Artificial Leaf Packs

Developers:

Ann M. Faulds
Claire Murray
Kathy Neville
Al Palmer
Alan Sexton
Samantha Mason
Kristen Travers
Stroud Water Research Center of
The Academy of Natural Sciences of Philadelphia
512 Spencer Road
Avondale, PA 19311

Grade Levels:

5 through 10

Discipline:

Life Sciences/Biology

Goals:

  1. Students will conduct a scientific, field oriented investigation that demonstrates the relationship between trees and the ecology of stream communities.
  2. Students will analyze the results of a leaf pack experiment that demonstrates and uses an experimental variable to draw conclusions about macroinvertebrates habitat quality and food preferences.
  3. Students will appreciate the value of replication of each experiment condition to produce meaningful experimental results.
  4. Students will learn to use artificial leaf packs to sample the biodiversity of local environmental quality.


Background:

Autumn is an important time for eastern woodland streams. Over the last twenty-five years, the Stroud Water Research Center staff has shown that tree leaves which fall into streams provide food as well as habitat for a wide variety of stream organisms, in particular the macroinvertebrates ("macros," small bottom dwelling insects and other creatures). In the nearby White Clay Creek alone, there are 50 to 60 species of aquatic insects that are found in leaves that collect along rocks and snags in the stream.

Macros that are adapted to leaf pack habitats have evolved life cycles that assure egg hatch is timed with the advent of FOOD. In some cases, an environmental or genetic cue triggers a fall hatching of eggs that have been dormant. The "leaf feast" that commences in the autumn continues through the winter until the last fragments of leaves are consumed in the early spring. Many of the stream macros live most of their life cycle underwater and emerge from the water only to live a short time as flying adults. The macros which colonize the leaf packs have been divided into functional feeding groups such as shredders, collector/gatherers, filtering collectors, scrapers, and predators.

Artificial leaf packs, or the purposeful introduction of leaves to streams, can be an important tool for understanding stream communities. The basic idea is that submerged leaves will become colonized by leaf pack dwelling organisms native to the area; the relative abundance and variety can be used as an indicator of the quality of the habitat. The Stroud Water Research Center was a pioneer in the use of artificial leaf packs for stream research to find which foliage stream insects prefer to eat, and which trees produce the most nutrition for the insects. The Stroud Center staff has developed a technique of using a plastic onion bag to hold a bundle of leaves in place in the stream. The bags securely hold the leaves in while allowing the macros to move freely in and out of the leaves. At the conclusion of the experiment, the macros are identified

Teacher's Notes:

Leaf packs can be used in science classes to compare macroinvertebrates feeding preferences, for instance: (1) native tree leaves vs. exotic tree leaves; (2) green leaves vs. dried brown leaves; (3) pesticide sprayed leaves vs. unsprayed leaves.

Leaf packs can also be used to compare the macros at different sites, for instance: (1) a meadow stream vs. a woodland stream (2) Above and below a sewage treatment plant; and, (3) a spring brook community vs. a river community.

And the list of possibilities goes on. Encourage students to invent their own investigations. As with most experiments, the more leaf pack replicates that are used, the more accurate the results.

Materials:

Materials for two treatments with three replicates each (6 leaf packs)
6 Plastic mesh onion bags
120 gm dried tree leaves
6 x 1m sections of rebar (reinforcement bar) or strong stakes (optional)
Nylon twine
6 Hollow cinder blocks or bricks
6 Photo copies of the macroinvertebrate identification boards (these can be laminated or used as disposable)
Sets of Macroinvertebrate Flash Cards
6 Water-proof tags (pieces of Tyvek® envelopes work well)
6 Large zip-lock bags
8 Leaf Pack Invertebrate Record Sheets
Graph paper
About 60 x 5 cm petri dishes
Tree identification guide

Tools
Rock hammer, or sledge hammer (optional)
Scissors
Thermometer
Fine point permanent marker
A balance that can accurately weigh 20 grams of leaves
Forceps and/or white plastic spoons
Spaghetti strainers or .5 - 1.2 mm sieve or plastic window screen
Cooler and ice packs
White plastic bowls (from the party supply section of the grocery store) or white enamel pans
Dissecting microscope or 10 X hand lens

Explanation:

Planting the Seeds for Streamside Reforestation

The Georgia Farm Preserve, located in Chester County, Pennsylvania, is a 332 acre farm that contains several small tributaries to the Brandywine Creek. The farm has been protected in perpetuity for environmental research by the Academy and the Stroud Center. The farm was bequeathed in 1990 by Dr. Morris Stroud, and conservation easements cover most of the property.

Currently at Georgia Farm, a team of researchers lead by Drs. Denis Newbold and Bern Sweeney is studying the importance of trees to streams. They are evaluating the extent to which streamside reforestation can mitigate the influences of surrounding land use, such as farming.

The Impact of Clearing Woodlands and the Benefits of Reforestation

Clearing trees can take a great toll on our streams and rivers. When woodlands are transformed into fields and pastures, cattle can easily break down stream banks, and grassy vegetation can reduce stream habitat. Due to lack of shade in a cleared area, temperature change occurs as sunlight warms up the stream water, making it too warm for some aquatic species. Furthermore, sediments and pesticides all find their way into streams when there are no trees to inhibit their transport.

Stroud Center research indicates that streamside woodlands are an important part of natural stream processes. A forest's shade provides habitats for fish and wildlife and encourages growth of beneficial algae. Tree roots help to stabilize stream banks and offer cover for insects which feed on leaf material falling into the stream. By shading out encroaching grasses, woodlands help keep stream channels wide and shallow, providing more surface area for bottom-dwelling organisms. Furthermore, the purity of water can be maintained, even improved by streamside forests. Trees and shrubs inhibit erosion by filtering and trapping sediments as well as by absorbing pollutants from overland runoff and groundwater zones.

Unanswered Questions

While there are clearly positive aspects of streamside reforestation, many questions remain to be answered. To what extent does a forested buffer zone cut down on the transport of sediments, nutrients and pesticides? How wide does the strip of forest along streams need to be? Which species of trees should be used? How should patches of forests be planted and managed by landowners? What are the costs for establishing such buffers and should the government subsidize such costs? Answers will make implementing effective buffer strips an easier task for land managers.

The Stroud Center: Searching for Answers

The Georgia Farm study focuses on a major pollutant of stream water, nitrogen. The scientists hope to learn more about paths of nitrogen transport from soil into streams in order to show how buffer zones may most effectively impede this transport. While nitrogen is used in many fertilizers as an essential nutrient for the growth and development of plants, an overabundance can greatly disturb stream ecosystems. Such "nutrient loading" is particularly pronounced in the Chesapeake and Delaware Bays, causing excessive oxygen-depleting algal growth and fish kills.

The research team is also focusing on the quantity and quality of leaves falling into the stream at Georgia Farm. Some exotic species, like multiflora rose, are not as nutritious as other native species of trees for insects in the stream. By researching which trees provide the most nutritious source of food for aquatic invertebrates, Stroud Center scientists can better recommend species of trees to plant in streamside forests.

Clearing trees can take a great toll on our streams and rivers. There are clearly positive aspects of streamside reforestations, but many questions remain to be answered. With the support of the U.S. Forest Service, Stroud Center scientists are quantifying the effects of streamside reforestation which will make implementing effective buffer strips an easier task for land managers.

Streamside Forest Diagram

The Benefits of Streamside Forests

1 Run-off Control
Rain that reaches cropland and pastures can soak into the ground or run off. Streamside forests can slow run-off and reduce sediment, fertilizers and pesticides that wash into streams.

2 Nutrient Uptake
Fertilizers and other dissolved impurities that originate in farmland can be taken up by tree roots. The nutrients can be changed into plant tissue by the trees instead of reaching the stream.

3 Stream Bottom
Streams that travel through woodlands are much wider than meadow streams. Wide, shallow woodland streams provide more habitat to support stream life.

4 Canopy Shade
The leaf canopy above a woodland stream provides shade that keeps the water temperature cool and encourages the growth of nutritious algae, including diatoms.

Leaf litter falls into the stream and collects in bunches called leaf packs, which provide food and habitat for small bottom-dwelling stream creatures.

Questions:

  1. How many different kinds of macros did your leaf pack contain? How many total organisms were there?
  2. Define what is meant by biodiversity and bioindicators. How do these terms apply to the leaf pack experiment?
  3. If there were no trees along the stream would you expect the number of macros to change? How would this affect the stream community?
  4. Why was it necessary to use 3 leaf packs for each experimental condition?
  5. How many leaf packs would be needed in order to investigate 2 separate variables at once?
  6. 6. Describe an investigation of your own design that would test 2 variables at the same time.


References:

  1. Cummins, K.W. and Wilsbach, M.A., Field Procedures for Analysis of Functional Feeding Groups of Stream Macroinvertebrates, Appalachian Environmental Laboratory, University of Maryland, Frostburg, Maryland, 1985.
  2. McCafferty, W.P., Aquatic Entomology, Jones and Bartlett Publishers, Inc., 1983.
  3. Merritt, R.W., and Cummins, K.W., An Introduction to the Aquatic Insects of North America, 2nd Edition., Kendall/Hunt Publishing Company, 1984.
  4. Needham, J.G., and Needham, P.R., A Guide to the Study of Fresh Water Biology, Holden-Day, Inc., 5th Edition, 1974.
  5. Pennak, R.W., Fresh Water Invertebrates of the United States, 3rd Edition, John Wiley and Sons, Inc., 1966.
  6. Reid, G.K., Pond Life, Golden Press, 1987.
  7. River Water Network, Guide to Macroinvertebrate Sampling, Montpelier, VT, 1993.
  8. Ward, H.B., and Whipple, G.C., Edmonson, W.T., Ed., Fresh-Water Biology, 2nd Edition, John Wiley and Sons, Inc., 1966.


Procedure:

Preparing the Leaf Packs for the Stream
A short time after autumn leaf fall, gather leaves and place them in a plastic bag, taking care to keep the leaf types separate. At least 3 onion bags of each leaf type, or experimental condition, should be prepared. When gathering, be sure to collect leaves of uniform dryness and type. This is most easily achieved as soon after autumn leaf fall as possible, before the leaves have become mixed or damp. Weigh 20 grams of leaves in each onion bag. Place a waterproof tag, labeled with the bag number and experimental condition, inside each onion bag. The open end of the onion bag can then be knotted closed. Securely tie the packs with nylon string to cinder blocks or bricks. Loop the string through the mesh so that the bag cannot separate from the brick.

Choosing a Stream Location The most macros will be attracted when the artificial leaf packs are placed in areas where natural leaf packs occur. These areas are usually "riffle" areas in which water moves swiftly and there are many rocks. Packs can be placed in other stream habitats, but, all packs should be placed in the same kind of habitat with about the same water depth and velocity.

Stream Location Diagram

Put the packs and bricks in a riffle area with the leaf packs facing upstream, so that as much surface area is facing the current as possible, thus simulating a natural leaf pack. Take care to submerge all the leaves. If cinder blocks are used, use a sledge hammer to drive a piece of rebar (sections of iron reinforcing rods) through the hollow part of the block securing it to the stream bed. If the experimental area is a spring brook, in a forested area, the storm flow is small enough that the rebar stakes are not needed; the packs will be secured by the weight of the brick. For all other locations the use of cement blocks and rebar are recommended. Bricks will be moved by high water in streams as narrow as two or three meters. Generally, a stream width of two to five meters is the most desirable. An ideal velocity would be between one and three meters/sec., and an ideal depth would be between 15 and 40 cm.

Ask students to fill-out the data record sheet indicating the bag numbers on the site map. Leave the packs in the water 4-6 weeks, checking them every few days and after storms to make sure they remain submerged. (Be sure to get the property owner's permission to use the stream location before setting-up the experiment.)

Collecting the Leaf Packs
Collect the packs and place them in zip-lock bags starting with the pack farthest downstream and work upstream. Pick the packs up quickly and uniformly because some of the insects are very quick and will try to escape. Place the zip-lock bags in an ice filled cooler to bring the packs inside. Most aquatic insects can be refrigerated or iced in coolers for several days without harm. Leave a little bit of water in the bottom on the bag making sure the animals are moist, but not submerged in water. Leaf packs should not be collected immediately after a storm, because some of the macros may have been washed downstream by the increased current. After a major storm, check the leaf pack for sediment load (you may have to gently "flush" it) and leave it in the stream for two more weeks or longer.

Setting-up the Lab for Sorting Set-up 6 areas for groups of students to work around, one area for each leaf pack. In the center of each area, place a macro ID sheet. Divide the forceps, spoons, hand lenses, sets of flash cards, Leaf Pack Invertebrate Record Sheet and petri dishes among the 6 stations.

Open the zip-lock bags and place the contents in a spaghetti sieve or on window screen. Look for a sieve with openings between .5 mm and 1.5 mm in size. Wash the leaf contents gently with water until all of the sediment is removed and the water appears clear, taking care not to spill any leaves or insects. Rinse the onion bag and the inside of the zip-lock bag into the sieve. Divide the sample evenly into white pans or individual white plastic bowls (one per student) filled with water. Cool tap water can be used, however, if the water contains chlorine the macros will not live very long - perhaps 45 minutes. When possible use cool stream water which will keep the insects alive for a longer period of time and return them to the stream when finished. Now the macros can be sorted and tallied.

Morphotyping
Because this lesson focuses on the importance of trees to streams, we recommend using the flash cards to enter the Data and Record Sheets for a "field guide" approach to identification rather than spending a lengthy amount of time on taxonomy and the use of dichotomous keys. The easiest way to separate the animals from the leaves is to remove the leaves one at a time from the water, inspecting the leaves to make sure there are no macros clinging. Ask students to sort the macros by kind. Place each group in a water filled petri dish next to the drawing of the animal it looks like.

PLEASE NOTE: Students may need to be reminded to be kind to their macros by putting water in the petri dish.

In most cases, the macros can live for an hour or more in covered petri dishes without harm. For a better look, students can place the petri dishes directly under a dissecting microscope or they can observe them with a hand lens. We have done our best to pick some of the most common stream macros, but if students find a unique macro that is not illustrated on the flash cards, then count it as a new group and place it on the flash cards near the picture that it most closely resembles; add the count of the new "morphotype" to the tally sheet. At the conclusion of the activity, ask the students to tally each kind from their leaf pack on to the Invertebrate Record Sheet. Combine the whole class's data onto two data sheets, one tally for each experimental variable. The tally of the number of individuals and the different kinds of macros can be used to generate a bar graph of the results (below).


Leaf Reference Bargraph

Variety of Macros at Two Locations Bargraph

Leaf Pack Data Sheet

Leaf Pack Invertebrate Record Sheet


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Last revision to this page on March 20, 1998

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