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?
Application:
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)
Post-assessment:
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:
|
The
Ecosystem