Ecology Organisms and Their Interactions Study Guide

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Updated on Sep 21, 2011


When we talk about the relationships among organisms in an ecosystem, the most important is how they relate to each other as predators and prey. The best means of illustrating these relationships is through food chains and food webs.

Food Chains

Food chains don't just show who eats whom, but instead, they represent the flow of energy contained in the chemical bonds of food molecules. When a fox eats a rabbit, the chemical bonds that make up the tissues of the rabbit's body will be broken down by the digestive system of the fox. This digestive process releases energy and smaller chemical molecules that the fox's body uses to make more fox tissue. Likewise, before the fox eats the rabbit, it uses the food energy in grass and plants to gain energy for its own life processes.

Let's look at a simple food chain such as one that includes grass, grasshoppers, frogs, and raccoons. Sunlight energy enters the food chain during a series of chemical reactions called photosynthesis. These reactions take place in plant tissue. The plant uses this sunlight energy to make food molecules, which are stored within the tissues of the plant. When the grasshopper eats the plant, it consumes some of the food molecules and uses these molecules in its own body. An organism at the next level of the food chain, such as the frog, then eats the grasshopper and derives energy from the tissue of the grasshopper. A series of these steps from one organism to the next is called a food chain.

Food Webs

A food web is a more complex view of energy transmission that includes more predator-prey relationships between more organisms. Food chains are parts of food webs. Each step along a food chain or within a food web represents what is called a trophic (or feeding) level. The first step in any food chain or the first trophic level is always a photosynthetic organism. These organisms, such as plants in terrestrial ecosystems and algae in aquatic ecosystems, use light from the sun, water, carbon dioxide, and a few minerals to produce food molecules. The chemical bonds in food molecules represent captured energy that can then be available to fuel the whole food chain. Organisms at this first trophic level are known as primary producers. See the following illustration of a food web.

Ecology Organisms and their Interactions

Energy and Food Webs

Energy becomes a part of the animal communities through those who eat plants. These organisms are called herbivores and, because they are at the second trophic level, are also known as primary consumers. Herbivores (primary consumers) eat plants and derive energy for their own life processes. However, much of the energy that transfers from the first trophic level to the second level (or from primary producers to primary consumers) is not turned into herbivore tissue but is instead lost as heat, used in the digestive process itself or used for movement by the herbivore. Much of the plant material is not even digested, so it passes through the digestive system and is excreted as waste. This waste material still contains much energy in the chemical bonds that make up the material.

Because much of the energy available from the primary producers (plants) does not become part of an herbivore's body mass, a given amount of plant material would not be able to sustain as many herbivores as you might think. As you move from one trophic level to another, it is usually estimated that only 10% of the available energy gets turned into body tissue at the next higher level. As a simple example, consider a field that has 100 pounds of plants. In terms of the energy available, you might think this field could provide for 100 pounds of rabbits, but the reality is that it would only be able to support a group of rabbits that weighs a total of ten pounds. The loss of 90% of the energy to other factors results in only one or two rabbits. If you then move to the next higher trophic level, a rabbit will only provide enough energy to sustain one pound of a fox's body. Again, the loss of 90% of the energy yields not even enough left over to completely sustain a fox, but only to sustain the fox for a short period of time.

In Short

Organisms interact with each other, and the most common interaction is within the structure of a food chain in which one organism is the food and energy source for another organism. All food chains start with plants as producers because they can photosynthesize and capture sunlight energy. Animals are consumers and eat either plants or each other. Several food chains can be interwoven to create a food web in which many organisms interact with each other.

Practice problems of this concept can be found at: Ecology Organisms and Their Interactions Practice Questions

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