Life in the Desert
Background: Terrestrial animals in the desert exhibit a number of different strategies to cope with scarce and unpredictable water sources. Some forage at night, when lower temperatures reduce the amount of water the animals’ bodies need for cooling. Some other mechanisms are gathering early morning dew and excreting small amounts of very concentrated urine.
Question: What strategies do terrestrial animals which depend upon limited or temporary water supplies need to survive?
Materials: You will need small sponges, water, a balance scale, and descriptions of desert plants and animals.
Procedure: 1. Provide students, working individually or in small groups, with a small sponge saturated with water. Explain to them that this represents a desert animal with a limited amount of available water. Over a 24-hour period, students should take care of their “animal” in a manner that will best conserve the water it contains, using only natural materials. Their “animal” must be in the open for at least four hours during that time to “feed.”
2. To measure the beginning moisture content, each student or group should use the balance to determine the mass of its sponge. A control sponge should be left unprotected for the experiment’s duration. Students should then plan a strategy and write it down along with predictions of what will happen.
3. During the 24-hour period, students should make and record observations. At the end of the allotted time, students again record the mass of their sponges. Students should compare with the previous mass and make inferences about the results in relation to real organisms with limited or temporary water supplies, such as lizards, pack rats, and coyotes.
4. Have individuals or groups share their experiments and results with the entire class. Afterward conduct a class discussion of methods, results, and how this relates to adaptations for survival in real organisms.
Life in a Pothole
In the following activity students will explore adaptive strategies that animal life dependent upon limited or temporary water supplies would need to survive.
Materials: sponges, water, balance scale and descriptions of desert plants and animals.
Procedure: Provide students, working individually or in small groups, with a small sponge, saturated with water. Explain to them that this represents a desert animal with a limited amount of available water. Their job is to conserve the water. Over a 24-hour period they are to take care of their “creature” in a manner that will best achieve this goal using only natural materials. Their "creature" must be in the open for at least four hours during that time to feed.
To measure the beginning moisture content each student or group weighs its sponge. A control sponge should be left unprotected for the experiment’s duration. They then plan a strategy and write it down along with predictions of what will happen. During the experiment students make and record observations. At the end of the allotted time students again weigh their sponges and record weights. Students should compare with previous weights and make inferences about the results in relation to real organisms with limited or temporary water supplies, such as pothole organisms. Iindividuals or groups will share their experiments and results with the entire class. Afterward conduct a class discussion of methods, results and how it relates to adaptations for survival in real organisms.
Extension: Ask students to predict how various lifeforms come to populate widespread potholes. Insects, toads and birds can use their mobility to select the potholes they wish. There is some preliminary evidence that at least the female toad returns to the pothole from which she hatched. During periods of heavy rainfall, some potholes could overflow into lower potholes, washing organisms along with it. This is probably not very common since most potholes are not in a downstream relationship with one another, and population inventories show a widely scattered, almost random pattern of species presence or absence. Birds may help populate potholes with organisms like snails, that could cling to their feet and be transported to the next water body the bird visits. Desiccated shrimp eggs and mites are almost identical in dimension to sediment grains, which, when dry, are blown about by the wind. The eggs and mites would be transported along with sediments. Wind distribution is consistent with the random population pattern.
This activity lets students demonstrate the greater water retention capacity of a healthy riparian system. Healthy streams generally have a shallow gradient and numerous meanders. Water slowly moves along, allowing it to soak deep into the banks, which act like giant sponges. They release water during periods of low waterflow, providing a buffer to the riparian-dependant plants and animals.
You will need a measuring cup, several flat-bottomed sponges, a cookie sheet, and a pan wide enough to accommodate the narrow end of the cookie sheet. First, simulate an unhealthy riparian area by laying dry sponges end to end in two rows the length of the cookie sheet, with about 5 cm of space between the rows. Place the end of the cookie sheet in the pan, and hold the opposite end at a low angle (don’t let the cookie sheet rest on the edge of a pan if the angle of the cookie sheet will be steep). Now gradually pour a cup of water down the trough between the sponge rows. Some water should seep into the sponges, but most should wash into the pan. Measure how much water is in the pan. Now simulate a healthy riparian system by laying pre-trimmed dry sponges in two parallel rows, but this time arrange them in a series of several curves. (Cut and piece wedges of sponge beforehand to create solid banks.) Again, gradually pour a cup of water down the trough and measure the amount that makes it to the pan. This time there should be much less water in the pan, because the healthy riparian system has soaked up water into its banks. You can further experiment with the angle of the cookie sheet to simulate the effects of different stream gradients.