Figure 3B. Export of total N from watersheds surrounding the North Atlantic Ocean as a function of net human-caused inputs to their watersheds. Human related inputs are synthetic N fertilizer application, N fixation by leguminous crops, atmospheric inputs of N, and net imports of N in food and feedstock. From P. M. Vitousek, J. D. Aber, R. W. Howarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger, and D. G. Tilman, 1997. Human alteration of the global nitrogen cycle: causes and consequences. Issues in Ecology 1: 1-15..and R. W. Howarth, G. Billen, D. Swaney, A. Townsend, N. Jaworski, K, Lajtha, A. Downing, R. Elmgreen, N. Caraco, T. Jordan, F. Berendse, J. Freney, V. Kudeyarov, P. Murdoch, and Z. Zhao-liang. 1996. Regional nitrogen budgets and riversine N & P fluxes for the drainages to the North Atlantic Ocean: Natural and human influences. Biogeochemistry 35: 181-226.
Pairs-share
In this "pairs-share" one student is given Fig. 3 A and the other is given Fig. 3B. There are separate explanations of the figures for each one. Each student is also given a set of questions. They are instructed to examine their figure alone, try to answer the questions posed to them as best they can, and then pair up with someone who has the other figure. The students explain their respective figures and discuss their respective questions. The idea is that they will better understand export of N from rivers into oceans with both sets of figures.
This "pairs-share" will take a fair amount of time. To save time, you can project the figures and lead a discussion using some of the questions below.
Figures 3A and 3B
These Figures synthesize large data sets from rivers around the world. In addition to helping students interpret the data, you should also encourage them to think about how the authors went about pulling together water chemistry data from rivers as different as the Mississippi, the Ganges, and the Amazon. Students are not used to thinking about data that a scientist collects, works up, and publishes. What are likely sources of the kind of data presented here? (E.g. departments of health and ministries of environment gray literature, EPA reports). What would be criteria for judgment of a valid data set to use in this synthesis? (E.g. samples collected throughout the year for at least several years, from different site, with good methodology).
Figure 3A is a log-log plot. For some reason students become fearful at the mention of "log" and many believe that they cannot understand what a log is. Log 10 is not a difficult concept of course; one simple technique that has worked for me is to ask students to use a calculator (or spreadsheet) and go back and forth between the numbers 0.1, 1, 10, 100, and 1000 and their logs. They quickly see that logs are no mystery at all.
In discussion ask students to describe how increased human density results in increased nitrate export in rivers. Get as much detail as they can provide and explain the biological or chemical mechanisms underlying their suggestions.
For Figure 3A and 3B it will be necessary to give a brief explanation of the statistics of correlation and perhaps of P values. (Correlation measures the degree to which two variables vary together; students may confuse this with regression, which expresses one variable as a function of the other). Also be sure that they understand the term "inputs per area".
Discussion Questions (some repeated from student instructions)
Figure 3A
What is meant by the term "export"? Where is the nitrate exported to?
Why are these scientists interested in the relationship between nitrate export and human density? Why is nitrate so important to look at in this way?
Why does nitrate export in rivers increase with increasing human population? What human activities result in increases in river nitrate concentrations?
What are some effects of high nitrate concentration in water?
Why are the data plotted on log/log scales? Is this "cheating"?
As much as 80% of the N estimated from human activities in North American watershed is not accounted for by export in rivers. Much is lost by denitrification in watersheds. What is dentirification and what types of habitats would you expect to have high rates of denitrification? Using this information, how might you decrease nitrogen loading rates into rivers?
Figure 3B
The data are labeled as flux (or input) per area? Per area of what? What does "flux" mean? What is total N (as opposed to nitrate for instance)?
The data points are export of nitrate from watersheds to the North Atlantic Ocean. What is the watershed for the data point labeled NE U.S.? For the North Sea? How does the water actually enter the Atlantic Ocean - where does it come from and where would you go to see "it" enter?
Synthesis of these data was an enormous undertaking for scientists from many nations. Why are these scientists so interested in the relationship between N flux and net anthropogenic input?
Jordan, T. E., and D. E. Weller. 1996. Human contributions to terrestrial nitrogen flux. BioScience 46: 655-664.
Student Assessment: Take home quiz.
According to the paper by Jordan and Weller (1996) several anthropogenic environmental variables are correlated with average nitrate discharge to rivers. Here are their correlation coefficients for 8 variables: net anthropogenic nitrogen input (0.76), agricultural biotic nitrogen fixation (0.62), atmospheric deposition (0.61), livestock waste (0.44), N fertilizer (0.36), human density (0.23), import of agricultural-product N (0.22), and percent cropland in watershed (0.08). In a one page essay, explain each of these variables as N sources to rivers. In your essay, also explain the concept of correlation as it applies in this example. (note: a correlation coefficient value of 1 is the highest value possible).
Evaluating an Issue: How do you know whether it is working?
On-going (also called formative) evaluation of the approaches your are using is critical to the success of student-active teaching. Why try out new ideas if you don't know whether or not they are working? This is a brief overview of formative evaluation. For more information, go to the Formative Evaluation essay in the Teaching Section.
Course Goals:
Formative evaluation only works if you have clearly described your course goals - because the purpose of the evaluation is to assess whether a particular technique is helping students reach these goals. For instance, most of us have "learn important ecological concepts and information" as a course goal. If I reviewed the nitrogen cycle in a class, for evaluation I might ask students to sketch out a nitrogen cycle for a particular habitat or system. Each student could work alone in class. Alternatively, I might ask students to work in groups of 3 and give each group a different situation (e.g. a pond receiving nitrate from septic systems, an organic agricultural field, an agricultural field receiving synthetic fertilizer). The students could draw their flows on a large sheet of paper (or an overhead transparency) and present this to the rest of the class.
The Minute Paper:
Minute papers are very useful evaluative tools. If done well they give you good feedback quickly. Minute papers are done at the end of a class. The students are asked to respond anonymously to a short question that you ask. They take a minute or so to write their response in a 3x5 card or a piece of paper. You collect these and learn from common themes. In the next class it is important that you refer to one or two of these points so that students recognize that their input matters to you. The UW - FLAG site (www.wcer.wisc.edu/nise/cl1/flag/) gives a good deal of information about using minute papers including their limitations, how to phrase your question, step-by-step instructions, modifications, and the theory and research behind their use.
© 2004 – Charlene D’Avanzo and the Ecological Society of America. Teaching Issues and Experiments in Ecology, TIEE Volume 1 © 2004 - Ecological Society of America (www.tiee.ecoed.net).
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