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THE FOOD CHAIN: ANIMALS TO HUMANS

The first concepts that I would like to talk about are biotic and abiotic parts of our environment. The biotic component is the living part of our environment. This includes all organisms that exist around us. The abiotic component is the non-living aspect of our environment. These are factors like temperature, sunlight, and participation. Ecology is the study of the interaction between these parts of an organism's environment.

Levels of Biological Organization:

All of the earth's "communities" are organized into a biosphere. A biosphere is the combination of the abiotic and biotic environments and how the "communities" interact with them. All of the organisms contained in the biosphere depend upon each other and the environment around them (4). There are three different divisions to the biosphere:

• Atmosphere: is the gaseous envelope around the earth.
• Hydrosphere: this is the earth's water supply.
• Lithosphere: consists of the soil and rocks on the earth's surface.

  Producers, Consumers, and Decomposers: "Who eats whom?"

The food chain is the way that energy is passed from one organism to another. At the beginning of the food chain are producers. The producers capture the sun’s light and convert it to a form of energy that they can use. From there, the producer’s energy is passed along to consumers who use the producer's energy to make their own energy. Decomposers feed off of consumes and producers which then completes the food chain. An example of energy flow is (8):

The energy flows up the food chain until it gets to the decomposer and then it is "lost." It is lost through biological activities such as metabolism, procreation, movement, and excretion.

Food chains are usually so complex that they will weave together to form food webs. A food web is a more realistic way to visualize how energy moves throughout the ecosystem. The flow of energy moves in one direction only which means that energy can not move backwards it can only go from one animal to the next. The only way that energy can move is as long as it was not already use by the previous organism to do biological work. When energy is lost it is lost as heat and can never be used again (4).

Trophic levels are each "link" or organism in the food chain or food web. The first Trophic level is made up of producers (autotrophs). This is where the energy begins to move after it is taken from the sun. The second Trophic level is made up of primary consumers (herbivores). The third level is made up of secondary consumers (carnivores), and the third is occupied by tertiary consumers (top carnivores). Each of the organisms that make up a level of the food chain also makes up a Trophic level (4).

Ecological pyramids show the progressive movement of energy through the food chain and energy moves from one Trophic level to the next (8).

Discussion:

Bioaccumulation and Biomagnification:

Bioaccumulation is the process by which organisms are affected by chemicals in their environment. Bioaccumulation is the way organisms take in chemicals and store them faster than they are able to break them down. We need to understand this process so we can protect ourselves as well as the plants and animals from the detrimental effects of chemicals. Bioaccumulation has become a major issue in the regulation of chemicals (10).

Some terms that are used in context with bioaccumulation are uptake and storage. Uptake describes how chemicals get into an organisms system. There are different ways that this can occur: absorption through the skin, breathing, or swallowing. This occurs without regard to the storage, metabolism, or excretion by the organism. This means that the chemicals is built up no matter how fast it can be metabolized and excreted. Storage is a term that tends to be confused with Bioaccumulation. It is the temporary deposit of a chemical in the tissue or organs of an organism.

A specific bioaccumulation process is bioconcentration. Bioconcentration is when the concentration of a chemical in an organism increases faster that in the amount in the air or water around the organism. Bioconcentration usually refers to a chemical that is present but that type of chemical is unknown to the organism. An example of a chemical like this would be PCB's. They are also easily absorbed by organisms from their surrounding environment. This is due to the fact that they don't decompose quickly in the water and soil (10).

Biomagnification:

Biomagnification is another aspect of bioaccumulation. This is when there is a higher level of chemicals in the organism than in its food. Biomagnification occurs when a chemical increases in concentration as it moves up the food chain. This is due to the dietary linkage between single celled plants and higher organisms. High levels of pollutants will collect in each of the Trophic levels. This is dangerous to the organisms that are higher up the food chain, because the chemicals have built up along the way. Biomagnification begins at any level of the food chain. The chemicals are absorbed from the ground and the water around the organism. Producers will absorb inorganic nutrients and store them quickly, because they need to gain a great deal in a short time. They will also take in more than they need and this is where the problem begins. The problem is that when pollutants such as DDT and mercury are present in the environment around the producer then this what is taken up in large quantities. These pollutants resemble the inorganic nutrients that they take in, so they can’t differentiate between them. As a result there is a higher concentration inside the organisms than in their surrounding environment. These chemicals are passed along when the producer is consumed by one of the consumers; the biomass is then passed along to that organism. As a result the chemicals will continue to build up as each organism is consumed. This is a usually seen in chemical that is fat soluble like DDT. But with pollutants that are water soluble, Biomagnification will not occur. This is because the pollutant is excreted through the loss of fluid (10).

DDT stands for dichloro biphenyl trichlorethane, and it is a chemical that shows all of the characteristics that chemicals need for Biomagnification. These characteristics are that they are not broken down easily and they are not water-soluble. DDT has a half-life of 15 years (see table). Even after 100 years there would still be a pound of DDT in the environment. The toxicity to humans is rather small and since it is safe to handle it has been used extensively since it was discovered (10). During WWII it was used to fight malaria which is often carried by mosquitoes. After the war DDT was used as an insecticide and it was so over used that the insects were becoming immune to it (10).

This table shows the break down of DDT over a 120-year period (10):

DDT is not the only chemical that biomangifies, PCB’s (polychlorinated biphenyl) are an example of a chemical that biomangifies. PCB’s are insoluble in water and doesn’t break down under high temperatures.

Because of this they are used as lubricants, fire retardant, insulation in transformers, and plasticizers. Problems are then it biomangifies, impairs reproduction, and is usually found in aquatic systems. PCB’s are highly resistant to decomposition and because of this it stays in the soil and water for years and accumulates in the food chain. The ways that PCB’s end up in the environment are industrial and municipal disposal and leaks and it is most dangerous to fish and invertebrates. It is fatal to animals even in small doses and in humans it can cause dizziness and liver failure. Also PCB’s are thought to be cancer causing agents and since these facts have been discovered its usage ahs been curtailed.

Humans Vs. The Food Chain:

Humans are dependent upon agriculture to feed many, many people. We depend upon certain species of plants to produce food and we get rid of plants that do not serve our purposes. This is an example of our agricultural ecosystem:

Problems:

There are problems that exist in these man made ecosystems. One problem is monocultures, which are fields with only one crop (easiest to take care of). But when the crop fails there is no other to take its place and insects are abundant. This then leads to the over use of insecticides. Another problem is the lack of nutrients in the soil. This is due to the fact that the dead plants are pulled up instead of allowing then to be absorbed back into the soil (7).

Natures Solution:

In natural ecosystems there is no problems with monocultures, because plants intermingle. As a result of this intermingling, it makes it more difficult for insects to find food. Also nutrients are recycled, because the decay of dead plants rejuvenates the soil.

Solutions:

No-till farming: this is when a new crop is planted over a dead crop that was there previously. With no-till farming it reduces the erosion of the soil and the loss of its nutrients.

Sewage as fertilizer: the problem with this is that sewage can be contaminated with house hold chemicals and therefore can biomagnify in plants.

Changes in the Antarctic Food Chain:

Even though the waters around Antarctica seem to harsh to live in, there is a great deal of life. The basis of the food chain is microscopic algae and it is consumed by krill, which in turn is consumed by larger animals. Some examples:

Humans have a great impact on this food chain. Whaling has been one of the problems that have affected the food chain and this has lead to a steady drop in baleen whales since 1986 (4).

The hole in the ozone layer has also had an impact on the food chain. This has led to ultraviolet rays which contain higher amounts of energy and is able to break certain chemical bonds (DNA). A study in 1992, showed that the increase in UV rays that penetrated the water's surface decreased the production of algae. This will then affect consumers along the entire food chain (4).