Community Stability Guiding Question: How do communities respond to a disturbance? LESSON 4 • Describe what happens to a community after a Reading Strategy As you read, create a flowchart disturbance. that summarizes the steps of both primary and secondary • Explain the conditions necessary for a species to succession. become invasive. 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 5.4 LESSON PLAN PREVIEW ranging from gradual phenomena like climate changes to sudden events Differentiated Instruction such as storms, floods, or fire. Disturbances can modify the composition, Pairs stop and discuss lesson structure, or function of an ecological community. How communities passages as they read. respond to disturbances is a measure of their stability—or lack thereof. Real World Students consider the consequences of introduc- Ecological Succession ing exotic species as they read. Following a disturbance, communities may undergo 5.4 RESOURCES succession. In Your Neighborhood Activity, Inva- sive Organisms Near You • Lesson 5.4 Worksheets • Lesson 5.4 Assessment A community described as being in equilibrium is generally stable and • Chapter 5 Overview Presentation balanced. Normally, species interactions and limiting factors hold their populations at or around carrying capacity. Sometimes, however, distur- bances throw a community into disequilibrium. Limiting factors shift, GUIDING QUESTION altering carrying capacities. Population sizes change, and the community FOCUS Tell students that a distur- enters a period of adjustment. bance is any change in an ecosys- Some communities return to their original state following a distur- tem. Have students brainstorm a list bance, but other communities are changed permanently. Usually, perma- of disturbances that can occur in ecosystems. Record their responses nent change happens in response to severe disturbances that eliminate all on the board. Then, for each dis- or most of the species in a community. When this occurs, a community turbance on the list, discuss how it experiences a somewhat predictable series of changes over time that might impact the communities in ecologists call succession. Ecologists recognize two traditional types of the ecosystem. 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. 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 BIG QUESTION Primary Succession When a disturbance is so severe that no vegeta- tion or soil life remains, primary succession occurs. In primary succes- How do organisms affect one another’s survival and environ- sion, a community is built essentially from scratch. Primary succession ment? takes place after a bare expanse of rock, sand, or sediment is exposed Application Use the concept of for the first time. This can occur when glaciers retreat, lakes dry up, or primary succession after a glacial volcanic lava or ash spreads across the landscape. Species that colonize retreat to talk about how the envi- the newly exposed land first are called pioneer species. Pioneer species ronment can affect organisms’ survival and vice versa. Have are well adapted for colonization. For example, they often have spores or students suggest at least two ways seeds that can travel long distances, helping them spread quickly across the environment can shape the the land. communities that grow in the area Lichens are particularly successful pioneers of bare rock. Lichens are and two ways organisms can alter formed by a mutualistic relationship between algae and fungi. The algae the environment during primary succession. provide food and energy via photosynthesis, while the fungi take a firm hold on rock and capture the moisture that both organisms need to sur- vive. As lichens grow, they release acids that break down the rock surface FIGURE 29 Primary Succession into the beginnings of soil. Once soil begins to form, small plants, insects, When a glacier retreated from Glacier and worms can move in. As new organisms arrive, they change the envi- Bay, Alaska, barren rock was exposed. ronment by providing more nutrients and habitat for future arrivals. As For more than 200 years, primary time passes, larger plants establish themselves, the amount of vegetation succession has been occurring. Today, a hemlock and spruce forest grows in increases, and species diversity increases. An example of primary succes- the area. sion is shown in Figure 29. Time 15 years 35 years 80 years 115+ years 150 Lesson 4 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 suc- ANSWERS cession usually occurs faster than primary succession. Consider the abandoned agricultural field in eastern North America, Reading Checkpoint Secondary succession is usually faster than shown in Figure 30. In the first few years after farming ended, the site primary succession because the soil, was colonized by grasses and herbs that were already in the area. (In and possibly some living things from comparison, it can take more than fifteen years for lichens to colonize the previous ecosystem, remain. 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, form- FIGURE 30 Secondary Succession ing a pine-dominated forest. This pine forest developed an understory of Secondary succession occurs after a disturbance, such as fire, flood, or hardwood trees that grow well under a canopy. Eventually, the hardwoods farming, removes most—but not all— outgrew the pines, creating the hardwood forest that grows there today. vegetation from an area. The typical series of changes in a plant community Reading Why does secondary succession usually happen faster than Checkpoint of eastern North America is shown primary succession? here. Secondary succession began in many areas when farm fields were abandoned. Time 3 years 5 years 40+ years Evolution and Community Ecology 151 Successful Succession? Analyze and Conclude 1 Obtain2 3 a clean4 jar5 with6 a 7cover8 and9 place a handful of dried 1. Infer Why did you use boiled or sterile plant material into the jar. water? 1 2 Fill3 the4 jar 5with6 boiled7 pond8 water9 or sterile spring water. 2. Infer Where did the organisms you saw 1 2 3 Cover4 5 the jar6 and7 place8 it9 in an area that receives indirect come from? light. 3. Draw Conclusions Was ecological succes- 1 2 3 4 Examine5 6 the7 jar8 every9 day for the next few days. sion occurring? Give evidence to support your answer. 1 2 3 4 5 When6 7the jar8 appears9 cloudy, prepare microscope slides of Evaluate and Revise water from various levels of the jar. Use a pipette to collect 4. Check your results the samples. against those of your classmates. Do they agree? Give possible explanations for any 1 2 3 4 5 6 View7 the8 slides9 under the low-power objective microscope differences. and record your observations. Succession in the Water ANSWERS Succession occurs in aquatic systems, too. Primary aquatic succession takes place when an area fills in with water Quick Lab for the first time. This can happen, for example, when glaciers retreat and 1. To make sure there weren’t any liv- leave depressions in the ground. Over time, aquatic communities become ing organisms already in the water 2. From the dried plant material established as the water becomes richer in nutrients. 3. Yes. The community changed over Disturbances to aquatic communities, such as floods or excess run- time. off, can lead to secondary succession. One classic example of secondary 4. Answers will vary. 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 2 Soil, fallen leaves, and nutrients support more plant decaying matter pile up on the growth. 3 Eventually the lake lls in, bottom of the lake. The lake creating a grassy meadow. becomes shallower and marshy. 152 Lesson 4 Climax Communities In the traditional view of succession that we FiGURE 32 Not So Stable? have described, the transitions between stages of succession eventually Beech-maple forests , such as this one in Vermont, were once classic examples lead to a climax community. A climax community is a stable community of stable climax communities. that “completes” the succession process. Ecologists used to think that each Ecologists now think that ecosystems region had its own characteristic climax community determined by the such as this might not be as uniform nor as stable as once thought. region’s climate. However, ecologists now know that not only climate but What Do soil conditions and other factors influence a community’s composition. you think? Further, various conditions can promote or inhibit a community’s pro- gression between succession stages. ANSWERS Many ecologists now view communities as temporary, ever-changing What Do You Think? Answers will associations among individual species—not as cohesive and predeter- vary, but students should apply mined units. Once a climax community is disturbed, there is no guar- information from the chapter to antee that the community will ever return to that climax state.