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Food Chains and Food Webs
"What's for dinner?"

Ortiz, Mary France L. BEED 2 – IRR

Partido State University

College of Education

ortizmaryfrance@yahoo.com

February 9, 2013

“Humans are so busy asking about who or what had built the pyramids.
For me, whoever had built pyramids, the message is so clear and simple, 
"we are higher than human race in the food chain.” 
― Toba Beta, My Ancestor Was an Ancient Astronaut

An ecosystem is a living community which depends on each member and its surrounding environment. The living part of an ecosystem is sometimes called a food chain.

Every participant in an ecosystem has an important part to play and if one becomes more dominant than the others, the ecosystem can develop problems.

We'll start with the producers. These are living things which take the non living matter from the environment, such as minerals and gases and uses them to support life.

Green plants are considered producers and they are at the beginning of the food chain. 

Next are the consumers. These living things need the producers to be their food. Animals that eat plants are called herbivores. They are considered consumers and are next in the food chain. Animals that eat other animals are called carnivores. They also considered consumers and are a link farther along on the food chain since they need the herbivores for their food.
Animals and people who eat both animals and plants are called omnivores, and they are also part of the consumer piece of the ecosystem.

Finally, the last part of the ecosystem is the decomposers. These are the living things which feed off dead plants and animals and reduce their remains to minerals and gases again. Examples are fungi, like mushrooms, and bacteria.

Every organism needs to obtain energy in order to live. For example, plants get energy from the sun, some animals eat plants, and some animals eat other animals.

            A food chain is the sequence of who eats whom in a biological community (an ecosystem) to obtain nutrition. A food chain starts with the primary energy source, usually the sun or boiling-hot deep sea vents. The next link in the chain is an organism that make its own food from the primary energy source -- an example is photosynthetic plants that make their own food from sunlight (using a process called photosynthesis) and chemosynthetic bacteria that make their food energy from chemicals in hydrothermal vents. These are called autotrophs or primary producers.

          A food chain is a linear sequence of links in a food web starting from a trophic species that eats no other species in the web and ends at a trophic species that is eaten by no other species in the web. A food chain differs from a food web, because the complex polyphagous network of feeding relations are aggregated into trophic species and the chain only follows linear monophagous pathways. A common metric used to quantify food web trophic structure is food chain length. In its simplest form, the length of a chain is the number of links between a trophic consumer and the base of the web and the mean chain length of an entire web is the arithmetic average of the lengths of all chains in a food web.

Next come organisms that eat the autotrophs; these organisms are called herbivores or primary consumers -- an example is a rabbit that eats grass.

The next link in the chain is animals that eat herbivores - these are called secondary consumers -- an example is a snake that eat rabbits.

In turn, these animals are eaten by larger predators -- an example is an owl that eats snakes.

The tertiary consumers are are eaten by quaternary consumers -- an example is a hawk that eats owls. Each food chain end with a top predator, and animal with no natural enemies (like an alligator, hawk, or polar bear).

The arrows in a food chain show the flow of energy, from the sun or hydrothermal vent to a top predator. As the energy flows from organism to organism, energy is lost at each step. A network of many food chains is called a food web.

The Food Chain

Every living thing needs energy in order to live. Everytime animals do something (run, jump) they use energy to do so.

Animals get energy from the food they eat, and all living things get energy from food. Plants use sunlight, water and nutrients to get energy (in a process called photosynthesis). Energy is necessary for living beings to grow.

A food chain shows how each living thing gets food, and how nutrients and energy are passed from creature to creature. Food chains begin with plant-life, and end with animal-life. Some animals eat plants, some animals eat other animals.

A simple food chain could start with grass, which is eaten by rabbits. Then the rabbits are eaten by foxes. 

Trophic Levels:

The trophic level of an organism is the position it holds in a food chain.

1. Primary producers (organisms that make their own food from sunlight and/or chemical energy from deep sea vents) are the base of every food chain - these organisms are calledautotrophs.
2. Primary consumers are animals that eat primary producers; they are also called herbivores (plant-eaters).
3. Secondary consumers eat primary consumers. They are carnivores (meat-eaters) and omnivores (animals that eat both animals and plants).
4. Tertiary consumers eat secondary consumers.
5. Quaternary consumers eat tertiary consumers.
6. Food chains "end" with top predators, animals that have little or no natural enemies.

When any organism dies, it is eventually eaten by detrivores (like vultures, worms and crabs) and broken down by decomposers (mostly bacteria and fungi), and the exchange of energy continues.

Some organisms' position in the food chain can vary as their diet differs. For example, when a bear eats berries, the bear is functioning as a primary consumer. When a bear eats a plant-eating rodent, the bear is functioning as a secondary consumer. When the bear eats salmon, the bear is functioning as a tertiary consumer (this is because salmon is a secondary consumer, since salmon eat herring that eat zooplankton that eat phytoplankton, that make their own energy from sunlight). Think about how people's place in the food chain varies - often within a single meal.

Numbers of Organisms: In any food web, energy is lost each time one organism eats another. Because of this, there have to be many more plants than there are plant-eaters. There are more autotrophs than heterotrophs, and more plant-eaters than meat-eaters. Although there is intense competition between animals, there is also an interdependence. When one species goes extinct, it can affect an entire chain of other species and have unpredictable consequences.

Equilibrium

            As the number of carnivores in a community increases, they eat more and more of the herbivores, decreasing the herbivore population. It then becomes harder and harder for the carnivores to find herbivores to eat, and the population of carnivores decreases. In this way, the carnivores and herbivores stay in a relatively stable equilibrium, each limiting the other's population. A similar equilibrium exists between plants and plant-eaters. 

How have humans affected the food chain? 

            When we spray pesticides, we put the food chain in danger.  By breaking one link on the chain means all of the organisms above that link are in threat of extinction (like the domino effect).  By hunting animals nearly to extinction, everything above the animal in the food chain is put in danger.  A 'chain reaction' in the food chain can be perilous!  Since the food chain provides energy that all living things must have in order to survive, it is imperitive that we protect it.

References:
http://www.goodreads.com/quotes/tag/food-chain

http://www.enchantedlearning.com/subjects/foodchain/

http://en.wikipedia.org/wiki/Food_chain

http://www.sheppardsoftware.com/content/animals/kidscorner/foodchain/foodchain.htm

http://www.planetpals.com/foodchain.html

http://library.thinkquest.org/11353/food.htm