Food Chain Lets start with the food chain, which is a series of organisms each dependent on the next as a source of food. Food chains start with a produce, an organism that is able to make its own food (ex: plant). Another name for a producer is an autotroph. Then an organism eats the plant, which would be considered the primary consumer, then that organisms is eaten by another organism, the secondary consumer, and then another organism eats that organism, tertiary consumer. All organisms that must consume another organism for their food is called a heterotroph, or consumer. The primary consumer, which always eats the producer could also be called a herbivore, because producers are plants, and a herbivore is an animal that only eats plants. An animal that only eats other animals are called carnivores, which mean that they only eat meat. And an animal that eats both plants and animals would be called an omnivore.
The following shows 3 different food chains. In the first one corn is our producer, which is eaten by a mouse (the primary consumer), and the mouse is eaten by the owl (the secondary consumer). The second food chain is little longer, therefore showing the tertiary consumer, which is the lion in this example. If you notice the arrows are pointing in the direction of energy travel. The mouse gets the energy from the corn, and the owl gets the energy from the mouse. The corn goes into the mouse's stomach, and the mouse goes into the owl's stomach.
Continue looking at the food chains above. What would happen to the fox population if all the rabbits were killed in a short time span? Without the rabbit, the fox would have nothing to eat, therefore the fox population would die. Since the foxes are no longer around, the lion doesn't have anything to eat, therefore the lion population would also die. If we look at the other end of the chain at the carrots, the rabbit population disappearance would also affect them. Without the rabbits eating the carrots, there would be more carrots. So everything in the food chain would be affecting in one way or another if there was a quick disappearance.
Because the rabbits were killed in a short time span the fox would not be able to pick up a new food source. If the rabbits were disappearing over a long period of time, the foxes would have more chance to start eating something else to supplement their diets.
Energy Transfer
The producer receives its energy from the sun, which is the ultimate source of energy. Meaning all the energy we use comes from the sun. Like I said earlier, the energy from the corn moves into the mouse. Not all the energy that the corn has is able to be transferred into the mouse, only about 10% of the available energy is transferred to each trophic level. A trophic level is the position an animal occupies in a food chain. And again only about 10% of energy is transferred from the mouse to the owl.
To find 10%, we can multiply the starting number by 10%. We need to change 10% into decimal form, with is 0.10. Say the corn has 2,000 joules of energy. We will multiply 2,000 x 0.10 = 200
A short cut is to move the decimal over one spot to the left. 2,000.0 to 200.00 or 5,500.0 to 550.00
The carrot has 8,800 Kcal of energy. The rabbit would have 880 Kcal of energy, the fox would get 10% of that, which is 88 Kcal of energy and the lion gets 10% of that, 8.8 Kcal of energy.
This information can also be shown in the form of an energy pyramid. The bottom of the pyramid is always the producer. The next layer of the energy pyramid is the primary consumer, the next level is the secondary consumer, then the tertiary consumer. As we go up the pyramid, the energy decreases. Along with the energy decreasing there is also a decrease in the number of animals available at each level. For example there are more producers than primary consumers, more primary consumers than secondary consumers, and more secondary consumers than tertiary consumers.
Food Web
Food chains and pyramids are a very simplistic way to look at an ecosystem. In reality most animals have several different food choices. To show a more realistic view of an ecosystem, we look at food webs. A food web is a series of interlocking food chains.
The following is an example of a food web. As you can see, the arrows still point to the direction of energy transfer. The rabbit eats both the plant and the mouse. The Jackal eats the goat and the rabbit, and is eaten by the lion. The goat is eaten by the jackal and the lion, and it eats the plant.
Practice Problems
1. Organisms are classified into trophic levels according to:
Where they live
the source of their nutrients
how much they weigh
2. What do omnivores eat?
3. Where do producers get their energy?
4. What is another name for a producer? For a consumer?
5. What percentage of energy is transferred to each trophic level?
6. What is the ultimate source of energy?
7. Where is the energy the highest in this food chain: Acorns -> Squirrels -> Fox
8. If a tree leaf has 350 joules of energy, how much will be transferred to the primary consumer?
9. Use the following food chain: Grass -> Rabbit -> Fox If the fox has 25 kcal of energy, how much will the grass and rabbit have?
10. Using the following food chain: Corn -> Mouse -> Snake -> Hawk If the mice were all eliminated by a poison, which other organisms would be affected and how.