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Destination: Galapagos Islands Cyber Field Trip

Cyber Field Trip Teaching Guide
Backyard Science: How Ecosystems Work

Student activity available at

This interactive activity will be available for students beginning Friday, December 11, 1998.

National Science Education Standards
Concepts and Terms
Safety Precautions
Critical Thinking Questions


The word ecology is derived from the Greek word oikos, meaning "place to live" or "house." Ecology is the study of how organisms interact with one another and their physical and chemical environments. In other words, it is the study of how things are interconnected in nature. Because the delicate ecosystems found in the Galapagos Islands exist nowhere else on Earth and are, in some areas, largely undisturbed by humans, the islands are a great place to make ecological observations.


5-8: Populations and Ecosystems, Diversity and Adaptations of Organisms
9-12: Interdependence of Organisms; Matter, Energy and Organization in Living Systems
5-8: Transfer of Energy


  1. To understand interdependecies that exist in ecosystems, such as on the Galapagos Islands.

  2. To understand the transfer of energy in ecosystems.

  3. To understand the concepts of species, populations, biological communities, food chain and food web.

  4. To identify and diagram a food web that exists within local ecosystems.


To understand the interconnectedness of organisms, it is necessary to study ecosystems, or biological communities interacting with one another and their environment. Ecosystems have many components that can be broken into two major categories, biotic and abiotic. The biotic or living parts of an ecosystem include species, populations and biological communities. The abiotic influences are non-living, such as temperature, light, soil and water.

A species is a group of similar organisms that can produce fertile offspring.

Populations are groups of individuals of the same species that occupy a given area at the same time. (e.g., One species of land iguana, subcristatus, is found on five different islands in the Galapagos. All of the members of this species found on any given island are a population of these particular iguanas.)

All of the populations of different species occupying a particular ecosystem make up a biological community.

Organisms within ecosystems can be classified into two different categories: producers, or autotrophs, which make their own food, as green plants do; and consumers, or heterotrophs, which cannot make their own food and must eat producers or other consumers. Consumers can be further divided into herbivores, which eat plants; carnivores, which eat animals; omnivores, which eat both plants and animals; and detrivores, which feed off dead and decaying organisms.

The sequence of who eats whom in an ecosystem is called a food chain. All ecosystems have complex feeding networks, made up of many different food chains. This feeding network is called a food web.

Scientists assign every organism in an ecosystem to a feeding level, or trophic level, depending on whether it is a producer or consumer and on which type of consumer it is. The producer is on the first trophic level; the primary consumer is on the second trophic level; the secondary consumer is on the third trophic level; the tertiary consumer is on the fourth trophic level; and the quaternary consumer, if present in a food chain, would occupy the fifth trophic level.

blank tropic level pyramid

An introduced species is an animal or plant that is not native to an area but has been accidentally or purposefully brought to the area by humans. The results of species introduction can be devastating. Dogs were not naturally found in the Galapagos, but were brought in by humans. Darwin once wrote of the iguanas of Santiago Island, "I cannot give a more forcible proof of their numbers, that by stating that when we left Santiago Island, we could not for some time find a spot free from their burrows on which to pitch a single tent." Today, these iguanas can no longer be found on Santiago, largely due to the introduction of dogs to the island.


  • 3 x 5 index cards
  • Colored pencils or markers
  • Calculator
  • Meter stick
  • String
  • Wooden stakes
  • Plant and insect field guides


  • When working in the field, follow all safety precautions given by your teacher.
  • Be especially cautious of poisonous plants and stinging or biting animals.


One way to measure the population density of an organism is to divide a large area into smaller sections, count the number of organisms in each section, add the numbers together and divide by the total area of all the sections. The best size for the individual section depends on the relative size of the organism you're studying. For soil-dwelling organisms or insects, the section can be extremely small, while for mammals, the section should be much larger. This activity can be accomplished even if you do not know the names of the organisms. You must only be able to distinguish one species from another.
  1. Designate an area of study that is one square meter by marking it off with string and wooden stakes.

  2. Identify three different types of plants in this area to use for your population density study.

  3. Sketch each plant and, if possible, use a field guide to identify its common name. If you are unable to identify the plants, simply label your sketches A, B and C.

  4. Now divide the area into ten equal sections and mark each with string.

  5. Count and record the number of each of your selected plants in the first section. Repeat the procedure for the other nine sections. Finally, record the total population of each plant for the entire area.

  6. Have one student record the class totals for each plant on the Data Chart (copy or print out the chart below).

    Data Chart
    Section Organism A Organism B Organism C
    Section 1      
    Section 2      
    Section 3      
    Section 4      
    Section 5      
    Section 6      
    Section 7      
    Section 8      
    Section 9      
    Section 10      

  7. Divide each total by the number of students in your class to calculate the population density for each of the three plants. Your answers should be in the form, "There are n [name of plant] per square meter."

  8. Create an ecological pyramid of organisms found in your area, using the format below.

    ecological pyramid

  9. Calculate the amount of biomass (organic matter) stored in each organism in the pyramid using the following rules:

    • Herbivores store about 15% of the previous trophic level's biomass.

    • Carnivores store about 10% of the previous trophic level's biomass.

  10. After completing your ecological pyramid, answer the following questions:

    • Which organism is the primary consumer? The secondary consumer? The tertiary consumer?

    • Is there a top carnivore in your pyramid? If so, what organism occupies this position? On which trophic level is the secondary consumer? The tertiary consumer?

    • Calculate the amount of biomass in percent for each trophic level.


  • Identify three abiotic factors that might exist in any ecosystem.

  • Identify three biotic factors that might exist in any ecosystem.

  • Explain the difference between population and species.

  • Identify specific problems relating to indigenous wildlife that are caused by introduced species.

  • What is the difference between a food chain and a food web?

  • What impact have introduced species had on Galapagos Island ecosystems?

  • Diagram a four-level pyramid illustrating a food chain that might exist in the Galapagos Islands. Differentiate between and label the trophic levels and the producer(s) and primary, secondary and tertiary consumers. What percent biomass would be available to each trophic level?


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