Illinois Natural History Survey - University of Illinois

The Naturalist's Apprentice: Stream Habitats--Leaf Packs and Surface Film

Students will be able to recognize two stream habitats.


Leaf Packs: an oblong pan about 1 1/2 inches deep (9" x 13" cake pans work well); a quantity of small stones and twigs; confetti (such as from a paper punch); small paper cups or envelopes to hold the confetti; a soda straw.

The Water's Surface: a clear glass bowl two-thirds full of water; a sewing needle; a straight pin with a large head.


Background Information:
Leaves and twigs fall into a stream from trees along the banks. This material is pushed downstream by the current and tends to accumulate in areas where the current is less strong or where it has become trapped against logs or rocks. These accumulations of leaves are known as leaf packs and provide an abundance of hiding and feeding areas for aquatic invertebrates.

The surface of a quiet marsh or pond presents no barrier to a raccoon searching for crayfish or a heron attempting to spear an elusive tadpole. To the small creatures of the world, however, the water's surface presents a firm but flexible surface, and many plants and animals are well-adapted for living at or near this surface. Surface tension occurs because water molecules are more strongly attracted to each other than to the air above. The surface of the water, therefore, is held in place from each side and from below, and this attraction results in a dense surface film of water molecules.


Extending the Activity  
If you have the opportunity, take students to a nearby stream and have them attempt to find the two habitats. Examine one or more leaf packs and observe what organisms live in this unusual habitat.



Stream Habitats

To demonstrate a leaf pack:

1. Place a few stones and sticks in a pan, including some piles of stones and sticks along the edges. Place a pile of confetti at one end of the pan and gently blow on it with a straw. The air flow represents flowing water. Blow only in one direction to move the confetti (leaves) down the pan (streambed).


2. The leaves (confetti) will become caught and pile up in areas around the stones and twigs. The confetti does not readily move down the stream and easily becomes caught. If you blow harder, some leaves will break free from the debris and flow by while others will become packed in even more firmly. The packed confetti represents leaf packs, accumulations of fallen leaves and other debris that collect in certain areas of a stream. Leaf packs provide feeding and hiding places for many aquatic macroinvertebrates, and they can be an important habitat in the stream community.


To demonstrate surface tension:

1. Carefully lower a dry needle onto the water's surface. It will float. Warning: If the needle is wet, or if it is not placed on the surface very carefully, it will sink. The fact that the metal needle floats on the surface film demonstrates surface tension.


2. Hold a straight pin by its point and push the head against the surface of the water. Observe what happens through the side of the bowl of water. The water dimples down, but if you push too far, the head of the pin will break through the surface, once again allowing the water's surface to become level. As you slowly pull the pin back out, notice that the water sticks to the bottom of the pin and raises up for a short distance before it breaks free from the pin head. This phenomenon also illustrates surface tension.


Answer the following questions:


* What types of adaptations would enable invertebrates or plants to take advantage of the surface tension of water? Sketch some of these designs for surface living.

* What would happen to the animals that live on the surface film of a pond or lake if the water were polluted with soap or other chemicals that altered the surface tension?

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