Food Chains, Food Web, Ecological Pyramids

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In an ecosystem, plants capture the sun's energy and use it to convert inorganic compounds into energy-rich organic compounds. This process of using the sun's energy to convert minerals (such as magnesium or nitrogen) in the soil into green leaves, or carrots, or strawberries, is called photosynthesis.

Photosynthesis is only the beginning of a chain of energy conversions. There are many types of animals that will eat the products of the photosynthesis process. Examples are deer eating shrub leaves, rabbits eating carrots, or worms eating grass.

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When these animals eat these plant products, food energy and organic compounds are transferred from the plants to the animals. These animals are in turn eaten by other animals, again transferring energy and organic compounds from one animal to another. Examples would be lions eating zebras, foxes eating rabbits, or birds eating worms.

This chain of energy transferring from one species to another can continue several more times, but it eventually ends. It ends with the dead animals that are broken down and used as food or nutrition by bacteria and fungi.

As these organisms, referred to as decomposers, feed from the dead animals, they break down the complex organic compounds into simple nutrients. Decomposers play a very important role in this world because they take care of breaking down (cleaning) many dead material. There are more than 100,000 different types of decomposer organisms! These simpler nutrients are returned to the soil and can be used again by plants. The energy transformation chain starts all over again.

Producers: Organisms, such as plants, that produce their own food are called autotrophs. The autotrophs, as mentioned before, convert inorganic compounds into organic compounds. They are called producers because all of the species of the ecosystem depend on them.

Consumers: All the organisms that can not make their own food (and need producers) are called heterotrophs. In an ecosystem heterotrophs are called consumers because they depend on others. They obtain food by eating other organisms. There are different levels of consumers. Those that feed directly from producers, i.e. organisms that eat plant or plant products are called primary consumers. In the figure above the grasshopper is a primary consumer.

Organisms that feed on primary consumers are called secondary consumers. Those who feed on secondary consumers are tertiary consumers. In the figure above the snake acts as a secondary consumer and the hawk as a tertiary consumer. Some organisms, like the squirrel are at different levels. When the squirrel eats acorns or fruits (which are plant product), it is a primary consumer; however, when it eats insects or nestling birds, it is a tertiary consumer. Consumers are also classified depending on what they eat; they can be herbivores, carnivores, omnivores or scavengers.

In looking at the previous picture, the concept of food chain looks very simple, but in reality it is more complex. Think about it. How many different animals eat grass? And from the Facts about Red-tailed Hawks page, how many different foods does the hawk eat? One doesn't find simple independent food chains in an ecosystem, but many interdependent and complex food chains that look more like a web and are therefore called food webs.

We described in the previous sections how energy and organic compounds are passed from one trophic level to the next. What was not mentioned is the efficiency of the transfer. In a highly efficient transfer almost all of the energy would be transferred -- 80% or more. In a low efficiency transfer very little energy would be transferred -- less than 20%. In a typical food chain, not all animals or plants are eaten by the next trophic level. In addition, there are portions or materials (such as beaks, shells, bones, etc.) that are also not eaten. That is why the transfer of matter and energy from one trophic level to the next is not an efficient one.

One way to calculate the energy transfer is by measuring or sizing the energy at one trophic level and then at the next. Calorie is a unit of measure used for energy. The energy transfer from one trophic level to the next is about 10%. For example, if there are 10,000 calories at one level, only 1,000 are transferred to the next. This 10% energy and material transfer rule can be depicted with an ecological pyramid that looks like the one below.

This pyramid helps one visualize the fact that in an ecological system there need to be many producing organisms at the bottom of the pyramid to be able to sustain just a couple of organisms at the top. In looking at the pyramid, can you guess how much larger the volume of each layer is as compared to the one just above it? Take a guess. It might not look like it but they are close to 10 times larger.

A basic pyramid shape often represents a typical food chain or food web. The pyramid represents the decrease in the amount of energy, the number of organisms and the biomass from the producer to the high - order consumer levels. The decrease in the numbers and in the biomass represent the fact that, due to energy loss, fewer organisms can be supported at each successive trophic level.

Pyramid of Energy

Energy is lost between each link in a food chain. Much of the potential energy at each level never reaches the next level. Where does the energy go as it moves through a food chain? Some of the energy that enters a food chain is used as each organism carries out its life functions (i.e. foraging, metabolic processes, reproduction, predator/prey behavior, etc.). Producers manufacture their own food source directly from sunlight by the process of photosynthesis. In order to carry out life functions, consumers acquire energy through the 'burning' or breaking down of food molecules they consume (eat). Thermal energy (heat) is produced as a result of the burning of these food molecules. More than half of the energy from each food molecule is lost as heat. Only about 10% - 20% of energy at each trophic level is available to pass on to the next level. In other words, at each level there is only about 10% available energy to put on new biomass (growth).

Pyramid of Numbers

The loss of energy at each trophic level also explains why there are usually fewer organisms in each higher trophic level. The total number of plants in a particular area would generally be higher then the number of herbivores that the plants support and the number of herbivores would be higher than the number of higher order carnivores.

Pyramid of Biomass

Biomass is the total mass of dry organic matter per unit of area. Each higher trophic level contains less biomass than the previous trophic level. Therefore a drawing or graph that represents the amount of biomass at each trophic level would also produce the basic pyramid shape. Biomass is related to the abundance of organisms at each trophic level.

Human Impact on Food Chains and Webs

Humans have the ability to have a great impact on ecosystems. Living organisms are a significant portion of any ecosystem, therefore any activity that affects an ecosystem is also likely to affect the organisms within that ecosystem. If organisms are affected the food chains webs that the organisms are a part of will also feel the affects.

Food Chain Length

Food chain in a Swedish lake. Osprey feed on northern pike, which in turn feed on perch which eat bleak that feed on freshwater shrimp. 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. Food chains were first introduced by the African-Arab scientist and philosopher Al-Jahiz in the 9th century and later popularized in a book published in 1927 by Charles Elton, which also introduced the food web concept.

Food chain length

Food chains are directionaling paths of trophic energy or, equivalently, sequences of links that start with basal species, such as producers or fine organic matter, and end with consumer organisms.

The food chain length is a continuous variable that provides a measure of the passage of energy and an index of ecological structure that increases in value counting progressively through the linkages in a linear fashion from the lowest to the highest trophic (feeding) levels.[7] Food chains are often used in ecological modeling (such as a three species food chain).

They are simplified abstractions of real food webs, but complex in their dynamics and mathematical implications. Ecologists have formulated and tested hypotheses regarding the nature of ecological patterns associated with food chain length, such as increasing length increasing with ecosystem size, reduction of energy at each successive level, or the proposition that long food chain lengths are unstable.

Food chain studies have had an important role in ecotoxicology studies tracing the pathways and biomagnification of environmental contaminants.

Food chain vary in length from three to six or more levels. A food chain consisting of a flower, a frog, a snake and an owl consists of four levels; whereas a food chain consisting of grass, a grasshopper, a rat, a snake and finally a hawk consists of five levels. Producers, such as plants, are organisms that utilize solar energy or heat energy to synthesise starch. All food chains must start with a producer. Consumers are organisms that eat other organisms. All organisms in a food chain, except the first organism, are consumers.

Food chains Importance:

Food chains and webs ensure that one particular species cannot become too large and therefore destroy the species it feeds on, creating a massive inbalance. This means that every creature has a predator and/or environmental threats so their population cannot increase to an amount that is unhealthy for the surounding ecosystem.

A Food chain Example:

This food chain begins with Kelp and seaweed. Krill eats kelp that is in the bottom of the food chain, then it gets eaten by mackreel and herrings. The killer whale is at the top of the food chain that means no animal can eat it. It eats squid, octopus, seal and mackreel. Tuna is good to eat by humans. Kelp, mackreel, octopus, killer whale, red algae, krill, squid, tuna, anchovy, seal, sea lion and herring are linked together in one food chain.

And here are some random other examples for food chains: There are a lot of food chains you can form.