Teacher Notes for
Changing Biological Communities – Disturbance and Succession1
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 Standards2:
 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
By Dr. Ingrid Waldron, Dept. Biology, University of Pennsylvania, © 2015. These Teacher Notes and the related Student
Handout are available at http://serendip.brynmawr.edu/exchange/bioactivities/succession .
2 http://www.nextgenscience.org/next-generation-science-standards
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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 (iwaldron@sas.upenn.edu). 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.
3
I am grateful to Ben Cooper for suggesting this approach which is modified from POGIL (Process-Oriented
Guided Inquiry Learning; https://pogil.org/about). I welcome any feedback about how this approach has worked in
your classroom.
2
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.
4
White-tailed deer eat a wide variety of plant foods, including green vegetation and grains from agricultural fields in
the summer, acorns and other nuts in the fall, and buds and twigs in the winter. Deer use thickets of shrubs for
hiding from predators (currently humans; previously wolves and large cats such as mountain lions or bobcats). Thus,
white-tailed deer thrive in regions that have diverse vegetation, including forests, shrubs and fields.
3
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 invasive5
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.14610248.2005.00764.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 iwaldron@sas.upenn.edu).
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/anthromesanthropogenic-biomes-world-v2-1900/anthromes-v2-1900-namerica.jpg).
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A non-native (exotic) species is called invasive if it spreads rapidly and competes successfully so its population
size increases substantially and disrupts the populations of species native to the area. Whether or not a species is
invasive can vary in different environments.
In some cases a non-native invasive species can facilitate the growth of a native species. For example, the growth of
the non-native invasive vine illustrated on page 4 of the Student Handout facilitated the growth of a native vine by
providing a "ladder" to carry the native vine to the tree canopy.
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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.
How could less grass and fewer other herbaceous plants under the shrubs contribute to more tree
seedlings under the shrubs?
Make a flowchart or drawing that illustrates how shrubs inhibit the growth of herbaceous plants
underneath them and how this effect facilitates the growth of tree seedlings under shrubs.
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The experiment provided evidence in support of the hypothesis that there are fewer herbaceous
plants under shrubs because shrubs produce shade and the seeds of herbaceous plants were much
less likely to produce seedlings in the shade. As shown in this flowchart, less grass and fewer
other herbaceous plants under the shrubs resulted in much more open ground where tree seeds
could land and germinate to produce tree seedlings. Also, less grass and fewer other herbaceous
plants resulted in less competition for water and soil nutrients. This demonstrates how facilitation
can be indirect (e.g. by inhibiting a competitor).
Shrubs
Shade
Less
growth of
herbaceous
plants
Less
competition
for space
and water
More tree
seedlings
Inhibition
Facilitation
Additional evidence from this research indicates that the leaf litter of the sumac shrubs contained
toxins which inhibited germination of the seeds of herbaceous plants (study reported in Ecology 60,
1979, pp. 956-965; additional evidence in The American Midland Naturalist 108, 1982, pp. 124-132). This effect
is an example of allelopathy, a common type of competition that influences succession.
Suggested Follow-Up Activities
This succession activity provides important background for understanding the increase of Lyme
disease in recent years in the Northeast and upper Midwest, as discussed in "The Ecology of
Lyme Disease" (http://serendip.brynmawr.edu/exchange/bioactivities/LymeDisease). This
analysis and discussion activity engages students in understanding the lifecycle and adaptations
of black-legged ticks and the relationships between these ticks, their vertebrate hosts, and the
bacteria that cause Lyme disease. Students use this background to analyze when and where
human risk of Lyme disease is greatest, why rates of Lyme disease have increased in recent
decades in the US, and ecological approaches to preventing Lyme disease.
You may want to have your students analyze succession in other ecosystems and discuss how
these other examples of succession are similar to and different from succession in abandoned
farm fields. You may want to present your students with the general principles concerning
succession presented in the Learning Goals on page 1 of these Teacher Notes and ask your
students to discuss how these general principles apply to the other examples of succession and
whether these general principles should be modified based on these other examples of
succession. The top of page 5 of these Teacher Notes provides relevant comments and links.
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