What Darwin Never Saw
Lesson Introduction
Different regions of the world have different soils and different plant life. The development of soil is influenced by temperature, rainfall, the plant life which decays and adds richness to the soil, and the organisms which live in the soil. Under a forest of deciduous trees, for example, the soil is rich in humus from the decaying leaves in autumn and is well mixed by the great number of creatures living on the forest floor. Coniferous trees drop their spiny leaves or needles all year around and tend to make the soil more acidic. Few animals live in this type of soil so it never gets well mixed in a coniferous forest.
The Galapagos islands began with no soil at all. Three million years ago, it was just ocean. Lava came up, cooled, and formed islands. Similar eruptions formed Hawaii, but people reached Hawaii, and they changed it. People didn't travel to the Galapagos to the same degree. That is what makes it such a great laboratory for ecologists (definition). During some seasons, it is fairly warm and dry, and certain types of seeds respond well. At other times, it is very wet, and other types of seeds grow better. The weather patterns are influenced by El Nino (definition). In this lesson, the students grow their own seeds in model grasslands, and use these terrestrial ecosystems (definition) to see the effects of vegetation on rate of evaporation, wind erosion, and water erosion.
Grade Level
6 through 12
Objectives
The students will:
1. make a model of a terrestrial ecosystem.
2. conduct an experiment with a control condition.
Time Allotment
two 40-minute sessions, with a period of two weeks for growing grass in between: use one session to build the terrestrial model and introduce the experiment; take time to water the grass and make some observations during the two week growing period; use a second session for investigating erosion
Materials
two-liter bottles, with the tops cut off about six inches high, two per group
garden soil
grass seed
spray bottles
water
fan
sunny spot in the classroom
video/video player
Books to Read
Galapagos: The Lost Paradise, by Peter Salwen, New York, NY: Bantam-Doubleday-Dell, 1990.
Weather Watch, by Valerie Wyatt, Reading, MA: Addison-Wesley Publishing, 1990.
Advanced Preparation and Teacher Notes
Arrange students in groups of four.
Procedure
Tap Prior Knowledge
1. Ask the students what is a habitat? Can they name some habitats? What makes them different from each other?
Share with Neighbor/Revise Ideas
2. In small groups, have the students discuss their responses, and come up with a description of a good habitat for growing vegetation. Focus the groups' discussion on the amount of rainfall different habitats need. How wet is the Galapagos region? Show the section of the What Darwin Never Saw video (from 48 00 to 54 40) which shows El Nino in action.
3. Scientists often study different regions of the world to find answers to local problems. Assign the role of botanist to each student and tell them that they will be investigating the growing conditions on the Galapagos islands. Share one of the biographies from our Career Connections with the students.
Engage Students in a Hands-on Activity
4. Distribute two 2-liter bottles with the tops cut off and some soil to each group. Have each group fill two of the containers with soil, then sprinkle the top with grass seed and lightly cover the seed with a thin layer of soil. Water thoroughly.
5. Find a place in the room which would simulate a good growing environment. Ask students what the place would need to have? Plenty of direct sunlight, at least.
6. Spray the models with six sprays of water three times per day until the grass sprouts and the height of the grass reaches 1 inch. This will take about two weeks.
7. Introduce the idea of experimental control to the students. When scientists are doing experiments they change one variable to see its effects. If more than one variable changes at the same time in an experiment, the outcome of the experiment may not be clearly attributable to any one of the variables. Spray one of the containers with water three times a day and pour two cups of water into the other one. Make observations. Does heavy rain affect one model more than another? Describing things as accurately as possible is important in science because it enables people to compare their observations with those of others. Encourage older students to keep a notebook that describes observations made, carefully distinguishes actual observations from ideas and speculations about what was observed, and is understandable weeks or months later.
8. Spray one of the models with water two times a day measuring the moisture with a toothpick at the beginning and end of class. Spray the other ten times a day. Note any differences in growing rates.
9. Continue the investigation by asking more questions. Does the grass slow down evaporation of water? Make observations. 10. Simulate wind erosion by continuing to spray as usual, but add the fan to represent the wind over the plants. Check at the beginning and end of the class period. Does the wind erode the soil in one container more than in another? What is the effect of one dry and one wet on the rate of erosion?
11. Simulate torrential rain by pouring one cup of water or spraying water directly and forcefully on the models and measuring the amount of soil runoff. Does heavy rain affect one model more than another?
Introduce the Scientific Principle/Concept
12. How might these factors represent what happens in the Galapagos? Plants allow the soil to stay in place when heavy winds or rains would otherwise erode it. On Daphne Major, for example, there is no place for the water to collect to soak into the dirt. It runs right off into the sea. Whatever is collected easily evaporates in the wind or hot sun. There is no soil to let the rain soak in. Also, clouds drop their rain on the south side of the taller and larger island of Santa Cruz, which is 8 kilometers windward of Daphne Major. Daphne Major hardly gets any of the moisture.
13. In the beginning, there was no life, no soil, and no growth at all. Along the coastlines, there was only barren lava. Soil slowly formed by the build up of organic matter. Cactus seeds then came from winds and birds. Animals came floating on pieces of land or ice. They got swept off course. Winds and waves carved out strange topographies.
14. How might these models represent what happens in the Galapagos? There is a mass of warm water in the Pacific Ocean which is called El Nino. It is called the Child because it visits the pacific Shoreline of South America around Christmas. An El Nino can change the wind and the weather around the world. Heat energy carried by ocean currents has a strong influence on climate around the world. El Ninos typically come every three to six years, and there are many events in nature have a repeating pattern. It last came in 1983. When it is fairly warm and dry on the islands, certain types of seeds respond well. At other times, it is very wet, and other types of seeds grow better. In years where the rain that usually is centered over Indonesia and the far western Pacific moves eastward into the central Pacific, there is unseasonable weather in many regions- as far away as Australia and Africa. Climate models are being used to predict growing seasons.
15. Human beings are part of the earth's ecosystems. Human activities can, deliberately or inadvertently, alter the equilibrium in ecosystems. It is no exaggeration to say that major changes in our global temperatures or global ocean currents would change the course of evolution on the Galapagos Islands. The weather pattern the Grants saw may be just the beginning.
Connect to Other Everyday Examples
16. Different plants are adapted to different soils. Plants that make their homes in dry soils have small narrow leaves to prevent too much water loss from evaporation. Those in damp soils usually have large leaves to help them lose excess water. The part of a plant which is under the ground is often as big as the part of the plant we can see. Prairie plants have deep root systems which allow them to survive in dry conditions and survive prairie fires.
Lesson Assessment
17. Take photographs of the grassland models for the students' portfolios. Also, have each student write a paragraph summarizing the groups' findings. Each entry should include a description of rain amounts, and erosion by wind and water.
18. Give students a chance to surf the World Wide Web to find out about growing conditions around the world:
