5.2: Food Chains and Food Webs

Science Background

Ecologist Versus Environmentalist

The media tend to present ecology and environmentalism as synonyms. This is not an accurate portrayal. Ecology is the science that deals with relationships between organisms and between organisms and the abiotic environment. Environmentalism is a philosophy (or group of philosophies) that tends toward a particular agenda, whether it be economic, political, or spiritual. True ecological studies do not have an “agenda” as such, apart from the pursuit of understanding about the natural world. Many environmentalists do base their outlook on ecological and other scientific principles, and most ecologists probably consider themselves environmentalists. But the two roles do not necessarily go together.

Similarly, ecology and environmental science are often equated in the popular perception, but these branches of science differ in their focus. Ecology is broader than environmental science, dealing with all relationships between living things and the non-living environment. Environmental science looks more specifically at interactions between humans and other living things and between humans and the non-living environment.

POSSIBLE MISCONCEPTIONS

Identify

Students may think that any group of organisms could be grouped together in a food chain or food web.

Clarify

Food chains and food webs describe how energy and nutrients are passed through a particular ecosystem. A food chain or food web cannot have any set of producers, herbivores, carnivores, and decomposers. For example, while an herbivore eats plants, no one herbivore is adapted to feed on all possible plants. Organisms from different ecosystems do not belong together in a food chain or food web.

Ask What They Think Now

At the end of the lesson, ask, Can you make a food web with the following organisms: tiger, human, zebra, penguin, algae, bamboo, panda? Why or why not? (No. Although the list contains a mix of producers, herbivores, and carnivores, these organisms belong in different ecosystems, so they do not have natural feeding relationships.)

How did you get to school today? Did you walk, ride your bike, or take a bus (Figure 1)? Did you walk from class to class during the day? Did you write in your notebook today? All of these activities require energy. We obtain energy from the food we eat. But how did energy get into our food in the first place?

**Figure 1** You obtain the energy you need to get to school from the food you eat.

Linking to Literacy

Before Reading: Using Text Features to Support Predicting
Before reading, take a few moments to scan the content. Look at the following text features: title, subtitles, illustrations, photos, highlighted text, and captions. What information can you learn from scanning these features before you read? Make a prediction about the kind of information you will learn about as you read this section.

The path energy takes in an ecosystem begins with the Sun. All plants grow by absorbing energy from the Sun and turning it into foods such as sugars and starches. The energy of the Sun is stored in the plant. When an animal such as a chicken eats grain (seeds of various grass plants), the energy stored in the plant is transferred from the starches in the grain to the chicken (Figure 2). When you eat chicken for dinner, some of the energy is passed on to you. So, energy starts from the Sun and passes from producers to consumers.

Energy is transferred from the sun to grains, to chicken feeding on the grain, and to humans having chicken for dinner. — **Figure 2** The energy from the Sun is stored by producers in the form of high-energy sugars and starches. When an organism consumes grain, some of the Sun’s energy is passed on.

Food Chains

A food chain is a model that shows how energy and nutrients flow from one organism to another in an ecosystem. Arrows show the direction of the energy and nutrient flow. The number of “links” in a food chain can vary, but the food chain always starts with a producer and ends with a consumer. A food chain can have just two components, a producer and a consumer (Figure 3(a)). Other food chains are more complicated (Figures 3(b) and (c)). A producer may be consumed by a herbivore (primary consumer), which is then eaten by a carnivore (secondary consumer). In some cases, another carnivore (tertiary consumer) eats the first carnivore.

food chain: a sequence that shows how energy and nutrients are transferred from one organism to another in an ecosystem

Food Webs

Food chains show one producer being eaten by one consumer and perhaps another consumer eating the first consumer. Food chains are not that simple in real ecosystems. Producers are usually eaten by many different consumers, and most consumers are eaten by more than one predator. For example, a squirrel eats several different types of seeds, fruits, and nuts. The squirrel may be eaten by a fox, a hawk, or a raccoon. The raccoon also feeds on frogs, clams, birds’ eggs, and corn. The fox will also eat mice and grasshoppers; the hawk will also eat frogs, mice, and snakes. Most organisms are part of several food chains. A model that shows the connections between several different food chains is called a food web (Figure 4). A food web starts with the producers in the ecosystem and then branches off into interconnected food chains that show who eats whom in the ecosystem. Food webs can quickly become very complex.

food web: a model that shows how food chains in an ecosystem are connected

Changes in Food Chains and Food Webs

If one species is eliminated from a food chain or web, it affects the other species in that chain or web. For example, consider the simple food chain: clover → rabbit → fox. If rabbits are removed from the food chain, then the population of foxes in that ecosystem will decrease because they will have no food (Figure 5). If one species disappears from a food chain, organisms that eat that species may eat other species instead. This will also affect the food web that the food chain is part of. The foxes, for example, may begin to eat mice and insects instead. When any part of a food web changes, it affects the flow of energy throughout the web.

**Figure 5** Foxes rely on rabbits as a source of food. If rabbits are not available to feed on, the population of foxes must find other organisms for food, or they will die.

Check Your Learning

Create two food chains from the following list. Use Figure 3 to help you.
- raccoon
- mallard duck
- mosquito larva
- algae
- Elodea
- freshwater clam
- human
- duckweed
SAMPLE ANSWER
algae → mosquito larva → mallard duck → human

algae → freshwater clam → raccoon
What is the role of producers in food chains and food webs?
SUGGESTED ANSWER Producers convert energy in sunlight into stored energy in food.
1. What are the similarities between food webs and food chains?
  SUGGESTED ANSWER Food webs and food chains are both models that show how energy and nutrients pass from one organism to another in an ecosystem.
2. What are the differences between food webs and food chains?
  SUGGESTED ANSWER In a food chain, each organism only has one source of food and is eaten by at most one other organism. In a food web every organism can have many food sources and can be the food source for more than one other organism.
In your own words, explain what may happen to a food web if one of the species of the web is removed.
SUGGESTED ANSWER If a species is removed from a food web, the food web could collapse, or it could reorganize depending on how many organisms relied on the species that had been removed.
What is the initial source of the energy in all food webs and food chains?
SUGGESTED ANSWER The initial source of energy for most food chains and food webs is sunlight. Producers use energy in sunlight to make food.