Trophic Cascade
Notes to faculty
Outcomes: Students should understand the concept of “trophic cascades” – the idea that
organisms consume other organisms on the trophic levels directly below them, which
can result in a cascading effect of high density (e.g.of predators), low density (of their
herbivorous prey), and high density (of the plants these herbivores eat). Therefore,
students should recognize that changes in a lower trophic level might be the indirect
result of changes two levels “up”. Also, depending on the discussion, students may
better appreciate the challenges of “imperfect” data and what scientists do about it.
What students do: They work with data from the article listed below the figure about the
wolf, elk, aspen trophic cascade in Yellowstone Park. First they read background
information about changes in wolves during the last century in Yellowstone. They are
asked to describe the food chain based on wolf/elk/aspen data and on 3 graphs show
the general cascading pattern for the 3 trophic levels. A final question concerns
assumptions scientists must make when data are spotty.
What to pay particular attention to:
o A common misunderstanding is that a trophic level is only affected by the level
directly adjacent. Therefore you need to be very clear how the cascade idea is
different from this assumption.
o The Yellowstone “story” as described below is pretty straightforward. However,
some students may “blank out” when faced with the Table. The groupwork
should help with this potential problem.
o The 3 graphs: You are looking for simple sketches showing general patterns of
trophic cascades and should say that.
o Some students may question the missing data. That could lead to a discussion
about limitations of research and varying degrees of confidence in data – and
what scientists do about this.
Information specific to the activity: Trophic Cascade, limitations of data in scientific
research.
Units & Quantitative Skills: Measurements for different trophic levels (e.g. percent
browsed, numbers of organisms; Data presented as numbers in a Table translated to
patterns in a graph.
Logistics:
o The Directions are for students working in groups, which can be done many
different ways. In small-medium sized classes, a few groups could present their 3
graphs (on overheads if you use them, the board, etc.) which will lead to
discussion. Students in large classes can work in informal groups before a
general discussion. You could sketch the 3 graphs yourself following students’
instructions and start the discussion this way.
o This is an example of a “sexy” food chain; students should be interested in the
wolves in particular – why they were extirpated and then reintroduced and effects
of the introduction. There are many photos on the web you can use (see below).
o
o
You can have students discuss the final question in their groups or just lead a
discussion yourself.
Good student questions include: would findings from one locale apply more
generally, are wolves the main predators on elk, are elk the main herbivores for
aspen, what other variables (e.g. climate) influence aspen or elk, why did elk and
aspen values decrease in 2006 when the wolf numbers decreased substantially,
what, exactly, are the wolves eating (e.g. calves), what about lags between an
action (predation) and its effect, how accurate are these numbers. While you will
not be able to address all their questions, a fairly quick reading of the materials
below and on the web should give you a good idea of the emerging “Yellowstone
Story”.
Resources:
o Ripple WJ and RL Beschta. 2007. Restoring Yellowstone’s aspen with wolves.
Biological Conservation 138: 514-519 (PDF).
o DW Smith et al. 2003. Yellowstone after wolves. BioScience 53: 330-340. (PDF)
o There is a good deal of information on the web: see for example, Science Daily
article (http://www.sciencedaily.com/releases/2007/07/070726150904.htm); and
PBS
(http://www.pbs.org/strangedays/episodes/predators/experts/yellowstonewolves.
html). Just type Yellowstone wolves elk aspen into your search engine and you
will find lots more, including background information.
o Case Study “The Wolf, the Moose, and the Fir Tree:
Who Controls Whom on Isle Royale? A case study of trophic interactions“
(http://ublib.buffalo.edu/libraries/projects/cases/Isle.html)
HIDDEN CURRICULUM ADDRESSED
• Principles: Conservation of Matter (Primary production/photosynthesis,
transfer of matter to consumers/decomposers/atmosphere)
• Processes: Transformation (consumption of one organism by another)
• Scale & Time: Ecosystem (long term processes)
• Forms & Representations: Ecosystems Matter & Energy Flow (Trophic
levels are abstracted collections of organisms performing the same food
“function” in an ecosystem; Box-and-arrow diagrams).
Directions for Students
Have someone in your group read the paragraphs below. Be sure you all
understand what is being said and the go on to examining the data in the Table.
Then address the questions.
Wolves, elk and quaking aspen trees are important components of In the
Yellowstone National Park ecosystem. In 1995 reintroduction of wolves to
Yellowstone gave scientists the opportunity to study effects of wolves on the food
chain. Grey wolves had been hunted out of Yellowstone in the early 1900s and
were locally extinct until 1995.
The table below are data from a riparian (next to rivers) valley in Northern
Yellowstone Park. Elk and wolves numbers (no.) are obtained from aerial
photographs. The aspen data are average values from counts in the field. The
numbers for aspen are percent of trees browsed (eaten).
YEAR WOLVES (no.)
ELK (no.)
ASPEN (%)
1992
0
16,000
Not available
1994
0
17,290
Not available
1996
24
Not available
Not available
1998
32
14,663
100
2000
40
13,650
95
2002
82
9215
75
2004
95
8335
40
2006
50
6000
10
Based on Ripple WJ and RL Beschta. 2007. Restoring Yellowstone’s aspen with wolves.
Biological Conservation 138: 514-519
Based on the data in the table, address the following questions. Be sure to write
down any questions of your own or things you are uncertain about or believe you
need more information about.
1. Specifically, what is the food chain in Yellowstone Park the ecologists were
studying – who is eating what? Used the ecological terms for the different levels
of the food chain. You can use words or drawing to show this.
2. In your own words, state the question(s) the scientists were asking with these
data.
3. Based on the data above, in the 3 graphs below show the relative changes the
numbers of aspen, elk and wolves in this part of Yellowstone Park. Include as
much information as you wish.
WOLVES
No.
1990
1995
2000
2005
2010
1950
2000
2005
2010
2000
2005
2010
ELK
No.
1900
ASPEN
No.
1900
1950
4. The data in the table above are not complete – for some years values for elk
and aspen are not available. Does this mean that you (and the scientists) cannot
come to any conclusions abut the wolf-elk-aspen interactions? What kind of
information would help you be more confident about your assumptions?