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Every ecosystem has its own food web. This activity focuses on a food web for the marine ecosystem in the coastal waters of the Galápagos Islands. The ecology of a marine food web is influenced by environmental factors such as the climate, the salinity of the water, the ocean currents, and the winds.
In most ecosystems, the first level of a food web is composed entirely of producers organisms such as plants that get their energy directly from the sun. In some others, there is an outside energy source such as thermal vents in the deep oceans. Food webs are divided into trophic levels; the organisms in a particular trophic level are all the same number of steps away from the primary producers on the lowest, or first, level.
The higher levels of a food web, by contrast, are composed of consumers organisms that get their energy by eating other organisms. The second level is composed of herbivores, which get their energy by eating producers; the third level is composed of carnivores, which get their energy by eating herbivores; and the highest levels are composed of carnivores that eat other carnivores. Students will need to understand this concept of energy flow to construct their marine food web.
Decomposers exist at every level of a food web. Decomposers are those organisms that get their energy by devouring the dead organisms from every food web level. Whether breaking down dead plants to the inorganic nutrients that make up the soils, or decomposing dead animals, these organisms are as important in a food web as all of the others. Decomposers help to replenish the nutrients that are essential to the producers and help to rid the ecosystem of dead organisms that would quickly pile up.
Explain the difference between a food chain and a food web using the organisms of the Galápagos as examples. Two examples of simple food chains are given below. Write these on the chalkboard. Explain to students that the arrows point in the direction of food (energy) flow. Point out that in the first example, the turtle eats seaweed that grows attached to rocks in shallow coastal areas. In the second example, small fish eats zooplankton
(microscopic animals) which in turn eat phytoplankton (microscopic plants)
and other zooplankton that drift in the water. In both cases, the first element of the food chain is a producer, whose energy comes directly from the sun.
Explain the food web basics described in the Background section above.
Once students have discussed the different levels and energy flows of the Galápagos marine organisms, distribute copies of the Student Printout Galápagos Marine Organisms. Have students review the diets of each organism and create a poster of a Galápagos food web, working either individually or in groups. Students can color the pictures on the printouts, then cut them out and glue them onto poster board to produce an accurate representation of a Galápagos food web.
Ask for volunteers to explain the Galápagos food webs they created, explaining where they find examples of producers, consumers, herbivores, and carnivores.
Teacher answer key to trophic levels
Zooplankton, green sea turtles
Humpback whales, anchovies and sardines
Galápagos penguins, groupers
Sea lions, fur seals
Galápagos sharks, killer whales
At which levels of the food web would one would expect to find:
Q. The largest population? The smallest population?
A. Populations are largest in trophic level 1, at the bottom of the food web, and grow smaller with each successive level. Thus, the algae comprise the largest population, and the top carnivores (trophic level 6) are the smallest population.
Q. The largest adult organism? The smallest adult organism?
A. Both questions refer to body size and weight. The killer whale in trophic level 6 and the humpback whale in level 3 are the largest organisms listed on the printout. The smallest creatures are in the lowest level, and the largest creatures are usually in the higher levels, but this is not always the case. For example, the blue whale is the largest consumer organism on Earth, yet it grazes on plankton and zooplankton.
Q. Which organisms might be described as top carnivores and why?
A. As the killer whale and Galápagos shark are the largest carnivores and nothing preys on them, they are considered top carnivores.
Discuss how changes in the environment could affect populations of organisms. Below are two examples of how environmental changes can have severe consequences on life in the Galápagos. Use these examples to elaborate how a food web is dependent on many different factors:
El Niño: Marine iguanas primarily eat ulva, a type of green algae that looks like lettuce (a producer in trophic level 1), which grows on rocks in the cool, shallow waters around the islands. If, during an El Niño, the ocean temperatures were to rise, the ulva could disappear. As other types of algae are not as nutritious for the iguanas, iguanas would suffer from malnutrition and weaken, dying from disease and predation before they could die of hunger.
Upwellings: As the wind moves surface water away from the coast, cold subsurface waters are brought up to the surface in a process known as upwelling. Upwellings bring cold, mineral- and nutrient-rich water from the deeper sea levels close to shore. Because algae thrive on such nutrient-rich water, their population would expand greatly and clear water would turn greenish from the population explosion. Herbivores that eat the algae, such as the marine iguana, would thrive, as would algae-eating sea urchins and bivalves. However, unlike the iguana, these organisms wear down the reefs on which they feed. Large reefs, therefore, cannot survive in an area in which upwellings occur. If something were to cause an upwelling in the vicinity of a large reef, that reef would eventually be worn down, and the other life forms that need a large reef to survive would have to move elsewhere or die.
This activity uses the following principles of the National Science Education Standards for grades 5-8:
Populations and Ecosystems
Populations of organisms can be categorized by the function they serve in an ecosystem. Plants and some microorganisms are producers--they make their own food. All animals, including humans, are consumers, which obtain food by eating other organisms. Decomposers, primarily bacteria and fungi, are consumers that use waste materials and dead organisms for food. Food webs identify the relationships among producers, consumers, and decomposers in an ecosystem.
For ecosystems, the major source of energy is sunlight. Energy entering ecosystems as sunlight is transferred by producers into chemical energy through photosynthesis. That energy then passes from organism to organism in food webs.
The number of organisms an ecosystem can support depends on the resources available and abiotic factors, such as quantity of light and water, range of temperatures, and soil composition. Given adequate biotic and abiotic resources and no disease or predators, populations (including humans) increase at rapid rates. Lack of resources and other factors, such as predation and climate, limit the growth of populations in specific niches in the ecosystem.
Structure of the Earth System
Global patterns of atmospheric movement influence local weather. Oceans have a major effect on climate, because water in the oceans holds a large amount of heat.
This activity is an online modification of a chapter from NSTAs publication Ecology and Evolution: Islands of Change. Visit the Ecology and Evolution Web page to learn more about the range of Galápagos-oriented activities that can be found in the full-length book.