Food Chains, Food Webs, Trophic Levels, Energy Loss
All ecosystems depend on the input of energy. This energy is in most cases from the sun. Living things are needed to convert solar energy from the sun into chemical energy for use by All energy originally comes from the sun living organisms.
Energy is used in a variety of ways. It is released from glucose in the process of respiration. Energy is used by organisms for: ◦ movement of muscles ◦ to keep warm ◦ To create proteins, enzymes, fats and hormones ◦ Growth and repair ◦ Chemical reactions in organs. ◦ Movement of chemical in and out of cells Where the energy is used most will depend on the organism and the stage of its life cycle.
Living things that convert solar energy to chemical energy include plants, algae and cyanobacteria. They use chlorophyll found within their cells to trap visible light (red and blue light) and use it to produce organic compounds from inorganic compounds taken from their environment. These organisms supply all the organic material on which other organisms in an ecosystem rely. These organisms are known as producers. Cyanobacteria
Photosynthesis can be represented by the word equation: carbon dioxide + water + sunlight → glucose + oxygen or as the balanced chemical equation:6CO2 + 6H2O → C6H12O6 + 6O2
Not all of the sunlight energy (ie. the sunlight that falls on the plant) is successfully trapped: ◦ Some is reflected off the waxy surface of the leaf ◦ It can be transferred to the water in the leaf ◦ Or it may be the wrong wavelength of light and can’t be absorbed by the chlorophyll in each plant cell. Overall a plant traps only about 2% of the sunlight that falls on it. The amount of energy that does get trapped (in glucose molecules) is called the gross primary productivity of the plant .
Gross primary productivity (GPP) represents the rate that producers capture and store energy in an ecosystem. It is a way of measuring the photosynthesis process. The rate of GPP is affected by how efficient photosynthesis is in a given ecosystem. ◦ Tropical rainforests for example have a very high GPP as the rainforest has everything plants need to photosynthesize. Not all GPP produced in a system is available to the organisms that depend on it (consumers). The amount of biomass available if referred to as net primary productivity (NPP).
Organisms such as ourselves cannot create organic compounds from inorganic materials and we are known as heterotrophs. Heterotrophs consume other organisms to obtain their organic material. They are known as consumers. They may obtain these materials by consuming the producer directly or by consuming an organism that has consumed a producer. These consumers include animals, fungi and many kinds of bacteria.
From the simple compounds and mineral nutrients they consume, consumers create the complex compounds they rely on for growth and repair of damaged tissue. The energy need to do this is supplied by the breakdown of some of the carbohydrates made in photosynthesis. The process is called respiration- the release of energy in the bonds of glucose. This process also produces heat which is lost to the surroundings.
As one organism feeds on another a food chain is formed. Each link in the chain is known as a trophic level. Each animal in the chain feeds on and therefore gains its energy from the organism preceding it in the food chain. In this way matter and energy are passed from one organism to the next.
Animals that feed directly on producers are known as herbivores (or omnivores) or first order consumers. Those carnivores (or omnivores) that depend on first order consumers are called second order consumers and so on. The top consumers are not usually preyed upon and die of old age, disease or injury. Animals that feed on the remains of dead animals are known as scavengers. Organisms that feed on dead or decaying remains and waste (detritus) are known as detritivores. Organisms that live on complex molecules by breaking organic material down are known as decomposers.
A simple food chain in reality is rare. Think about what would happen if just one of the species in the food chain ran out- the whole system would collapse. Instead a herbivore will feed on a large number of plants and in turn will be eaten by a range of consumers and so on. These networks of food chains form a food web. An organism in one food chain may have a different trophic level in another food chain so it can be hard to assign an organism to one trophic level. Food webs are dynamic as ecosystems are open and organisms can move in and out bringing and taking matter and energy with them.
The food chain represents the transfer of energy from one organism to the organism that eats it and so on. However at each trophic level not all of the energy is transferred. Some is lost from the food chain as heat energy released during cell respiration and some as chemical energy in wastes and any left-over dead animal or plant matter (detritus). This loss of energy means that as you move along the food chain less energy is available to pass on to the next level. Fortunately energy continually flows into the system as producers continue to trap the energy of sunlight.
Three hundred trout are needed to support one man for a year. The trout, in turn, must consume 90,000 frogs, that must consume 27 million grasshoppers that live off of 1,000 tons of grass. -- G. Tyler Miller, Jr., American Chemist (1971)
The percentage of energy at one trophic level that ends up in the next trophic level is referred to as trophic efficiency.
As energy is lost at each trophic level, the more trophic levels in a food chain the more energy is lost and the more lower order organisms must be consumed by the higher order organism for it to obtain its energy requirements. This also makes it more likely that interruptions to the lower order organisms will affect higher order organisms. The loss of energy also limits a food chain as eventually much of the energy will be lost.
The amount of energy that is available to be passed from one organism to the next varies with the organism. Insects for example pass on more energy as they are ectothermic and do not use energy trying to maintain a constant body temperature. A useful rule is the 10% rule which states that about 10% of the energy at one trophic level is passed on to the next. The remaining 90% is lost as heat energy and chemical energy in wastes. Measuring trophic efficiency has high relevancy to agriculture. Refer to Figure 11.14 on page 353. Some of the highest trophic efficiencies are found in the ocean. It has been observed that zooplankton feeding on phytoplankton (producer) receive up to 40% of the energy found in the phytoplankton.
The total mass of the organic matter at each trophic level is called biomass Biomass is just another term for potential energy – energy that is to be eaten and used. The transfer of energy from one level to another is very inefficient (10% Law)
An ecological pyramid shows the relationship between consumers and producers at different trophic levels in an ecosystem Shows the relative amounts of energy or matter contained at each trophic level The Pyramid shows which level has the most energy and the highest number of organisms