Field Ecology on a Shoestring

Alverno Science Inquiry Activities on the Internet

Activity: Use inexpensive methods to test hypotheses about the environment
Level: Varies, can be used at any level
Time: Varies, but allow at least 2 hours for most activities
Objectives: After successfully completing this activity, the student will be able to:

Author: Rebecca Burton, feel free to email questions and comments from you and your students.

Introduction

Many schools have access to some outdoor site, even if it is just the school soccer field. While there are definite advantages to having a lot of sophisticated measurement equipment, many schools cannot afford it. The techniques listed here can be used to test a variety of hypotheses without spending a lot of money on equipment. The only expensive items used in these activities are balances. If these are not available at your school, you might contact a scientific business or a more affluent school or college to see if they have older models which they no longer use. Hints are often given on how to incorporate quantitative techniques into the activities. Of course, graphing, unit conversion, and summary statistics will probably be useful in any of these activities.

Creation and maintenance of this web site was made possible by NSF-ILI Grant DUE 9750658.

Overview

Any combination of the techniques described on this site can be used for discovery laboratories. The general structure of any of the activities might follow this pattern:

  1. Discuss an interesting concept or area
  2. Identify interesting questions
  3. Formulate a hypothesis
  4. Identify dependent and independent variables
  5. Design a test of the hypothesis
  6. Collect data
  7. Use data to evaluate the hypothesis
  8. Present findings through oral presentations, posters, lab books, or lab reports

| Soil Invertebrates | Soil Properties | Vegetation Sampling | Water Properties |


Looking at invertebrates in the soil

Students can make hypotheses about where the most individuals or the most diverse communties might be. They can compare sites with different soil characteristics, different amounts of traffic, different sides of a building, etc. They can also compare different methods for looking at invertebrates.

Equipment needed (includes all four methods):

Most of these methods will require activities two different days, a few days apart. The amount of time needed depends on soil temperature and your local population of soil organisms. If soil is too cool, there will be little insect activity.

Drop boards should be placed at least few days in advance to allow animals to move in. When you're ready, have one student lift the board while another student counts or collects the animals.

For pitfall traps, bury cans so that the lip is flush with the ground. Invertebrates will fall in. This may take a few days. Be sure to locate these traps where people will not step in them. Save the soil for backfill.

To find small invertebrates, make your own Berlese funnel (see figure). Cut the bottom off a clean, empty milk jug. Invert the milk jug and place it in an empty coffee can with both ends cut out. Duct tape the milk jug onto the coffee can to make it stable. Place netting of some sort inside the jug to keep dirt from falling through (but allow microinvertebrates to go through). Place a small jar with alcohol in it inside the coffee can. Put a measured amount of dirt inside the milk jug. Put a lamp above the jug.

Microinvertebrates will move away from the heat and light of the lamp and fall into the jar of alcohol. This process may take two or three days. The animals can then be observed through a dissecting microscope or magnifying glass.

To find larger insects, a volume of soil can be placed in a dishpan. When water is added and soil is broken up, most will float to the surface.

When using this method or the Berlese funnel you might calculate the volume of soil used and calculate the number of animals per unit of soil, then extrapolate to your entire study site.


Soil Properties

Equipment needed (includes all methods):

Soil Profile
Use a shovel or trowel to make a hole. Make a sketch of the layers you see. Note the following: depth of layers, colors, grain size, density, and moisture content (dry, moist, or wet). Record the depth of each layer and whether the transitions are abrupt or gradual. You might calculate the relative depth of each layer.

Soil Moisture
Take a sample and seal it in a plastic bag for later measurement. In the classroom, weigh the moist soil, transfer it to sheets of foil and dry it by air-drying (in dry climates), under a lamp, or in a toaster oven (available at second hand stores), then re-weigh it. Calculate the percent water content of the sample.

Soil Density
Dig up some soil. Weigh it. Fill a graduated cylinder with sand. Pour the sand into the hole, keeping track of the volume of sand necessary to fill it. This will give you soil density (mass/volume). To compare soil hardness in different areas, drop a weighted metal stake from a consistent height and measure the depth to which it penetrates the soil. Watch your feet! Is density or moisture correlated with soil hardness?

Soil pH
Make a solution of soil and a small amount of distilled water. Be consistent in the proportions used. Use pH strips to determine the pH of the solution. This a not true measure of the soil's pH, but it is useful for relative values. Since the pH scale is a log scale, the answer won't be too far off. This might be a good opportunity to discuss the nature of a log scale and exponents. Most soil nutrients are cations. What pH ranges will result in retaining more of these positively charged ions?

Soil Type
Soil is classified using a
textural triangle, which has sand at one point, loam at another, and clay at another. A sample might fall anywhere in that triangle. The following method will give you a rough idea of the soil type of any sample:
Sand: leaves no film on your fingers
Loamy sand: leaves a slight film
Sandy loam: can be molded when wet, but can't form a cylinder by rolling between palms
Loam: somewhat sticky when wet, can make a cylinder, but not a ring
Sandy clay loam: sticky, some sand obvious
Clay loam: very sticky, sand hard to find
Clay: can be polished by rubbing, can take a shape and hold a fingerprint


Vegetation Sampling

General sampling methods
Equipment needed:

Getting a random starting point
Any sampling scheme requires randomness. One way to get it is by measuring along two adjacent sides of an area and choosing random numbers from a random number table to get coordinates for a starting point. Another method is throwing a plastic disk with a line marked on it to get a direction, then using a random number table to get a travel distance in that direction. A discussion of how to get random starting points and directions might lead to a good discussion of the difference between "random" and "haphazard".

Linear transects
Transects are lines with sampling points at regular distances. Transect points should be far enough apart so that the given feature (e.g. a particular tree) will not be counted twice. Note that the use of transects is appropriate only if the objects in question are fairly uniformly distributed with respect to the transect. To get a sample with true statistical validity, you must do multiple transects that start from random points and go in random directions. Mark the starting points with a flag. Have one person use the compass to sight a straight line in the desired direction and have someone else flag the end of the transect. Use the measuring tape to determine the location of each stop.

Point-quarter sampling
To get density of plants, mark randomly-determined points with flags. Each point represents the center of the compass, which will let you divide the area into quadrants (NE, SE, SW, NW). From that center point, measure the distance to the nearest plant of interest in a quadrant. Note the distance from point to plant, plant species, and the area the plant covers. Record data for only the closest plant per quadrant.

To get density of plants, compute the mean for all point to plant distances. The area in which a plant occurs is the square of the distance, so if it is an average of 5 m (linear) from point to plant, one has to cover 25 square meters to get one plant. (This method tends to overestimate cover if plants are uniformly distributed.) If you have more than one plant species, calculate the relative density of each species. To get coverage, multiply the area covered per plant by the density of the plant.

Plot sampling
Requires hoop, frame (made of PVC or wood), or an area marked with flags

Set out plot frames or measure plots out and mark them. Plot points must be randomly determined. The plot is sampled exhaustively to get percent cover, relative cover, or relative frequency. Multiple plots are used. This is a very tedious method, so small frames are advised.

Percent cover
A densitometer is used to measure percent cover. Commercial ones are expensive, but you can make your own using a toilet paper tube. Have one person hold the tube level. Have another student look directly up (or down) the tube. Two people are used because the tube must be level and it is difficult for one person to do this. The distance from eye to tube must be consistent as well. Along transects, stop at pre-determined intervals and determine whether trees (or shrubs) appear in the sight. If any portion of a tree (or shrub) is in the field of view, it is counted. Percent cover is determined by dividing the number of "hits" by the number of sampling points. The data are binomial. In other words, each point can only be a "yes" or a "no", not a "three" or a "45%". It can be used with the open end up to survey overhead vegetation or held with the open end down to survey shrubs or ground cover.

Measuring Tree Height
Equipment needed:

This is a simple application of geometry. To get the height of a tree (H), measure the distance between you and the tree (D). Hold a meter stick upright between you and the tree a comfortable distance from your eye (d). Now hold the stick steady and look at the top of the tree, simultaneously noting the mark on the meter stick that is along that same line. On the meter stick, the distance from your eye up to that point is "h". In other words, if your eye were level with the 20 cm mark, and the line from your eye to the top of the tree crossed the meter stick at 80 cm, h = 60 cm. From geometry we know that H/D=h/d, so H = (hD/d). You might want to try this a few times or with a few people as it is a bit difficult to get the technique right.

Leaf Area
Equimpent needed:

In order to compare leaf size on different areas of a tree or on trees of different sizes, it is not always necessary to know the exact area of the leaf. The leaves of trees of the same species will be approximately the same shape. An estimate of leaf area can be measured by measuring the greatest length and width of the leaf and mutliplying. For more accuracy, students might study a leaf and break it into simple geometric shapes, then apply standard measures of area. They might also try cutting the leaf material into a square, measuring and weighing the square, then using weight to estimate area.


Water Properties
Note: use extreme caution to avoid contact with water, which you should assume is contaminated. Do not allow students to be in a position where they might fall in. Have adequate adult help to supervise students.

Equipment needed (includes all methods):

General Characteristics
Students can note type of water body (stream, lake, etc.), substrate (Rocky, sandy, muddy, concrete, etc.), direction of flow, and type of bank. If practical, they might measure the dimensions. A meter stick attached at right angles to a pole might be used to measure the depth. Note any water color or odor.

Water pH
Water pH can be measured using test strips. These are extremely popular with younger students.

Water Temperature
Temperature can be measured at the surface and at other depths. Note that streams might have very different temperatures in different areas, for example in a deep hole versus a shallow, sunny area. Thermometers break easily, and older ones are made of glass containing toxic mercury. If you can't obtain a safer thermometer, you might want to do the measuring yourself and have student do the recording only.

Water Clarity
Water clarity is often measured using a Secchi disk. You can easily make one using a white plastic disk, such as a margerine tub lid or a plastic plate. Draw two perpendicular lines to divide the plate into quarters. Color two opposing quaters black with a permanent marker. Punch a hole through the center of the plate and pass a small rope or heavy cord through it. Tie a knot just above the plate to keep it from moving up. Tie some sort of weight (such as a rock) to the cord on the underside of the plate. To use the disk, lower it into the water (wear latex gloves). When it disappears, mark or hold the area of the cord that is in contact with the water. Measure the distance from your mark to the disk to find the depth where the Secchi disk disappeared. Now lower it into the water past where it disappears and raise it until it appears. Measure the depth at which the disk appeared. The mean of these two depths is the Secchi depth.

Stream flow
Stream flow rate can be measured by timing the trip of something with approximately neutral bouyancy. An orange works well. Measure out a 10 m distance and mark the begining and end. Have an orange dropper, timers, and students at the end to net out the orange. Do repeated trials. This is a good time to talk about means and variation.

Stream flow can also be measured in terms of pressure. Attach a plastic plate to line or wire at three points and attach this to a spring balance. If you don't have a spring balance available, tie the point where these lines connect to a spring. Tie a cord handle to the other end of the spring. Rig a plastic ruler beside the sping or be ready to measure it by holding the ruler next to the spring. You might have a variety of plates and a pair of scissors handy so that you can adjust plate size to stream force and depth. Students can measure the "relaxed" spring and compare that to the stretched spring. Back in the classroom, you can calibrate your spring using known weights. By measuring the area of the plate, you can get the kilograms per square centimeter of force exerted by the stream.


Alverno Discovery Activities on the Internet
Last update: 4/19/02 by Rebecca Burton, Dept. of Biology, Alverno College