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Why So Many Frogs?

Lesson Objectives

By the end of this activity, students will:
  1. Be able to explain the different stages in the growth of a frog.
  2. Analyze and collect data to make generalizations about a larger population.
  3. Determine the survival rate of a population of tadpoles under controlled conditions.
  4. Explain why organisms produce large numbers of young.

National Science Standards

This lesson addresses the following national content standards found in the McRel Standards Database at
  1. Knows the organization of simple food chains and food webs (e.g., green plants make their own food with sunlight, water, and air; some animals eat the plants; some animals eat the animals that eat the plants)
  2. Knows that plants and animals progress through life cycles of birth, growth and development, reproduction, and death; the details of these life cycles are different for different organisms
  3. Knows how the interrelationships and interdependencies among organisms generate stable ecosystems that fluctuate around a state of rough equilibrium for hundreds or thousands of years (e.g., growth of a population is held in check by environmental factors such as depletion of food or nesting sites, increased loss due to larger numbers of predators or parasites)
Grade level: 5-7

Tools and Materials Needed
  1. Copy of the program "The Living Edens: Costa Rica, Land of Pure Life"
  2. Masses of fertilized frog eggs. A mass of 300 eggs is needed for a class of 30. If these can not be obtained locally, they may be purchased though a number of biological supply companies. (Carolina Biological Supply Company 1-800-334-5551)
  3. Spinach (cooked well)
  4. Source of fresh water.
  5. Centimeter ruler.
  6. Ziplock snack size bags.
Estimated Time to Complete Lesson
This activity will take several weeks to complete. It will take one to two periods to watch the video and discuss the concepts. Depending on the type of frog eggs that you acquire, you will obtain results over a period of four to six weeks.

Teaching Strategy

Background Information

The video provides a dramatic section on the reproduction strategy of the Olive Ridley Turtle. Once a year, these endangered turtles swarm onto the shores of Costa Rica and other countries to lay thousands of eggs in a period of about one week. The turtles then return to the sea leaving the eggs unguarded. Predators immediately come into the area and begin to uncover and eat many of the eggs. It is only through the sheer number of eggs that enough remain untouched to produce young turtles that then try to run the predator gauntlet to the sea. Throughout the world, this pattern is replicated in some form by many other species of the plant and animal kingdoms. Dogs produce litters; most plants produce seeds in large numbers and even humans in many areas produce large families. Most students are familiar with the sound of thousands of frogs each spring. Adult frogs emerge from the mud where they lay dormant and lay large egg masses in puddles of water. About 25 % of these eggs never hatch and birds, fish, salamanders and other predators eat many. However, enough grow and mature to ensure the continuation of the species. Unfortunately, the populations of many frog and amphibian species are on the decline for reasons not yet fully understood.

The study of the life history of a frog is fun and engaging for students from elementary school to high school and in many ways parallels the history of the Olive Ridley Turtle in the video. Frogs and toads usually lay their eggs in March or April when ice melts. Frog eggs occur in masses and toad eggs are found in long strings. The eggs develop rapidly and are easy for students to care for. When laid, many of the eggs (about 25%) will not be fertilized and will not develop. Fertilized eggs are easy to spot since the dark larvae of the frog will be on the top of the egg. The lighter area underneath is the egg sac. Interestingly, if the eggs are turned over, the frog larvae will quickly adjust themselves to be back on the top of the egg where they can absorb sunlight. Once the tadpoles hatch, they will like to hide themselves at the bottom of the tank but will come to the surface for food. Food should be added daily for a short time and the excess removed so the water stays clear. While boiled spinach or lettuce works well, liver or hardened egg yolk may be used as they become older. An additional death rate of 25% among the tadpoles is not unusual.

The tadpoles will quickly become larger. After a couple of weeks, the rear legs will begin to appear and the length of the tail will increase. At this point, reduce the depth of the water to a couple of inches and provide both sticks and rocks for the new frogs to begin to stand on. When the front legs begin to appear, the tadpoles may stop eating as the tail is absorbed for a food source.

It is possible to continue keeping the frogs as they become older but the amount of care increases significantly and it is recommended that they be released. If you plan to keep fully formed frogs within the classroom, consult a reference guide for the requirements of the species that you intend to keep.

As the tadpoles pass through the various stages, a number will die due to environmental conditions or mutations. It is not unusual for between 50% and 75% not to reach adulthood. In the outside world, even more would perish from predators. If an egg mass of 500 eggs were laid, probably 125 would not grow due to being unfertilized, another 125 would not make it through the tadpole stage and an additional 125 would not survive the transition from water to land. Of the 125 that would have the possibility of reaching adulthood, perhaps 90% would perish due to predators leaving only 5 to 10 frogs to reproduce and become the next generation.

  1. Watch the video "The Living Edens: Costa Rica, Land of Pure Life" with special emphasis on the first 7 to 8 minutes of the film. Discuss the number of eggs that are produced by the turtles and the dangers that are faced by the young.
  2. Ask students if new generations of other species face the same or similar dangers as they grow. Have small groups of 2 to 4 students take a species from your area and speculate on the dangers that face their young. Pictures of the organism could be drawn on newsprint with the dangers listed to the side with short presentation given to the class.
  3. Obtain a large aquarium containing pond water and some aquatic plants. Place frog egg masses into the aquarium and have students estimate the total number of eggs in the mass. Students should be able to explain how they determined the number of eggs. Students should be encouraged to pick up the mass.
  4. Take clear plastic cups for students to withdraw small groupings of eggs from the aquarium, and have students start a journal of daily observations. Students should record the size of an egg, draw a picture and record observations. Have students keep a chart with an estimated size of the tadpoles. This is best done in groups of four. At the end of each observation, students return the eggs to the aquarium. (You may have to cut the egg masses with scissors to get small grouping of eggs.)
  5. The instructor should emphasize how to care for the eggs each day. Discuss the sensitivity to soap, chemicals (especially chlorine). All water added to eggs or tadpoles must sit in the open for a day or be boiled and aerated. Tadpoles are fed spinach that has been boiled to a slimy consistency.
  6. As the eggs develop, students should estimate the numbers that have not grown. (This will be about 25%.)
  7. As the eggs hatch into tadpoles, students can be given 10 tadpoles in a Ziplock plastic bag to take home and raise. They are to continue the daily journals for a period of two weeks. Students can place the tadpoles in either glass bottles or plastic containers. Half of the water in the container should be replaced twice a week.
  8. As the tadpoles develop legs, students will need to place sticks or rocks in the container so they can begin to climb on them. Frogs should be released or returned to school when both front and back legs have grown.
  9. Students will present a final report that includes their journal along with a paper describing the development of frogs from egg through tadpole to small frog.
  10. A number of the developing tadpoles will fail to grow to maturity. When the final reports are submitted, students should mark on a class collection chart the number of tadpoles that they cared for and the number that survived to be released. The class numbers should be used to calculate the % of individuals that did not survive to adulthood. (This will be about 25%)
  11. Discuss the findings with the class. Identify that no predators were present and that the number of individuals that would survive would be lower in the wild.
  12. Use the numbers generated as a class to estimate how many individuals in a 500 egg mass would grow to maturity both with and without predators.
Helpful Web Sites
World Wildlife Federation Olive Ridley Page

Part of the "Species Under Threat" section, this site gives a description of the turtle and its current status. Students will find other threatened and endangered species to explore.

Comprehensive turtle site discussing the Olive Ridley turtle and many other species. Teachers and students may copy and use the diagrams, worksheets and pictures.

Exploratorium: Frogs
General information on frogs with many resources and links.

Site from the National Biological Information Infrastructure. Information on the decline of many organisms and extensive links.

Assessment Recommendations

Each student should complete a journal that includes daily observations, a size chart and drawings. The drawings should show the development of the tadpoles with labels where appropriate. Some sample labels would include leg buds, gills, egg sac and eyes.

The journals should be assessed periodically and submitted for a final grade that would be assessed by a rubric developed to reflect the number drawings, the detail of the drawings and the detail of the descriptions. A number of possible rubric entries are listed below and can be weighted to match your class objectives. Assigning each section a value of 20% makes it easy to base your grade on a 100 point scale. Students should also write a paragraph explaining why organisms in the wild produce large numbers of offspring. Since students will be discussing their findings on a daily basis, a class presentation of what they found is not needed.

Teacher and peer assessment can be facilitated by providing a rubric to score successful completion of lesson components. Rubrics may be generated online at Teach-nology: Rubric Generator at

Rubric Entries
  1. Daily written observation entries
  2. Growth chart
  3. Drawings of developing tadpoles
  4. Calculation and justification of answer for the number of surviving frogs at the various stages.
  5. Class participation and raising a tadpole
  1. Determine the number of seeds produced by the plants in a field by counting a small number and multiplying to determine an approximate number for the field.
  2. In the past, people commonly had large families with eight or more children. Students may interview their grandparents and older members of the community to find what diseases effected children in the past and what changes have occurred in society and medicine in the last century.

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Volcanos and Soil Formation

Lesson Objectives

By the end of this activity, students will:
  1. Understand why newly formed lava has little plant life.
  2. Use a model to determine why organic soil formation is a slow process.
  3. Grow a plant in "fresh" volcanic soil and observe the effect.
  4. Explain why one generation of plant life allows subsequent plant life to grow in an area.

National Science Standards

This lesson addresses the following national content standards found in the McRel Standards Database at
  1. Knows that changes in the environment can have different effects on different organisms (e.g., some organisms move in, others move out; some organisms survive and reproduce, others die)
  2. Knows that all organisms (including humans) cause changes in their environments, and these changes can be beneficial or detrimental
  3. Knows how matter is recycled within ecosystems (e.g., matter is transferred from one organism to another repeatedly, and between organisms and their physical environment; the total amount of matter remains constant, even though its form and location change)
  4. Knows components of soil and other factors that influence soil texture, fertility, and resistance to erosion (e.g., plant roots and debris, bacteria, fungi, worms, rodents)
Tools and Materials Needed
  1. Copy of the program "The Living Edens: Costa Rica, Land of Pure Life"
  2. Radish seed, birdseed or other fast-growing seeds.
  3. Small volcanic rocks. The volcanic rocks can be found at suppliers of materials for gas grills. If necessary, most any small pebbles will work.
  4. Journal
  5. 2 liter pop bottles or larger glass bottles
  6. Bag of peat moss or humus
  7. Camera (optional)
Grade Level: 5-7

Estimated Time to Complete Lesson

This activity will take from 6 weeks to 6 months to complete depending upon how many generations of plant that you raise. It would be possible to start this project with one class raising several generations and then store the bottles. Future years could reuse the same bottle and compare results to pictures of the plants and rocks from previous years.

It will take one to two periods to watch the video and discuss the concepts. You should plan one day to distribute and assemble the pop bottles. The first generation of plants should appear within two weeks and die fairly quickly. If you choose to continue adding seeds and observing subsequent generations, this activity will easily fill a semester or more.

Teaching Strategy

Background Information

The central mountains of Costa Rica consist of a chain of large, active volcanoes that erupt on a repeated basis. The lava that is ejected range from small pebbles to red-hot boulders larger than a car. Several of these boulders are shown in the film breaking into smaller pieces as they roll down the steep volcanic slopes in a large skree slope. Later, the film shows the bleak, unvegetated land that has been formed, the edges of the lava field as plants start to move into the area and finally the cloud forests that will come to dominate in the area. While the scenes from this film show plant succession in a volcanic area, succession from barren ground to climax vegetation occurs in all habitats, although with different plants. This activity should be related to the succession pattern in your area.

When newly formed volcanic lavas cool, or when an area of land is completely denuded by fire, construction or other means, it takes some time for plants to reestablish themselves. However, given enough time a few pioneers will begin to come into an area. These pioneer plants help loosen the rocks and soil and provide a bit of fertilization when they die. With each generation of plant life, additional nutrients become available that can be used by future generations of plants coming into the area.

In volcanic areas, lichens are normally the first plants to come into a lava field. They are adaptable to a wide variety of water and heat conditions and begin to break down the rocks into soil with the acid they produce. Students may be able to see this in your area by finding a lichen-covered rock and lightly peeling the lichen with a knife blade. They should find a few grains of sand that came from the larger rock. Unfortunately, lichens grow very slowly and are not suitable for a simulation on the student level.

The broken rock material formed by the lichens becomes mixed with the decayed plant remains and starts to collect in small crags and hollows. As this process occurs over many generations and more rock particles are added by weathering and the action of plant roots, soil with usable nutrients begins to form in the area. When conditions become right, newer varieties of plants will begin to grow. Over a period of time, the area will go through a series of plant types until it reaches a final stable vegetation cover called the climax vegetation. The climax vegetation will differ in each location due to differences in the minerals in the soil, temperature ranges, water availabilities and other climatic factors.

Once students study soil formation in a classroom setting, they should be taken to several areas to compare the differences in the soil layers, soil mineral makeup and life in the soil. A soil test kit that is available from any garden supply store will allow students to do fertility testing though more sophisticated kits may be found in science catalogs.

  1. Watch the video "The Living Edens: Costa Rica Land of Pure Life" with special attention to the section about the volcanic chain 23 minutes into the movie. Note the rock debris field, the pictures of the lichens on the rock and the views of the grasses, ferns and trees at the edges of the lava field.
  2. Take a 2 liter plastic pop bottle and cut the top off where it starts to have vertical sides. The removed piece will resemble a funnel that will fit into the top of the lower section when turned upside down.
  3. Put two inches of volcanic rock or other stone into the bottom of the container. Add about an inch of water. Add five or ten seeds.
  4. Twice a week, add three or four additional seeds but do not add additional water. In volcanic areas, water will collect underground and occasional rain will wet the surface of the rocks. (In this simulation, the volcanic rocks will absorb enough water from the bottom of the container for the seeds to sprout but we want them to die in a short period of time.)
  5. The seeds will sprout and then die. This organic material will build up over a long period of time. Students should keep a journal of observations. It is somewhat easier for the journals to be kept in the room rather than having the students try to keep them over the period of the observations.
  6. If the students raise enough generations of plants, they will begin to observe that the plants begin to live for a longer period than they did at first. However, this will take many generations of plants. To simulate the formation of soil, small amounts (a tablespoon) of peat moss, humus or topsoil can be added between each generation. The additional material will allow roots to grow better and the plants will be healthier. Students should think of the added soil as representing 10 or more generations of plants.
Helpful Web Sites
Teacher's Guide to Craters of the Moon National Monument: Plant Succession
Part of the Teachers Guide to Craters of the Moon National Monument. This section discusses plant succession on the lava fields of the Monument. More detailed information on aspects of the park can be found on the site.

Volcano World
One of the Internet's premier sites for everything about volcanoes.

Snapshots in Time
When Mt. St. Helens erupted in 1980, it devastated the surrounding areas and killed most of the plant and animal life. These pages document the changes during the last 20 years as plant succession has begun to reclaim the area.

Assessment Recommendations

Each student should complete a journal that includes both repeated observations of the plants and pop bottle. Students should record plant heights and information about the health of the plants. Students should be able to make statements generalizing what they found from the activity and relating it to the soil formation in your area. Due to the length of time of the activity, it is recommended that the journal records and participation constitute any grade and that objectives relating to soil be assessed within other parts of the class.

Teacher and peer assessment can be facilitated by providing a rubric to score successful completion of lesson components. Rubrics may be generated online at Teach-nology: Rubric Generator at

  1. Students can use the bottle to make a terrarium of plants found in your area.
  2. Starting the seeds with the water an inch over the rocks and adding aquatic plants can make a representation of pond succession. As you continue to add seeds, let the water evaporate. When the water lowers to a level below the rocks, the aquatic plants will die and the seeds will begin to grow.
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Lesson 2: Volcanoes Lesson 1: Frogs