FACULTY NOTES

Based on the background information provided, introduce this next section with another "mini-lecture," in which you give a little more detail about why the two hypotheses might be valid. This will help the students in determining what to look for in the next set of figures.

You should have half of the student pairs address one hypothesis, while the other half addresses the second hypothesis. This will allow them to spend more time understanding and interpreting the figures. For logistical purposes, I recommend having students continue to work in pairs on this exercise, but assigning pairs to one or the other hypothesis and figure set. This will allow you to use this exercise in almost any size class, from very small (10 students) to large (50 students or more). In a large classroom, simply divide the room in half and tell student pairs on one side of the room to address hypothesis one, while the others address hypothesis two. In very small classes, you could be less formal about having students work in pairs, and simply have students work in two groups of 3-4 (5 max.) per group.

The students will need to be able to refer back to the graphs in the first figure set in order to compare some of the time series data.

Group One (Climate change hypothesis, Figure 2-1):

Based on the data presented here (Figure 2-1a and b), the climate hypothesis can be rejected. There were many years during the study interval when peak flows were greater than the 5-year return frequency event of ~290 m³/second (Fig. 2-1a). A "5-year return frequency" means that, on average, about once in five years the peak flow of a stream will reach this level (What this flow level is will vary depending on stream size, geologic setting, climatic regime, etc.). The main point to get across here is that cottonwood seedlings are most likely to become established after a spring flood event of at least this magnitude, because they require newly scoured point bar/meander surfaces with a new layer of sediment deposited for ideal germination and growth. Those conditions have existed many times during the period of little or no cottonwood recruitment in Yellowstone. In addition, during more recent times when the woody vegetation began to rebound, moisture (i.e. snowpack) and streamflow levels (in both spring and summer) were not particularly different from their long-term averages (Fig. 2-1b). In fact, if anything, conditions were slightly drier than average, which we would expect to cause a decrease in willow height during those years, instead of the observed increase.

Group Two (Browsing change hypothesis, Figure 2-2):

Based on the data and images presented in Figure 2-2a and b, the grazing hypothesis is not rejected. Comparison of willow heights inside and outside the ungulate exclosure shown in the repeat photographs (Fig. 2-2a) indicates a very large difference in the earliest photo, and no obvious difference in the most recent photo, during the time of increasing willow height shown in Figure 1-2. Likewise, direct measurements of percentage browsing on willow stems and overall willow height from 1998 to 2002 show a fairly abrupt reduction in browsing pressure.

Students may need extra help interpreting the data in Figures 2-1a and b, although giving them an introduction to the ecology of cottonwoods should alleviate some of their confusion. Students generally do not have much difficulty in interpreting Figure 2-2a and b, although observant students may note that the repeat photos were not taken at the same time of year, and so they may question the validity of the comparison. This is a great opportunity for discussion about the urgent need for long-term data in ecology, and the utility of repeat photography as a qualitative tool for reconstructing site histories, including the pros and cons of the approach.

Call on a few pairs of students to present their conclusions regarding rejection or acceptance of the hypotheses based on the data. A brief discussion with the whole class usually will follow, to make sure all students understand why one hypothesis is rejected and one is not. Student pairs may then be asked to revise or rewrite their hypothesis to address the next question: why was there a change in browsing pressure by elk on riparian vegetation during the 20th century?

There are several options for assessment of this part of the exercise. Revised hypotheses could be evaluated using the rubric from figure set 1, or students could be asked to write a "minute-paper" summarizing what they learned from the two figures, and how hypotheses were rejected or not. Alternatively, in a short essay quiz, you could have the students respond to a question such as this:

What sort of data/results would have allowed you to accept (not reject) the climate change hypothesis for lack of riparian vegetation growth in the 20th century?