Example of Using the Power of Story: Eastern Deciduous Forest- Mutualisms and Nutrient Cycling

5E cycle #2 Mutualisms and Nutrient Cycling

This next cycle continues with eastern deciduous forest, by adding the mutualisms of mycorrhizae and plants, ants dispersing herbaceous seeds, birds and small mammals as frugivores, and nutrient cycling with examples of pit-and-mounding effects on understory, effects of logging, effects of acid rain including calcium depletion and nitrogen saturation, and effects of earthworm invasion. The mutualisms and nutrient cycling are together because these mutualisms reflect response to nutrient poor environments. Thus, this cycle focuses on nutrient cycling but also reinforces concepts of population dynamics and species interactions.

Major concept: Nutrient cycling
Running themes: evolution of ecosystems, human impact, introduced species, species (interspecific) interactions, life history traits

Engage phase, part 1:

If you were to rake away the leaves on the forest floor, you would see that the forest floor is not flat. You would find that much of the surface consists of pits and mounds, even in the areas that were covered by glacial ice sheet. How do you explain that?

In class, list ideas from students, or post questions on Blackboard for students to consider before class. Re-visit after mini lecture.

Likely ideas from students: frost heave (no), before it was a forest it was grassland with gophers (no), digging by other animals (no)

Now provide mini-lecture about pits and mounds, physical characteristics of pits and mounds, what that means for plants. See outline for Eastern Deciduous Forest.

Have students pair-and-share about what else might create patches of different environmental conditions for plants growing within a forest. List ideas on overhead.

Time ~20 minutes

Engage phase, part 2:

Woodland ants gather seeds and small insects for food. Some seeds are routinely gathered but then dumped on the trash piles of ant colonies. Why would ants expend the time and energy transporting items as big as they are over many meters and then discard them?

In class list ideas from students, or post questions on Blackboard for students to consider before class. Re-visit after mini lecture.

Likely ideas from students: ants gather everything they can and then others at nest decide to discard (no), ants cannot discriminate well (no)

Now provide mini-lecture about ants gathering seeds with elaiosomes, fate of seeds, establishment and success of such seeds. See outline for Eastern Deciduous Forest. Re-visit question.

Have students pair-and-share ideas about what kind of environmental conditions promote this kind of mutualism. List ideas.

Time ~ 20 minutes

Engage phase, part 3:

In contrast to fruits that ripen in the fall, many summer fruits exhibit three rather than two distinct color changes. For example, blackberries and wild cherries, both fruits of summer, go from green to pink/red to black; blueberries go from green to pink to blue. Why is this such a common pattern for summer fruits but not for fall fruits?

In class list ideas from students, or post questions on Blackboard for students to consider before class. Re-visit after mini lecture.

Likely ideas from students: color changes are chemical changes which are affected by temperature, different colors for mature fruit in fall so sequence needed for final color is different, different species so just different (all are possible, but probably not the answer)

Now provide mini-lecture about frugivory by birds and small mammals in eastern deciduous forest. Re-visit question.

Have students pair-and-share ideas about what kind of environmental conditions promote this kind of mutualism, then have them describe and draw the seed shadow for fruiting plants that produce fruits in: 1) summer, 2) fall with fruits eaten then, and 3) fall with fruits held on plants through winter.

List ideas on overhead.

Time ~20 minutes

Explore phase, part 1:

What are mycorrhizae? How wide spread are mycorrhizae? What kind of plants have an association with them? What exactly do plants gain by the association? What happens to plants that don't have the association? Can mycorrhizae ever be harmful to plants? What exactly do mycorrhizae get from plants? Can mycorrhizae exist without plants? Do agricultural practices enhance or discourage mycorrhizal-plant associations? Does logging affect mycorrhizal community and, thus, the mycorrhizal-seedling interactions? Does acid rain affect mycorrhizal-plant associations? What would the world be like without mycorrhizae? Need at least a sentence explanation for each.

Post questions on Blackboard and have students research their answers before class.

In class, play "challenge" game. Need a bag of candy. Explain game to students. Instructor asks first question. First person with hand up gets to answer question. Answer must be one clear and concise sentence (not a "paragraph sentence"). If someone challenges that he/she can answer it better, then puts hand up. If answer is better (as judged by instructor or a panel of students "thumbs up or down"), then instructor announces that it is better. If someone else wants to challenge, then continue. Instructor decides how far to let this go. Candy given to winner(s). Then go to next question. Instructor should make this fun and keep a fast pace. Objective: Students do some research on their own. Covers topic with students engaged. Students have to listen to answers to ascertain whether they can answer better, and it forces them to articulate clearly and concisely. This is a good game to play early in course. Students then never know when you might provide a little more reward for them coming prepared to class!

Ask students to post to Blackboard: From a plant's perspective, draw a concept map incorporating the terms: pits, mounds, ants, elaiosomes, frugivory, mycorrhizae. What are the organizing principles and themes that you used?

Time ~ 20 minutes

Explore phase, part 2:

What happens to the nutrients locked up in a forest if the plants are killed? Post a multiple choice on overhead. Choices: a) lost through physical decomposition, b) lost through microbial decomposition, c) absorbed by soil, d) carried off by water into stream system, e) about equally by all of above, f) don't know

Get a hand tally and write numbers by each choice.

Describe and discuss the study done on Hubbard Brook Experimental Forest (described in most ecology textbooks). Pose more questions throughout. Re-visit the multiple choice question and take another hand tally. Record the numbers. [Most of the loss is via water runoff; plants aren't present to hold nutrients in system.] Ask where the nutrients that are carried off by water to the stream system end up. List ideas on overhead. Is massive transport of nutrients from one ecosystem to another a good thing? [Most ecosystems are naturally nutrient-poor so the point is that there is massive transport to another ecosystem (e.g., lake), which usually originally was naturally in balance as a nutrient-poor system and, thus, now balance is upset (unnatural eutrophication).]

Time ~ 20 minutes

Explain phase:

Briefly discuss/review what soil is, how long it takes to develop, soil layers, and how cation exchange capacity works and why it is important. Briefly discuss or review what an element cycle is, why we use the word "cycle", and illustrate with the nitrogen cycle. Ask students to review their concept maps for this 5E unit (#2) to identify pieces of nutrient cycling in forest. [It is everywhere, but students need to explain.] Discuss the effects of different organisms on nutrient cycling for the forest.

Ask students to list other ecological concepts involved (so far) in this "story"; ask for definition and example. Then have them draw a concept map of the hierarchy of all of these concepts. [There is no one or right answer. Mapping shows individual perception. What is important is that students have this practice and develop reasonable linking and sufficient complexity.]

Remind students how to use standard criteria for evaluating such a "map". Have students either self-evaluate or as pairs evaluate each others' maps. Briefly discuss the evaluation (what students learned from it). Again, students should keep copies of all maps they produce in a course portfolio.

Time ~ 55 minutes

Elaborate phase:

The problem with acid rain is the sulfuric acid created. Yes, because....? or No, because....? We have solved the acid rain problem with the recent Clean Air Acts. Yes, because.... or No, because....?

Have students discuss as pair-and-share. List summary of opposing ideas on overhead.

Mini-lecture on acid rain, calcium depletion and nitrogen saturation. Revisit the questions posed at the outset. Have pair-and-share groups re-discuss and offer answers. List summary of ideas on overhead. Statement #1: The problem with acid rain is the sulfuric acid created. [The problem is not just with sulfuric acid. More complicated......] Statement #2: We have solved the acid rain problem with the recent Clean Air Acts. [Because of the complications, the problem persists and will continue to do so for some time......]

Time ~ 20 minutes

Evaluate phase:

Native earthworms don't usually occur in northeastern forests. These forests are now being invaded by exotic earthworms. Why are there not native earthworms in northeastern forests? Why is there a concern about exotic earthworms in northeastern forests? What long-term effects might exotic earthworms have on these forests?

a) First, have students record their answers to the questions above. Likely ideas from students: no natives because too cold (no), no natives because too many predators (no, not stopping exotics), exotics eat different food (no), exotics have no predators/parasites here (no); concern about exotics because not part of forest (so what?), concern because causing problem (such as?), disrupting food web (how so?). Then out-of-class, students read an article or posted summary (on reserve; our library has "electronic reserve"), or in-class have students read a short description.

Sample reading assignment: Of all the animals that live in soil, earthworms are the best known and usually the most important. The general view is that earthworms are beneficial and, other than collecting them for fishing, people do not deliberately kill them. Earthworms consume dead plant matter; what they can't assimilate is deposited as casts (a mix of organic and mineral debris). Pleistocene glaciations eliminated earthworms from most of Canada and northern continental United States and probably to the edge of permafrost (i.e., some distance south beyond the edge of the ice sheet). Re-colonization has been slow, so native earthworms are not usually found in the deglaciated areas. At present through accidental and purposeful introduction, there are 45 exotic earthworm species in North America, along with the 100+ native species. Many of the exotic species have successfully established in the northern forests. Within habitats, earthworms only occur where the vegetation, soil texture, organic content and moisture are favorable. At any given place as many as a half dozen earthworm species may co-exist because resources are partitioned through different feeding strategies reflecting different behavioral, morphological and physiological adaptations. For example, some inhabit and feed on plant litter and others inhabit the mineral soil within the rhizosphere. Research from agricultural systems shows that earthworms can improve soil via increasing rates of plant litter decomposition, increasing nutrient transformation and plant uptake of nutrients, improving soil porosity, and enhancing water infiltration and solute transport. However, studies in forest indicate that earthworms may also have negative effects on soil via moving or burying surface materials that would protect soil surface from erosion, depositing casts that seal surfaces, increase compaction of soil surface, dispersing weed seeds and pathogens (e.g., foot-and-mouth disease in Europe), increasing losses of soil nitrogen through leaching and denitrification, and increasing loss of soil carbon through enhancement of microbial respiration. Earthworms can eliminate the forest floor (organic layer), which is the key component to stability of forests (e.g., protects against erosion, protects seeds from predators, facilitates forest regeneration after disturbance). One forest project showed that presence of earthworms reduced fine root biomass, soil nitrogen content (also nitrogen mineralization and nitrification rates), soil phosphorus, mycorrhizae rate of infection of plants, and "soil" respiration. Another study showed that when deer density and earthworm densities were both high, there was a decline in native understory (e.g., lily and orchid populations) and increase in exotic plant species (e.g., grasses).

Sources for reading material.8 Online sources that have photographs or illustrations of soil without worms versus with worms.9

b) Compare and contrast movement of nutrient atom (e.g., nitrogen) through forest ecosystem in earthworm-free versus earthworm-invaded situation. Students should indicate where movement of atom is slowed down or sped up (i.e., locked up for some period versus freed sooner) in the earthworm-invaded situation compared to the earthworm-free situation. This task can be done by constructing a concept map in class or outside class, or filling out a concept map form on a multiple choice exam. Another method is the individual-then-group quiz (a proven method that promotes both individual learning and teamwork).7

Total class time: typically a full class period (~50 minutes)


Outside of class, have students re-draw their concept map-food web for eastern deciduous forest. It will be quite complicated now. Students should use sub-maps (e.g., earthworms on major map should refer to a submap to show earthworm effects on forest). There is no best way to do this and students can (and should) use their creativity (e.g., using colors to distinguish concepts from facts, transparent overlays, thematic series of maps, powerpoint features). The instructor, and/or student (self-evaluation) and/or student group can evaluate using standard criteria.3 Students should keep the set of maps developed in this series (from beginning to end) in a portfolio and asked to reflect on these either at this time or at end of course.

Total course time for this 5E cycle: ~ two weeks

Examples of how to implement the power of story:

Eastern deciduous forest: Lyme disease, masting, and gypsy moths
Eastern deciduous forest: Mutualisms and nutrient cycling
Outlines for Eastern deciduous forest material
References for Power of Story examples
Sample concept maps


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Last Updated: 1/28/15