Chapter 20 Species interactions II: analysis and poster

Skills this lab builds. 4DEE: Core Ecological Concepts → Communities. Ecology Practices → Data analysis and interpretation; Working collaboratively; Communicating and applying ecology. BioSkills: Communication & Collaboration → “Use a variety of modes to communicate science (e.g., oral, written, visual)”; Process of Science → “Analyze data, summarize resulting patterns, and draw appropriate conclusions.”

This is the second half of the greenhouse experiment you planted earlier in the semester. The design, planting and weekly measurement protocol are in Species interactions I: designing and planting your experiment. This chapter covers the harvest, the analysis and the poster.

By now you have six to eight weeks of weekly measurements on a set of pots you designed yourselves. This chapter is where that turns into a result.

20.1 Final measurements

At the end of the experiment, plants come out of the pots. Working group by group:

1. Take your last set of height measurements before you disturb anything, using the same ruler-at-the-soil-surface method you have used every week. Photograph the pots one last time.
2. Remove the plants from the pots. Work gently — you need the roots intact, and roots break where you can’t see them.
3. Measure root and shoot lengths for each plant, in millimetres, keeping them associated with the pot and species they came from. A root length recorded without knowing which pot it belongs to is not data.
4. Place the biomass in a drying oven. Dry weight is a much better measure of how much a plant actually built than fresh weight, because fresh weight is mostly water and water content varies with how recently the pot was watered.
5. Record everything in the same data sheet you have been using all along. Resist starting a new file for the harvest data — one file, one structure, is what makes the analysis straightforward.

Your TA will tell you how long the material stays in the oven and when to come back and weigh it.

20.2 Analyzing and visualizing your data

You now have the two things a statistical test needs: a question you wrote down in week 1, and data collected to answer it.

• For a reminder about summarizing your data — means, medians, spread, and the figures that show them — see Descriptive statistics.
• For the logic of hypothesis testing, p-values and what a test is actually telling you, see Basic statistical testing.
• To work out which test your particular design calls for, and to get R code you can modify, see Selecting statistical tests.

Work from your own question backwards. What you measured, and whether your groups are independent, determine the test — not the other way around. If you find that your design doesn’t support the test you had in mind, say so in your poster. A clearly explained limitation is worth more marks than a test applied to data that doesn’t meet its assumptions, and your instructors can tell the difference.

Remember that a growth time series gives you more than one way in. You can compare final heights between treatments, or compare growth rates, or ask when the treatments began to diverge. These are different questions and they do not always give the same answer.

20.3 Final week

20.3.1 Downloads

Please download:

• Poster instructions
• Peer evaluation

Once you have all of your data, you will prepare a poster with your group detailing your experiment and its outcome. For the poster you will follow standard scientific writing style (see handout). Using the statistical lessons and R scripts that you have received earlier in the semester, analyze your data and create figures. Remember the correct methods for reporting results and writing figure lessons.

Figure legends matter here more than anywhere else in the course, because on a poster the figure is what people actually read. The seven components of a good legend are listed in Descriptive statistics.

20.3.2 Turn-in

Submit your poster as a PDF to your lab instructor through canvas (or via email, if instructed by your TA). Only one person in your group needs to submit. Each group member must also submit a peer evaluation, assessing the contribution of each group member.

Please review the poster instructions and rubric to ensure that you receive full points for this assignment.

Other notes:

• You do not need an abstract on this poster.
• Participation points will be determined by both your performance during the presentation, and by your group mate’s assessment of your participation level.