Ecosystems and the Biosphere: Energy Flow Through the Ecosystem and the Recycling of Matter Teacher Lecture Notes Overview: An ecosystem is all the living organisms in a given area as well as the abiotic factors with which they interact. All of the organisms living on Earth need energy to carry out life processes such as growth, movement, and reproduction. In an ecosystem, the ultimate source of energy is the sun. The sun’s energy is converted from one form to another and passed through the various levels of the ecosystem. The flow of energy through an ecosystem is crucial to the ecosystem’s ability to sustain life. I. Energy Flow Through the Ecosystem A. Sunlight is the main energy source for life on Earth. 1. Without this energy from the sun, life on Earth could not exist. 2. Of all the sun’s energy that reaches the earth, less than 1% is actually used by living organisms. 3. This 1% is used by the organisms that are capable of photosynthesis. Photosynthesis is the process by which green plants (and a few other organisms) use the light energy from the sun to convert water and carbon dioxide into glucose and oxygen. 4. The organisms on Earth that cannot carry out photosynthesis rely on the energy that has been stored in the organic compound glucose (a type of sugar) as their source of energy. 5. Not only does photosynthesis provide food in the form of sugars and starches for many of the organisms on Earth, but it also removes carbon dioxide from the atmosphere and releases oxygen into the atmosphere. B. Autotrophs 1. Autotrophs are organisms that have the ability to use energy from the sun to convert simple inorganic substances into complex organic substances. 2. Autotrophs convert carbon dioxide and water into carbohydrates (glucose). 3. The only organisms that are autotrophs are the green plants, the algae, a few species of bacteria and a few protists. 4. On land, the green plants are the main autotrophs. In aquatic ecosystems, algae are the main autotrophs. Photosynthetic bacteria (cyanobacteria) are also important oxygen producers. 5. Autotrophs are also called “producers”. 6. The autotrophs are essential to the flow of energy through the ecosystem. 7. Our very life is dependent on these autotrophs. Without them, we would not have food to eat or oxygen to breathe. 8. A few autotrophs can produce food in the absence of light. Through a process called chemosynthesis, these autotrophs use the energy contained in the chemical bonds of inorganic molecules such as hydrogen sulfide to produce food. Chemosynthesis is a process that is carried out by several types of bacteria. C. Heterotrophs 1. Many organisms cannot directly use the energy from the sun as the autotrophs do. 2. These organisms acquire their energy from other organisms. 3. Heterotrophs are organisms that cannot make their own food. They rely on other organisms for their energy and food supply. 4. 5. Heterotrophs are also called “consumers”. There are many types of heterotrophs: a) Herbivores obtain energy by eating only plants. b) Carnivores obtain energy by eating other animals. c) Omnivores obtain energy by eating both plants and animals. d) Detritivores feed on plant and animal remains, animal wastes and other dead matter. Examples of detritivores are vultures, mites, earthworms, snails, and crabs. e) Decomposers are a class of detritivores that cause decay by breaking down organic compounds. Decomposers include bacteria and fungi. Some of the molecules released during decay are consumed by the decomposer, and some of these molecules are returned to the soil or water. The action of the decomposers makes the nutrients contained in the dead bodies and wastes of organisms available to autotrophs. The process of decomposition recycles chemical nutrients. f) What would happen if there was no decomposition? All life on Earth would cease as detritus (dead organic matter) piled up and the supply of chemical elements needed to build new organisms was exhausted. Chemical elements such as carbon, nitrogen, and phosphorus must be recycled to be used again in new organisms. D. In summary, energy enters the ecosystem in the form of sunlight. It is converted to chemical energy by autotrophs and passed to heterotrophs in the form of organic compounds (molecules of glucose). II. Feeding Relationships A. What happens to energy in an ecosystem as one organism eats another? The energy flows in a one-way path through the ecosystem. Energy enters the ecosystem in the form of sunlight. Photosynthetic organisms convert the sun’s energy into molecules of glucose. This energy is then passed on to the animals that eat the plants and to the animals that eat other animals. B. Energy flows through an ecosystem in one direction, from the sun to autotrophs and then to various heterotrophs. C. Food Chains 1. The energy stored by producers in the form of glucose molecules can be passed through an ecosystem along a food chain. 2. A food chain is a series of steps in which organisms transfer energy from one organism to another by eating and by being eaten. 3. All food chains begin with an autotroph. 4. Examples: a) grass à mouse à snake à hawk b) marine algae à zooplankton à small minnow à squid à whale D. Food Webs 1. In an ecosystem, the feeding relationships between organisms are much too complex to be shown in a single food chain. 2. Many consumers eat more than one type of food. More than one species may feed on the same organism. 3. There are many complex interactions between many different food chains. 4. Food web: The network of complex interactions formed by the feeding relationships among the various organisms in an ecosystem. 5. A food web links all the food chains in an ecosystem together. E. Trophic Levels 1. Each step in a food chain is called a trophic level. The trophic level indicates the organism’s position in the sequence of energy transfers. 2. The first trophic level in a food chain is always made up of producers. These organisms are referred to as primary producers. 3. The second trophic level is occupied by the herbivores that feed on the producers. These organisms are referred to as primary consumers. 4. Predators of herbivores belong to the third level. These organisms are referred to as secondary consumers. 5. A tertiary consumer eats the secondary consumer. 6. Each consumer depends on the trophic level below it for energy. 7. Most ecosystems contain only three or four trophic levels. III. Productivity of the Ecosystem A. The productivity of the ecosystem can be measured in two ways: 1. Gross primary productivity 2. Net primary productivity B. Gross primary productivity: 1. Gross primary productivity is the rate at which the producers (autotrophs) in an ecosystem capture energy. 2. Gross primary productivity is the amount of light energy that is converted to chemical energy by photosynthesis per unit time. 3. The photosynthetic organisms in the ecosystem capture the energy from the sun and store it in molecules of glucose. C. What does the plant do with the glucose it produces? 1. About half of the glucose is used immediately in cellular respiration. Respiration is the conversion of glucose into molecules of ATP, the energy source for a cell. 2. Some of the glucose molecules are used as raw materials (building blocks) for the building of other organic compounds within the cell. 3. Much of the glucose is stored by the plant for future use. D. Biomass is a term that is used to describe the amount of organic material in an ecosystem. Producers add biomass to an ecosystem by making organic compounds. E. Net primary productivity 1. The energy stored as biomass is available to other organisms in the ecosystem. 2. Net primary productivity is equal to gross primary productivity minus the amount of energy used by the producers for respiration. 3. Remember: Gross primary productivity is the total amount of glucose produced by photosynthesis. Some of this glucose is used immediately by the plant in cellular respiration. The remaining glucose is stored and is available to consumers. 4. Net primary productivity is the most important measurement because it represents the amount of chemical energy (glucose) that will be available to consumers in the ecosystem. 5. Net primary productivity varies greatly from one ecosystem to another. For example, net primary productivity in a tropical rain forest is about 25 times greater than the net primary productivity in a desert of the same size. 6. Rain forests account for 5% of the Earth’s surface, but account for 30% of the world’s net primary productivity. 7. In terrestrial ecosystems, three factors determine the net primary productivity: a) light b) temperature c) amount of rainfall An increase in these three factors generally leads to an increase in the amount of photosynthesis taking place, and therefore, an increase in productivity. 8. In aquatic ecosystems, productivity is limited by two factors: a) light b) the availability of nutrients. F. Energy Transfer Between Trophic Levels 1. The amount of energy or matter in an ecosystem can be represented by an ecological pyramid. 2. Ecological pyramid: A diagram that shows the relative amounts of energy or matter contained within each trophic level in a food chain or a food web. 3. Roughly 10% of the total energy consumed in one trophic level is passed to the organisms in the next trophic level. 4. The pyramid shape of the diagram below represents the low percentage of energy transfer from one trophic level to the next. Label each section of the ecological or energy pyramid.