TEACHING ALL VOLUMES SUBMIT WORK SEARCH TIEE
VOLUME 1: Table of Contents TEACHING ISSUES AND EXPERIMENTS IN ECOLOGY
Experiments


Introduction (written for students):

      Some ecologists describe what they do in a way that sounds simple but in practice is pretty difficult: ecologists first see and describe patterns in organisms and then attempt to explain why those patterns exist. Examples of different patterns are high or low abundance of animals, or whether a site has many of the same type of plants. Mechanisms to explain these patterns might concern the amount of rainfall in a year or heavy grazing by some herbivore.

       In this field investigation, you will have the opportunity to hone your "what" and "why" skills in a situation where the ecological patterns are fairly evident. You will study trees on the sharply contrasting environments of the north and south slopes of the Holyoke Range. This is a study of climate, topography, and the distribution, abundance and diversity of organisms in communities. We wouldn't expect to find a tree that is common in New England, like white birch, everywhere on the globe; organisms exist within fairly narrow ranges of rainfall, temperature, soil type, and other physical-chemical factors. On a global scale, climatic regions are characterized by distinct vegetation types such as the grasses and sedges of the tundra or trees that we find here in the northern deciduous forest. On a local scale, topographic features such as mountains result in localized climatic differences and also variations in soil. The unusual situations where mountains run east-west in northern temperate zones provide a terrific opportunity to determine how sharp gradients in temperature, soil moisture, and soil type influence vegetation characteristics such as dominance, presence/absence, or abundance of tree species. Such a study also allows us to examine what is known about adaptations of organisms to factors including cold, drought, and poor quality soil. Finally, knowledge that major events like hurricanes also alter the structure of tree communities on mountain slopes leads us to consider the additional role of disturbance in understanding patterns of organisms in communities.

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Materials and Methods (written for faculty):

Study Site

       We are fortunate here in Amherst to have a small mountain range called the Holyoke Range within a 5 minute drive. This range in unusual in that it runs east-west and therefore has north and south slopes that often differ dramatically in daily irradiance and temperature At your location you might compare mowed/unmowed or grazed/ungrazed fields, polluted/unpolluted areas, burned/unburned or old/young forests etc. (see examples in links below)

click to see a relief map
{image from http://www.mtholyoke.edu/proj/cel/ref/place/natplaces/holyoke_range.shtml}


Data Collection Methods

       During the first 3 hour lab session, we walk up to a fairly easily accessible spot of the ridge of the range. Along the way I teach students how to identify the 10 or so dominant trees. When we reach our destination, I ask the students to form groups of 2-4, walk around on the upper parts of both slopes, make a list of 10 or so observations, and return in 30 minutes. After we re-group, the students begin to talk about their observations and some of the more obvious ecological questions that come from these observations. They inevitably note differences in wind, temperature, and light on the two sides and also that some of the trees appear to be different. With my guidance in sharpening their questions, they typically ask ones such as: Are the tree types, densities, and sizes different on the two slopes? Is air temperature different? If the tree types are different on the north and south aspects, is this due to climatic differences or are there other variables that are likely important as well?

       Back at school (same day or next class), we work on the experimental design. Their time in the field is restricted to one afternoon. In regard to the tree sampling, some students often have had some experience with ecology in high school, and the concept of sampling by quadrats usually comes up early on. I suggest how to do this simply in this setting (e.g. a circle drawn in the snow; one student holds one end of a rope and another walks around the circumference). I also recommend a quadrat size (in this case 10 m diameter). The students usually decide to distribute the samples randomly, but fairly closely together, and they decide how to do that simply in the field (e.g. walking a set distance from a center point). See below for links describing various plant sampling techniques and analyses.

       Since 1996, I have used recording thermometers sold by On-Set (On-Set Computer Corp. (www.onsetcomp.com), 536 MacArthur Blvd, Pocasset MA, 02559, 508-759-9500). These are very easy to use and allow us to collect a rich temperature data set that is especially helpful for this investigation. A small team of students takes on this aspect of the work; they decide how to arrange the thermometers (I have 10) through the air column (e.g. on a line strung between trees or on the trees themselves), in replicate locations, in the tree sampling sites on both sides of the range. These students also choose the frequency and duration of sampling, and they are responsible for temperature data reduction and presentation as figures. These data are shared with the other students.

       During the second lab period the students work in teams of 2-3 and collect the tree data (type, number, diameter) in as many quadrats as possible. First we review the sampling methods. Then I give each team a plastic bag containing a clip board and data sheets that I make up, copies of pages from a good plant ID book with drawings, pencils, DBH tape, meter tape - and maybe some chocolate candies if it is cold! (Organization of all of this is quite important; you also need to make sure that everyone knows how to use the DPH tapes, data sheets, etc.). At the same time the temperature group sets up their recording thermometers.

       I also use walkie-talkies to keep track of teams and so that students can discuss problems (e.g. are we counting dead trees?) together. This also makes the lab more fun.


Data Analysis

       During weeks 3 and 4 in lab, the students work in task teams (tree types, tree size, temperature) to enter the data on spreadsheets (I use Microsoft's Excel) and begin producing figures and tables. At the end of the 4th week, the teams present their findings, now in final figure or table form, to the entire group. All the data, crunched or raw, is available to everyone through the college's computer network.

       If you need to teach your students how to use excel, there are some good tutorials online (for examples see below). In my experience some students know or quickly learn to use spreadsheets; you can pair them with less experienced students, but make sure that everyone gets a chance to work with the data. Be sure to give students enough time to learn how to use the spreadsheet and make simple, mock figures similar to ones they will be making later. This is also a useful way for them to think about the figures/tables they can make to address the questions they are asking.

       For comparison of density and number of species on the two slopes my students use a simple chi-square test (for tutorials see below). I go over the basic idea of the chi-square fairly quickly and leave more detailed descriptions for a statistics class. A hand-out is useful (e.g. www.hum.utah.edu/communication/classes/sp03/3710-1/handouts.pdf). For size comparisons we use t-tests (see tutorials below and http://demeter.hampshire.edu/%7Embruno/general/statistics.html for a hand-out). Despite complaints that t-tests cannot be done correctly on Excel, my statistician friends assure me that it is ok for a simple paired t-test for this lab.

       The figures my students make are usually simple histograms. For instance, one figure is density (mean ± std. deviation) of common tree species on the N and S slopes. For diversity I only use species counts because there is so much data analysis as is.

       I focus on temperature because this is an early winter project in New England. The ground is frozen and often covered with snow. In other situations soils could be compared (e.g. water content simply by weight before and after drying or loss on ignition for organic content; see physio-chemical analyses in http://groups.ucanr.org/danranlab/Methods_of_Analyses545/#soil3). Light is also easy to measure accurately; I have used LI-COR meters with success.


Report Format

       Each student writes a research-style paper (abstract, introduction, methods, results, discussion, references). In class, I go over what should and should not be included in each section and give them handouts about how to write scientific research papers. In this course, by the time their first lab paper is due, students have already read 4-5 primary ecology papers and so they are familiar with the structure. The students are encouraged to work on the papers together to discuss the references I put on reserve or what the data might indicate. The references I give them include other similar studies done elsewhere, geologic and soil maps of the range, and other types of ecological studies of the Holyoke Range.

       Students' research papers are structured according to the following criteria (see also links below)

       Abstract: includes ecological context, why this study would be of interest to ecologists, specific questions addressed, overview of methods, specific results and conclusions from these data.

       Introduction: explains ecological context, geologic and ecological setting, other relevant studies and findings, specific questions addressed (2-3 pages).

       Methods: enough detail so that study could be repeated but not so much as to be tedious to read (1 page).

       Results: clear explanations of tables and figures with no interpretation; tables and figures are clean and professional (1-2 pages).

       Discussion: nutshell summary of overall results, interpretation of results, comparison with other studies, professional discussion of limitations of the study, suggestions for follow-up studies (3-5 pages).

       References: in Ecology format, 5-10 references expected.

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Questions for Further Thought and Discussion:


  1. Consider local vs. regional distribution of tree communities. Would we expect that the community on the north slope of the Holyoke Range to look just like communities far north of here (say in northern Vermont) and the community on the south slope to be typical of tree communities in Maryland or Virginia? Why or why not?


  2. We are doing our study in midwinter. Design a year round study to address the questions we are asking. What times of year would be especially important to study and what would you do?


  3. Students in beginning ecology courses when asked why a plant or animal is found in a certain place often say something like "it likes it there?" Critique that statement.


  4. Ecology texts are often divided into sections called autecology, population ecology, community ecology, and ecosystems ecology. If you were studying the Holyoke Range from each of these points of view, what kinds of questions might you ask and what types of studies would you do?


  5. Suppose you noticed a typically northern tree like hemlock growing on the south slope. What kinds of studies would you do to determine why this was the case?


  6. Consider a totally different ecological situation where zonation is clearly evident. Pick any example either from your observations or from the literature, and describe the situation in detail. Design experiments to test possible causes of the pattern zonation in your example.



*** Note: Answers to many of these questions and numerous other comments by the contributing author can be found in the "NOTES TO FACULTY" page.

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References and Links:


Writing Research Papers


Plant Sampling Techniques


Chi-squared Tutorials


t-test Tutorials for Excel


Excel Tutorials


References

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Tools for Assessment of Student Learning Outcomes:

       Student assessment is based mainly on the quality of the research paper as outlined in the "report format" section of this lab. Effectiveness of students as group members and leaders is also important in this class and is noted in an evaluation written by me at the end of the semester.

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Tools for Formative Evaluation of this Experiment:

       There are numerous approaches to course evaluation. Here are examples of several that could be used with Experiments or Issues.

An extensive discussion on Evaluation appears in the Teaching section of this site.

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