not grow in an area where pine trees don t usually live. But what if a new organism arrives from elsewhere? Sometimes these nonnative, or exotic, organisms can turn into invasive species. An invasive species is a nonnative organism that spreads widely in a community. Invasive species are one type of community disturbance and a major problem in many parts of the world. The zebra mussel is an example of an invasive species. Reading Checkpoint What is an invasive species? Figure 32 Not So Stable? Beech-maple forests, such as this one in Vermont, were once classic examples of stable climax communities. Ecologists now think that ecosystems such as this might not be as uniform nor as stable as once thought. What Do You Think? Answers will vary, but students should apply information from the chapter to justify their responses. Reading Checkpoint A nonnative organism that spreads widely in a community What Do you think? Are invasive species all bad? Some people have questioned the notion that all invasive species should be considered a problem. Is it always bad to change a native community? Does it make a difference whether the invasive species arrived on its own or through human intervention? Evolution and Community Ecology 153

6 Reading Checkpoint They were intentionally introduced to help control the cane beetle population. What Makes a Species Invasive? Not all exotic species turn invasive. Some exotic species may remain small and localized, eventually dying out. Others may simply exist without causing problems. Species only become invasive when such limiting factors as predators, parasites, or competitors are not present in their new environment. As a result, population growth of the exotic species is not held in check. The community is thrown out of balance as native species are eliminated through predation or herbivory, or simply out-competed for resources. Examples of Invasive Species There are many examples of introduced species that have turned invasive and have had major ecological effects. As illustrated in the examples that follow, invasive species can be introduced to an area intentionally or by accident. The Zebra Mussel A wide variety of techniques to control the spread of zebra mussels has been tried, including removing them by hand, applying toxic chemicals, drying them out, introducing predators and diseases, and stressing them with heat, sound, electricity, carbon dioxide, or ultraviolet light. However, most of these are only short-term fixes that don t make a dent in the huge populations of the mussels in the Great Lakes. Figure 33 Zebra mussels Figure 34 Cane toads The Cane Toad The cane toad is a toxic organism native to Central and South America. In other parts of the world, notably in Australia, it s an invasive species. Cane toads were brought to Australia intentionally in 1935 to help rid sugar farms of the cane beetle pest. Unfortunately, the cane toad did not eat the cane beetle as predicted, but instead bred rapidly and began to spread. Without its native predators, like the black rat and water monitor, the cane toad population has been growing exponentially. Many native species have been out-competed by the toads, and their poisonous skin often kills animals that try to eat them. Reading Checkpoint Why was the cane toad introduced to Australia? 154 Lesson 4

7 Kudzu Plants can be invasive, too. Kudzu was introduced to the United States from Japan in At the time, the United States government encouraged farmers in the Southeast to plant kudzu to prevent soil erosion. However, kudzu quickly became invasive. The lack of freezing temperatures and natural predators enabled kudzu to spread and literally cover the land from Texas to southern New Jersey. Figure 35 Kudzu The Honeybee Not all invasive species are bad. The European honeybee is thought to have evolved in Africa, and then spread to Europe and Asia. Colonists to North America brought the bees with them, and they rapidly spread, becoming invasive. However, they are far from harmful! European honeybees pollinate most of America s commercial crops, providing billions of dollars to our economy. What Can Be Done? Today, many ecologists think that invasive species are the second-greatest threat to species and natural systems, behind habitat destruction. In 1990 the U.S. Congress passed the Nonindigenous Aquatic Nuisance Prevention and Control Act, which, in 1996, became the National Invasive Species Act. Since then, funding has become widely available for the control of invasive species. However, in most cases, preventing the introduction of invasive species is a much better investment than trying to control them once they re here. Figure 36 European honeybee Lesson 4 Assessment For answers to the Lesson 4 Assessment, see page A 7 at the back of the book Compare and Contrast What are the major differences between primary and secondary succession? 2. Infer The cane toad was brought to Australia from the island of Hawaii. The toad had been introduced to Hawaii some time earlier, but has not had the same kind of destructive effects there as it has in Australia. What could explain why the toad has not become invasive in Hawaii as it did in Australia? 3. A federal agency has put you in charge of responding to the zebra mussel invasion. Based on what you ve learned in this lesson, how would you try to control the mussel s spread and impact? Evolution and Community Ecology 155