Food Chains, Webs, and Pyramids
A food chain shows how each living thing gets its food. Some animals eat plants and some animals eat other animals. For example, a simple food chain links the trees & shrubs, the giraffes (that eat trees & shrubs), and the lions (that eat the giraffes). Each link in this chain is food for the next link. A food chain always starts with plant life and ends with an animal.
Coyote Raccoon Squirrel Oak Tree
Most food chains only have 4 or 5 organisms in them.
There cannot be too many links in a single food chain because the animals at the end of the chain would not get enough food (and hence energy) to stay alive.
Most animals are part of more than one food chain and eat more than one kind of food in order to meet their food and energy requirements. These interconnected food chains form a food web.
A change in the size of one population in a food chain will affect other populations.
This interdependence of the populations within a food chain helps to maintain the balance of plant and animal populations within a community. For example, when there are too many giraffes; there will be insufficient trees and shrubs for all of them to eat. Many giraffes will starve and die. Fewer giraffes means more time for the trees and shrubs to grow to maturity and multiply. Fewer giraffes also means less food is available for the lions to eat and some lions will starve to death. When there are fewer lions, the giraffe population will increase.
Food Pyramids Indicate the number of organisms at each trophic level. Usually there are less organisms as the trophic levels increase but there are some exceptions. One of these exceptions is Mosquitoes. An energy pyramid is the graphical representation of the trophic levels (nutritional) by which the incoming solar energy is transferred into an ecosystem. The source of energy for living beings on Earth is the Sun.
For example, the amount of energy acquired by the herbivores is equivalent to the ingestion of one gram of organic material from photosynthetic organisms. Each subsequent amount of energy (green rectangle) in the pyramid (towards the peak) is the energy that would be obtained by each gram of organic material originated from the underlying level. Detritivores or detritivorous
are organisms that fed on remnants of organic matter, like corpses, excrements, etc. Detritivores take advantage of ca. 57% from the energy stored by the producers.
Patterns and Controls of Primary Production in
the World's Ecosystems
The world's ecosystems vary tremendously in
productivity, as illustrated in the following figures. In
terms of NPP per unit area, the most productive
systems are estuaries, swamps and marshes,
tropical rain forests, and temperate rain forests.
If we wish to know the total amount of NPP in the world, we must multiply these values by the area that the various ecosystems occupy. In doing that, we find that now the most productive systems are open oceans, tropical rain forests, savannas, and tropical seasonal forests. Areas that are warm and wet generally are more productive. Overall, the amount of water available limits land primary production on our world, in part due to the large areas of desert found on certain continents. Agricultural crops are especially productive due to "artificial" subsidies of water and fertilizers, as well as the control of pests.
Page 302 Questions 1‐3
Draw a food web with four food chains each with four organisms and state the food chains