Teacher Notes for
Changing Biological Communities – Disturbance and Succession[1]
This analysis and discussion activity helps students to understand how biological communities change during succession after a disturbance. Students analyze research evidence and explore how the interactions between different types of plants and animals influence succession. Students use their understanding of the processes involved in succession to construct and evaluate models of succession in abandoned farm fields. Students also analyze the effects on succession of climate and non-native invasive plants.
Learning Goals
· A biological community consists of all the types of organisms that live in an area. The different types of organisms interact in a variety of ways including competition and consumption.
· Although biological communities often appear stable from year to year, major changes are common over periods of many years. A biological community may be disrupted by a disturbance (e.g. due to human activity such as farming). After a disturbance ends, natural processes lead to gradual change in the biological community. This process is called succession and often extends over many decades.
· During succession, the organisms in a biological community change the environment in ways that may facilitate or inhibit the growth of other organisms. For example, the growth of trees or vines may reduce the growth of other types of plants that are less successful in competing for sunlight. The changing types of plants in a biological community during succession can provide food that can support new types of animals in the community. Conversely, animals that eat plants can reduce the growth and reproduction of plants and thus influence the rate of plant succession.
· Succession often restores the biological community that was present before a disturbance. However, under some circumstances, succession does not restore the original biological community. For example, disturbance may allow an invasive non-native species to become established which can interfere with restoration of the original biological community.
· An ecosystem includes the physical environment as well as the biological community in an area. The physical environment affects how succession proceeds. For example, in regions with limited precipitation, succession often culminates in grasslands, whereas, in regions with more precipitation, succession often culminates in forests. This explains why the central part of the US has a grassland biome, whereas the eastern part of the US has a forest biome.
· Different models of succession show different aspects of succession.
In accord with the Next Generation Science Standards[2]:
· This activity will help students to meet the Performance Expectation:
HS-LS2-6. "Evaluate the claims, evidence and reasoning that the complex interactions and ecosystems maintain relatively consistent numbers and types of organisms in stable conditions, but changing conditions may result in a new ecosystem."
· This activity helps students to understand the Disciplinary Core Idea, LS2.C, Ecosystem Dynamics, Functioning and Resilience:
"If a modest biological or physical disturbance to an ecosystem occurs, it may return to its more or less original state (i.e. the ecosystem is resilient), as opposed to becoming a very different ecosystem. Extreme fluctuations in conditions or the size of any population, however, can challenge the functioning of ecosystems in terms of resources and habitat availability. Moreover, anthropogenic changes (induced by human activity) in the environment – including habitat destruction, pollution, introduction of invasive species, overexploitation, and climate change – can disrupt an ecosystem and threaten the survival of some species."
· In this activity students engage in several scientific practices – interpreting data, constructing explanations, engaging in argument from evidence, and developing and using models.
· This activity will provide the opportunity to learn about three crosscutting concepts – stability and change; systems and system models; and cause and effect.
· This activity can be used to reinforce a student understanding about the nature of science:
"Science knowledge is based on empirical evidence."
Instructional Suggestions and Background Information
It will be helpful if your students are familiar with biological communities and the types of interactions within biological communities before beginning this activity.
To maximize student participation and learning, I suggest that you have your students work individually, in pairs, or in small groups to answer a question or small group of questions; then, after each question or group of questions, have a class discussion of student answers to probe their thinking and guide them to a sound understanding of the concepts and information before moving on to the next section. You may want to try having your students work in groups of four, each with a specific task. One student is the facilitator (responsible for keeping the group on task and time management). Another student is the reader (reads information out loud). Another student is the recorder (records the answers in the Student Handout that the group turns in). The fourth student in each group is the spokesperson (reports out to the class during class discussions).[3]
A key is available upon request to the author (). The following paragraphs provide additional background information and instructional suggestions.
To foster student understanding of the biological processes involved in succession, this activity focuses on in-depth analysis of succession in one context, abandoned agricultural fields. In the early stages of succession after an agricultural field has been abandoned, the biological community consists primarily of plants that (1) have seeds that disperse readily over substantial distances and (2) grow and reproduce rapidly. These pioneer plants may provide shade and increase soil nutrients which may facilitate the growth of plants that are observed later in the succession. On the other hand, pioneer plants may compete for sun and water and secrete chemicals that inhibit new types of plants from becoming established. In the later stages of succession, large long-lived plants that are good competitors for light and water become more abundant. Often a forest develops (if there is sufficient rainfall and not too much fire or herbivory). Sometimes, succession is interrupted; for example, a tree may die of disease or fall in a windstorm and this creates a small gap in the forest with increased sunlight where succession begins anew.
The stages shown in the figure on page 1 of the Student Handout typically overlap during succession. For example, young trees develop among the herbaceous plants and shrubs. This type of succession has been observed in temperate areas with sufficient rainfall in North America, Europe and Japan.
The graph on page 2 of the Student Handout is based on data from a study of abandoned farm fields in the Piedmont area of northwestern New Jersey. This graph (and the accompanying text box) illustrates several generalizations:
· Change during succession is gradual.
· During most years of succession in abandoned farm fields, multiple types of plants coexist (e.g. herbaceous plants, shrubs and trees, at least tree seedlings or saplings).
· Succession to forests in abandoned farm fields takes multiple decades, mainly due to the slow development of trees.
These generalizations are discussed in questions 2, 3 and 6.
If your students are not experienced interpreters of graphs, it will obviously be helpful to make sure they understand what each axis and symbol represents. To help students understand the concept of percent area covered by a particular type of plant, you could ask them to estimate the percent of area in the photo covered by herbaceous plants. (I would estimate 100% since no bare ground or woody plants are visible.) You also may want to ask your students to link different periods in the graph to the corresponding parts of the picture of succession on page 1 of the Student Handout.
Results from this study showed that annual plants became less common in the later stages of succession. In the one-year-old fields, 28% of the 94 species of plants were annuals; in contrast, in the 60-year-old fields, 13% of the 104 species of plants were annuals. Plants that thrive in full sun and tolerate relatively dry conditions were more abundant early in succession, whereas plants that tolerate partial shade and prefer moister soil were more abundant later in succession. (Similar trends have been observed in many other studies of succession.)
The researchers found that the forests observed 60 years after a farm field had been abandoned differed significantly from a nearby fairly undisturbed mature forest which contained some trees that were at least 250 years old. This mature forest had taller trees, especially oaks, whereas the 60-year-old forests had mainly red cedar. Another striking difference was the poison ivy vines festooned over many of the red cedar trees at 60 years, but almost absent in the mature forest.
The rate of succession is influenced by multiple factors. For example, the rate of succession can be slowed by the effects of deer and rodents that eat seeds and tree seedlings.[4] The rate of succession is more rapid if there are nearby sources of seeds (e.g. trees in hedgerows) and if the native plants have seeds that disperse more widely and rapidly (e.g. seeds that are dispersed by wind or birds). Also, the rate of succession is more rapid in the southeastern US than in the northeastern US, in large part because there are more growing days per year due to the warmer temperatures.
Question 6 in the Student Handout emphasizes the point that many decades are required before succession can produce a forest, so very long-term evidence is needed to show that succession has failed to restore a forest biological community. This type of long-term evidence that succession does not always restore the original biological community is provided by the example on the top of page 4 of the Student Handout. This example illustrates how non-native invasive[5] plants can interfere with the expected process of succession and also shows how chance factors (such as where a seed for a non-native invasive plant happens to germinate) can have a dramatic effect on succession. Other factors that can interfere with the restoration of the original biological community include soil degradation, an absence of seed sources for native plants, removal of predators, and the introduction of non-native pathogens (e.g. a fungal blight that killed chestnut trees in North America during the twentieth century). To some extent these problems can be prevented, e.g. by preventing the introduction of non-native species. Also human intervention can help to restore native biological communities; for example, during the twentieth century tree planting and the establishment of state and national forests contributed significantly to succession that restored large areas of forest in the northeastern US. An interesting review of restoration ecology summarizes additional ecological concepts needed to understand and design successful restoration (http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2005.00764.x/pdf).
Question 8 in the Student Handout is designed to reinforce student understanding of how the biological processes discussed thus far interact to influence succession in abandoned farm fields. By actively synthesizing the information, students should improve their understanding of how the growth and reproduction of individual organisms and the interactions between individuals from different species in a biological community contribute to succession. Your students may find this question quite challenging, especially if they have not had previous experience with interpreting and making flowcharts. To help your students develop skills and understanding, you may want to have them work in pairs or small groups to answer question 8, then share their different versions of the overall flowchart, and discuss the relative merits of different ways of representing the processes involved in succession (including the version of the flowchart shown in the key, which can be obtained by writing ).
Question 9 in the Student Handout encourages students to link differences in the amount of precipitation to differences in succession and biomes. Other factors can also influence succession and biomes. For example, fires and grazing animals can help to maintain grasslands by killing tree seedlings and saplings or preventing them from growing. Although fire destroys the dry stems and leaves of grasses, the deep roots of grasses often remain unharmed and grasses can regrow from these roots after a fire. Thus, repeated disturbance can contribute to the maintenance of grasslands. A helpful overview of a broad range of terrestrial and aquatic biomes is available at http://www.ucmp.berkeley.edu/glossary/gloss5/biome/. It should be noted that much of central and eastern United States does not have the natural biomes shown in the Student Handout. Instead, much of this area is covered by anthropogenic biomes (e.g. croplands and urban areas; illustrated at http://sedac.ciesin.columbia.edu/downloads/maps/anthromes/anthromes-anthropogenic-biomes-world-v2-1900/anthromes-v2-1900-namerica.jpg).
This activity has focused on more in-depth analysis of succession in abandoned farm fields in order to foster student understanding of the biological processes involved in succession. Some of the generalizations would not apply in other contexts. For example, the idea that "Often, succession gradually restores a biological community so it becomes similar to the community that existed before the disturbance" would need to be modified in discussing primary succession. Although succession in abandoned farm fields extends over multiple decades, succession in some aquatic environments such as streams may occur much more quickly (in weeks or months). Two useful introductions to other examples of succession and general principles of succession are available at http://www.countrysideinfo.co.uk/successn/index.htm and http://sky.scnu.edu.cn/life/class/ecology/chapter/Chapter20.htm. The focus on plants and animals in this activity is intended to facilitate student understanding, but has the disadvantage of omitting the important roles of prokaryotes and fungi. Also, this activity focuses on change during multiple decades, but does not consider the broader changes that can occur over centuries and millennia (e.g. due to climate change).
Optional Additional Questions
If you would like to introduce your students to the roles of observational and experimental evidence in studying succession, you can use the following optional additional questions.
In former farm fields in the central US researchers measured the plants under sumac shrubs and in nearby comparison areas. The researchers also did an experiment to evaluate the effect of shade on the ability of herbaceous plants to sprout from seeds in the soil. Their results are shown in this table.
Observations-There was much less grass and fewer other herbaceous plants in areas under shrubs than in equal-sized nearby areas that received full sunlight.
-Under the shrubs the grass was much shorter and there was much more exposed open ground.
-There were many more tree seedlings in areas under shrubs, with almost no tree seedlings in the comparison areas with full sun.
Experiment
-The researchers removed the plants from seven pairs of adjacent plots and used screens to create artificial shade over one plot in each pair. One year later they counted the number of herbaceous plant seedlings that had grown from the seeds in the soil. Only half as many herbaceous plant seedlings grew in the shaded plots.
Propose a hypothesis to explain why there were fewer herbaceous plants under shrubs compared to nearby areas with full sun. Give evidence to support your hypothesis.