This is a good Data Set to use in discussions on variability within ecosystems and on how limiting resources can influence primary productivity. Prairie ecosystems provide a great opportunity to investigate the influences of different factors, and their interactions, on productivity and biomass, because the majority of the aboveground vegetation grows and dies each season. Finally, this exercise will hopefully illustrate the unique characteristics and species inherent in this disappearing ecosystem.
How much time your students will need to make these figures in part depends on their familiarity with Excel. Although the directions include step-by-step instructions, students new to Excel will of course need more time. Students should work in groups of 3-4 during one or more lab sessions, or outside of lab for homework. Data Set 1 is small and will not take much time. The directions for Data Set 2 ask students to make 5 figures, which could take a lot of time. To save time, you could provide the first figure or two and ask students to make the rest. There is a good deal of information to keep track of here and the Worksheet is designed for that purpose.
Many undergraduates have little experience interpreting figures and therefore will need help with this. For suggestions about a two-step process (observation followed by interpretation) see the TIEE essay “Interpreting Figures and Tables”.
Each teacher will have to decide how much feedback to give students, and how much to ask from them, as they progress through the lesson. A good way to make this decision is to consider the intended outcomes for the exercise. What do you want students to know and be able to do? This should help you decide how often, for example, you want to stop the class and ask students to discuss their progress and questions with each other. In addition to helping students better understand the material, this type of discussion gives you valuable formative feedback on student understanding. Student evaluation can be based on several products including the worksheet, the figures, or both. If students work in groups, you will have to decide if the group submits a combined piece of work or if each student submits their own. There are pluses and minuses for each. The suggestion for evaluation described below is an essay.
This exercise contains three datasets for students to explore productivity patterns in tallgrass prairie. Each successive dataset builds on the previous one. Therefore, don’t start with Dataset 2 without first completing Dataset 1. Dataset 1 and 2 are appropriate for undergraduate courses, and Dataset 3 integrates the relationships outlined in the previous two datasets and requires more thought. For this reason, Dataset 3 is more appropriate for graduate students or an advanced undergraduate ecology course.
Dataset 1: Contains the mean aboveground productivity plus standard error for total, grass, and forb components separated by annually and 20-year burns. These data illustrate that burned prairie is more productive than unburned largely because burning increases grass productivity. When fire is withheld, forbs increase, and grasses decrease. After the students finish their first set of graphs, assess how they have done and facilitate a discussion of their hypotheses. You could expect that if this experiment was repeated for another 16 years, the mean productivity value in annually burned prairie could range from 520 to 420 g-m-2 or between 420 and 344 g/m-2 in the 20-year burn. As a general rule, if the top of one error bar doesn’t overlap with the bottom of another, then a true difference between treatments likely exists.
Dataset 2: Includes total precipitation as an explanatory variable of annual productivity. The strength of the relationship between these two variables (precipitation and productivity) is investigated by sequentially separating the data into multiple burn regimes, topographies, and vegetation types. When finished, grasses in upland, annually-burned sites are identified as being the most water-dependent.
Dataset 3 (additional data): Contains the complete record of productivity values for all burn frequencies, topographies, and vegetation types. In addition, a third fire frequency is added (4-year) to investigate the effects of intermediate disturbance in this grassland.
For the second component of this dataset, a table of additional predictor variables beyond just total precipitation amount is included to investigate the influence of changing precipitation patterns (rather than just means alone) on aboveground productivity. There are a large number of relationships that the students can compare, and largely, most will be non-significant, but important trends can be identified (e.g., increasing variability in precipitation pattern decreases productivity). This dataset provides an excellent opportunity for students to think about how a changing climate will influence productivity and how records of long-term data can serve as an alternative to experimental manipulation.
There are many questions in the Student Instructions. Questions dealing with broader aspects may include but are not limited to: