Food Chains and Webs

Food Chains

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Vocabulary

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Discussion Starter

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Food Webs

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Essential Questions for Food Chains and Food Webs

1. How is energy passed from one animal to another?
2. Why do food chains rarely go over four trophic levels?
3. Why do all food chains start with something that photosynthesizes?

Background Information on Food Chains and Food Webs

Every food chain starts with energy from the Sun. Green plants are autotrophic, meaning they create their own food using a chemical reaction called photosynthesis. During photosynthesis, plants take carbon dioxide from the air and water from the ground through their roots, which react to produce glucose and oxygen.

The word equation for this reaction is carbon dioxide + water → glucose + oxygen
The symbol equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The glucose that plants make is used for respiration and can also be stored, often as starch. When the plant is consumed by another living thing, some of this stored energy is passed on. Energy is lost at each trophic level, as not all the energy is used for growth and stored in the living being. Some of the energy is used for respiration and other life processes, so this energy is ultimately released into the atmosphere as heat. Some of the energy in the food is lost as waste, such as feces. The shorter the food chain is, the more efficient the energy transfer is and less energy is lost to the environment.

A living thing that photosynthesizes is called a producer. On land, this is normally a green plant. In the oceans, the producer is seaweed or phytoplankton, which are microscopic organisms that use the Sun’s energy to create food. Food chains end in bacteria called decomposers, who extract the chemical energy out of the remains of living things. They are nature’s way of recycling and without them, the planet would be a lot messier. In every habitat, there is an apex killer, who is a well adapted killing machine.

Take this example food chain: Grass → Caterpillar → Sparrow → Hawk. The grass is the producer; it is a green plant that uses photosynthesis to create glucose. The caterpillar is the primary consumer. It is an herbivore that only eats plants. The next animal on the food chain is the sparrow. The sparrow is an omnivore, meaning it gets its nutrients from both plants and animals, and it is called the secondary consumer. The sparrow is prey for the hawk. The hawk is a predator. It is well adapted for the job, as it has incredible eyesight that allows it to spot its prey from far away. Its sharp claws allow it to grab its prey midair. The hawk is the apex predator, meaning there is no other animal above it on the food chain.

The populations of these animals are all connected. If there is a drought one year and the amount of grass decreases, the number of caterpillars could be affected. If the number of caterpillars reduces, then this could affect the number of sparrows, which could in turn affect the number of hawks. The arrows in the food chain shows the flow of energy from one living thing to another. They point from the organism being eaten to the feeder. In addition to energy and matter being passed from one organism to the next, there are nonliving parts of an ecosystem that can provide matter to living things, like air, water, and minerals.

Ecosystems are vast and animals rarely exist in a single food chain. Few animals rarely eat just one type of food; instead they get their nutrients from different sources. This also varies depending on the time of year and location of the animal. A fox in northern Alaska will eat different food than a fox in Massachusetts. Food webs are a more accurate way to show the flow of energy from one living thing to another. More complicated feeding relationships can be shown as food webs with different trophic levels. Students will need to be able to define the boundaries of the ecosystem they are describing when they create food webs. For example, does their model describe the ecosystem of part of a forest or an entire forest?

The Next Generation Science Standards push the importance of getting students to develop and use models to understand phenomena. In the real world, scientists will make models to aid their understanding of a system or part of a system. Models are used in Science to make predictions and communicate ideas or data to other people. There are a range of activities in these lesson plans that focus on that particular skill. Students will easily be able to create their own models to describe how matter is cycled and energy flows among living and nonliving parts of an ecosystem. This gives you a great opportunity to discuss the limitations of using models, giving students the opportunity to evaluate and refine them.

To look in more detail at how carbon is cycled among the biosphere, atmosphere, hydrosphere, and geosphere, see the carbon cycle lesson plans.